[v3,resend,1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

Commit Message

Joel Fernandes Nov. 8, 2018, 4:15 a.m. UTC

Android uses ashmem for sharing memory regions. We are looking forward
to migrating all usecases of ashmem to memfd so that we can possibly
remove the ashmem driver in the future from staging while also
benefiting from using memfd and contributing to it. Note staging drivers
are also not ABI and generally can be removed at anytime.

One of the main usecases Android has is the ability to create a region
and mmap it as writeable, then add protection against making any
"future" writes while keeping the existing already mmap'ed
writeable-region active.  This allows us to implement a usecase where
receivers of the shared memory buffer can get a read-only view, while
the sender continues to write to the buffer.
See CursorWindow documentation in Android for more details:
https://developer.android.com/reference/android/database/CursorWindow

This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
which prevents any future mmap and write syscalls from succeeding while
keeping the existing mmap active. The following program shows the seal
working in action:

 #include <stdio.h>
 #include <errno.h>
 #include <sys/mman.h>
 #include <linux/memfd.h>
 #include <linux/fcntl.h>
 #include <asm/unistd.h>
 #include <unistd.h>
 #define F_SEAL_FUTURE_WRITE 0x0010
 #define REGION_SIZE (5 * 1024 * 1024)

int memfd_create_region(const char *name, size_t size)
{
    int ret;
    int fd = syscall(__NR_memfd_create, name, MFD_ALLOW_SEALING);
    if (fd < 0) return fd;
    ret = ftruncate(fd, size);
    if (ret < 0) { close(fd); return ret; }
    return fd;
}

int main() {
    int ret, fd;
    void *addr, *addr2, *addr3, *addr1;
    ret = memfd_create_region("test_region", REGION_SIZE);
    printf("ret=%d\n", ret);
    fd = ret;

    // Create map
    addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)
	    printf("map 0 failed\n");
    else
	    printf("map 0 passed\n");

    if ((ret = write(fd, "test", 4)) != 4)
	    printf("write failed even though no future-write seal "
		   "(ret=%d errno =%d)\n", ret, errno);
    else
	    printf("write passed\n");

    addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr1 == MAP_FAILED)
	    perror("map 1 prot-write failed even though no seal\n");
    else
	    printf("map 1 prot-write passed as expected\n");

    ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE |
				 F_SEAL_GROW |
				 F_SEAL_SHRINK);
    if (ret == -1)
	    printf("fcntl failed, errno: %d\n", errno);
    else
	    printf("future-write seal now active\n");

    if ((ret = write(fd, "test", 4)) != 4)
	    printf("write failed as expected due to future-write seal\n");
    else
	    printf("write passed (unexpected)\n");

    addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr2 == MAP_FAILED)
	    perror("map 2 prot-write failed as expected due to seal\n");
    else
	    printf("map 2 passed\n");

    addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0);
    if (addr3 == MAP_FAILED)
	    perror("map 3 failed\n");
    else
	    printf("map 3 prot-read passed as expected\n");
}

The output of running this program is as follows:
ret=3
map 0 passed
write passed
map 1 prot-write passed as expected
future-write seal now active
write failed as expected due to future-write seal
map 2 prot-write failed as expected due to seal
: Permission denied
map 3 prot-read passed as expected

Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Reviewed-by: John Stultz <[e-mail]>
Signed-off-by: Joel Fernandes (Google) <[e-mail]>
---
v1->v2: No change, just added selftests to the series. manpages are
ready and I'll submit them once the patches are accepted.

v2->v3: Updated commit message to have more support code (John Stultz)
 	Renamed seal from F_SEAL_FS_WRITE to F_SEAL_FUTURE_WRITE
						(Christoph Hellwig)
	Allow for this seal only if grow/shrink seals are also
	either previous set, or are requested along with this seal.
						(Christoph Hellwig)
	Added locking to synchronize access to file->f_mode.
						(Christoph Hellwig)

 include/uapi/linux/fcntl.h |  1 +
 mm/memfd.c                 | 22 +++++++++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

Comments

Joel Fernandes Nov. 9, 2018, 8:49 a.m. UTC | #1

On Wed, Nov 07, 2018 at 08:15:36PM -0800, Joel Fernandes (Google) wrote:
> Android uses ashmem for sharing memory regions. We are looking forward
> to migrating all usecases of ashmem to memfd so that we can possibly
> remove the ashmem driver in the future from staging while also
> benefiting from using memfd and contributing to it. Note staging drivers
> are also not ABI and generally can be removed at anytime.
> 
> One of the main usecases Android has is the ability to create a region
> and mmap it as writeable, then add protection against making any
> "future" writes while keeping the existing already mmap'ed
> writeable-region active.  This allows us to implement a usecase where
> receivers of the shared memory buffer can get a read-only view, while
> the sender continues to write to the buffer.
> See CursorWindow documentation in Android for more details:
> https://developer.android.com/reference/android/database/CursorWindow
> 
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> which prevents any future mmap and write syscalls from succeeding while
> keeping the existing mmap active. The following program shows the seal
> working in action:
> 
[...] 
> The output of running this program is as follows:
> ret=3
> map 0 passed
> write passed
> map 1 prot-write passed as expected
> future-write seal now active
> write failed as expected due to future-write seal
> map 2 prot-write failed as expected due to seal
> : Permission denied
> map 3 prot-read passed as expected
> 
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Reviewed-by: John Stultz <[e-mail]>
> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> ---
> v1->v2: No change, just added selftests to the series. manpages are
> ready and I'll submit them once the patches are accepted.
> 
> v2->v3: Updated commit message to have more support code (John Stultz)
>  	Renamed seal from F_SEAL_FS_WRITE to F_SEAL_FUTURE_WRITE
> 						(Christoph Hellwig)
> 	Allow for this seal only if grow/shrink seals are also
> 	either previous set, or are requested along with this seal.
> 						(Christoph Hellwig)
> 	Added locking to synchronize access to file->f_mode.
> 						(Christoph Hellwig)


Christoph, do the patches look Ok to you now? If so, then could you give an
Acked-by or Reviewed-by tag?

Thanks a lot,

 - Joel


>  include/uapi/linux/fcntl.h |  1 +
>  mm/memfd.c                 | 22 +++++++++++++++++++++-
>  2 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 6448cdd9a350..a2f8658f1c55 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -41,6 +41,7 @@
>  #define F_SEAL_SHRINK	0x0002	/* prevent file from shrinking */
>  #define F_SEAL_GROW	0x0004	/* prevent file from growing */
>  #define F_SEAL_WRITE	0x0008	/* prevent writes */
> +#define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
>  /* (1U << 31) is reserved for signed error codes */
>  
>  /*
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 2bb5e257080e..5ba9804e9515 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
>  #define F_ALL_SEALS (F_SEAL_SEAL | \
>  		     F_SEAL_SHRINK | \
>  		     F_SEAL_GROW | \
> -		     F_SEAL_WRITE)
> +		     F_SEAL_WRITE | \
> +		     F_SEAL_FUTURE_WRITE)
>  
>  static int memfd_add_seals(struct file *file, unsigned int seals)
>  {
> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
>  		}
>  	}
>  
> +	if ((seals & F_SEAL_FUTURE_WRITE) &&
> +	    !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> +		/*
> +		 * The FUTURE_WRITE seal also prevents growing and shrinking
> +		 * so we need them to be already set, or requested now.
> +		 */
> +		int test_seals = (seals | *file_seals) &
> +				 (F_SEAL_GROW | F_SEAL_SHRINK);
> +
> +		if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> +			error = -EINVAL;
> +			goto unlock;
> +		}
> +
> +		spin_lock(&file->f_lock);
> +		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> +		spin_unlock(&file->f_lock);
> +	}
> +
>  	*file_seals |= seals;
>  	error = 0;
>  
> -- 
> 2.19.1.930.g4563a0d9d0-goog

Michael Tirado Nov. 9, 2018, 8:02 p.m. UTC | #2

On Fri, Nov 9, 2018 at 9:41 PM Andy Lutomirski <[e-mail]> wrote:
>
>
>
> > On Nov 9, 2018, at 1:06 PM, Jann Horn <[e-mail]> wrote:
> >
> > +linux-api for API addition
> > +hughd as FYI since this is somewhat related to mm/shmem
> >
> > On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> > <[e-mail]> wrote:
> >> Android uses ashmem for sharing memory regions. We are looking forward
> >> to migrating all usecases of ashmem to memfd so that we can possibly
> >> remove the ashmem driver in the future from staging while also
> >> benefiting from using memfd and contributing to it. Note staging drivers
> >> are also not ABI and generally can be removed at anytime.
> >>
> >> One of the main usecases Android has is the ability to create a region
> >> and mmap it as writeable, then add protection against making any
> >> "future" writes while keeping the existing already mmap'ed
> >> writeable-region active.  This allows us to implement a usecase where
> >> receivers of the shared memory buffer can get a read-only view, while
> >> the sender continues to write to the buffer.

Oh I remember trying this years ago with a new seal, F_SEAL_WRITE_PEER,
or something like that.

> >
> > So you're fiddling around with the file, but not the inode? How are
> > you preventing code like the following from re-opening the file as
> > writable?
> >
> > $ cat memfd.c
> > #define _GNU_SOURCE
> > #include <unistd.h>
> > #include <sys/syscall.h>
> > #include <printf.h>
> > #include <fcntl.h>
> > #include <err.h>
> > #include <stdio.h>
> >
> > int main(void) {
> >  int fd = syscall(__NR_memfd_create, "testfd", 0);
> >  if (fd == -1) err(1, "memfd");
> >  char path[100];
> >  sprintf(path, "/proc/self/fd/%d", fd);
> >  int fd2 = open(path, O_RDWR);
> >  if (fd2 == -1) err(1, "reopen");
> >  printf("reopen successful: %d\n", fd2);
> > }
> > $ gcc -o memfd memfd.c
> > $ ./memfd
> > reopen successful: 4
> > $
> >

The race condition between memfd_create and applying seals in fcntl?
I think it would be possible to block new write mappings from peer processes if
there is a new memfd_create api that accepts seals. Allowing caller to
set a seal
like the one I proposed years ago, though in a race-free manner. Then
also consider
how to properly handle blocking inherited +W mapping through
clone/fork. Maybe I'm
forgetting some other pitfalls?



> > That aside: I wonder whether a better API would be something that
> > allows you to create a new readonly file descriptor, instead of
> > fiddling with the writability of an existing fd.
>
> Every now and then I try to write a patch to prevent using proc to reopen a file with greater permission than the original open.
>
> I like your idea to have a clean way to reopen a a memfd with reduced permissions. But I would make it a syscall instead and maybe make it only work for memfd at first.  And the proc issue would need to be fixed, too.

IMO the best solution would handle the issue at memfd creation time by
removing the race condition.

Andrew Morton Nov. 9, 2018, 8:36 p.m. UTC | #3

On Wed,  7 Nov 2018 20:15:36 -0800 "Joel Fernandes (Google)" <[e-mail]> wrote:

> Android uses ashmem for sharing memory regions. We are looking forward
> to migrating all usecases of ashmem to memfd so that we can possibly
> remove the ashmem driver in the future from staging while also
> benefiting from using memfd and contributing to it. Note staging drivers
> are also not ABI and generally can be removed at anytime.
> 
> One of the main usecases Android has is the ability to create a region
> and mmap it as writeable, then add protection against making any
> "future" writes while keeping the existing already mmap'ed
> writeable-region active.  This allows us to implement a usecase where
> receivers of the shared memory buffer can get a read-only view, while
> the sender continues to write to the buffer.
> See CursorWindow documentation in Android for more details:
> https://developer.android.com/reference/android/database/CursorWindow

It appears that the memfd_create and fcntl manpages will require
updating.  Please attend to this at the appropriate time?

Actually, it would help the review process if those updates were
available now.

Jann Horn Nov. 9, 2018, 9:06 p.m. UTC | #4

+linux-api for API addition
+hughd as FYI since this is somewhat related to mm/shmem

On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
<[e-mail]> wrote:
> Android uses ashmem for sharing memory regions. We are looking forward
> to migrating all usecases of ashmem to memfd so that we can possibly
> remove the ashmem driver in the future from staging while also
> benefiting from using memfd and contributing to it. Note staging drivers
> are also not ABI and generally can be removed at anytime.
>
> One of the main usecases Android has is the ability to create a region
> and mmap it as writeable, then add protection against making any
> "future" writes while keeping the existing already mmap'ed
> writeable-region active.  This allows us to implement a usecase where
> receivers of the shared memory buffer can get a read-only view, while
> the sender continues to write to the buffer.
> See CursorWindow documentation in Android for more details:
> https://developer.android.com/reference/android/database/CursorWindow
>
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> which prevents any future mmap and write syscalls from succeeding while
> keeping the existing mmap active.

Please CC linux-api@ on patches like this. If you had done that, I
might have criticized your v1 patch instead of your v3 patch...

> The following program shows the seal
> working in action:
[...]
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Reviewed-by: John Stultz <[e-mail]>
> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> ---
[...]
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 2bb5e257080e..5ba9804e9515 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
[...]
> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
>                 }
>         }
>
> +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> +               /*
> +                * The FUTURE_WRITE seal also prevents growing and shrinking
> +                * so we need them to be already set, or requested now.
> +                */
> +               int test_seals = (seals | *file_seals) &
> +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> +
> +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> +                       error = -EINVAL;
> +                       goto unlock;
> +               }
> +
> +               spin_lock(&file->f_lock);
> +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> +               spin_unlock(&file->f_lock);
> +       }

So you're fiddling around with the file, but not the inode? How are
you preventing code like the following from re-opening the file as
writable?

$ cat memfd.c
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <printf.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>

int main(void) {
  int fd = syscall(__NR_memfd_create, "testfd", 0);
  if (fd == -1) err(1, "memfd");
  char path[100];
  sprintf(path, "/proc/self/fd/%d", fd);
  int fd2 = open(path, O_RDWR);
  if (fd2 == -1) err(1, "reopen");
  printf("reopen successful: %d\n", fd2);
}
$ gcc -o memfd memfd.c
$ ./memfd
reopen successful: 4
$

That aside: I wonder whether a better API would be something that
allows you to create a new readonly file descriptor, instead of
fiddling with the writability of an existing fd.

Jann Horn Nov. 9, 2018, 9:19 p.m. UTC | #5

On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <[e-mail]> wrote:
> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> <[e-mail]> wrote:
> > Android uses ashmem for sharing memory regions. We are looking forward
> > to migrating all usecases of ashmem to memfd so that we can possibly
> > remove the ashmem driver in the future from staging while also
> > benefiting from using memfd and contributing to it. Note staging drivers
> > are also not ABI and generally can be removed at anytime.
> >
> > One of the main usecases Android has is the ability to create a region
> > and mmap it as writeable, then add protection against making any
> > "future" writes while keeping the existing already mmap'ed
> > writeable-region active.  This allows us to implement a usecase where
> > receivers of the shared memory buffer can get a read-only view, while
> > the sender continues to write to the buffer.
> > See CursorWindow documentation in Android for more details:
> > https://developer.android.com/reference/android/database/CursorWindow
> >
> > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> > which prevents any future mmap and write syscalls from succeeding while
> > keeping the existing mmap active.
>
> Please CC linux-api@ on patches like this. If you had done that, I
> might have criticized your v1 patch instead of your v3 patch...
>
> > The following program shows the seal
> > working in action:
> [...]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Reviewed-by: John Stultz <[e-mail]>
> > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> > ---
> [...]
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 2bb5e257080e..5ba9804e9515 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> [...]
> > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> >                 }
> >         }
> >
> > +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> > +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> > +               /*
> > +                * The FUTURE_WRITE seal also prevents growing and shrinking
> > +                * so we need them to be already set, or requested now.
> > +                */
> > +               int test_seals = (seals | *file_seals) &
> > +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> > +
> > +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> > +                       error = -EINVAL;
> > +                       goto unlock;
> > +               }
> > +
> > +               spin_lock(&file->f_lock);
> > +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > +               spin_unlock(&file->f_lock);
> > +       }
>
> So you're fiddling around with the file, but not the inode? How are
> you preventing code like the following from re-opening the file as
> writable?
>
> $ cat memfd.c
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <printf.h>
> #include <fcntl.h>
> #include <err.h>
> #include <stdio.h>
>
> int main(void) {
>   int fd = syscall(__NR_memfd_create, "testfd", 0);
>   if (fd == -1) err(1, "memfd");
>   char path[100];
>   sprintf(path, "/proc/self/fd/%d", fd);
>   int fd2 = open(path, O_RDWR);
>   if (fd2 == -1) err(1, "reopen");
>   printf("reopen successful: %d\n", fd2);
> }
> $ gcc -o memfd memfd.c
> $ ./memfd
> reopen successful: 4
> $
>
> That aside: I wonder whether a better API would be something that
> allows you to create a new readonly file descriptor, instead of
> fiddling with the writability of an existing fd.

My favorite approach would be to forbid open() on memfds, hope that
nobody notices the tiny API break, and then add an ioctl for "reopen
this memfd with reduced permissions" - but that's just my personal
opinion.

Andy Lutomirski Nov. 9, 2018, 9:40 p.m. UTC | #6

> On Nov 9, 2018, at 1:06 PM, Jann Horn <[e-mail]> wrote:
> 
> +linux-api for API addition
> +hughd as FYI since this is somewhat related to mm/shmem
> 
> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> <[e-mail]> wrote:
>> Android uses ashmem for sharing memory regions. We are looking forward
>> to migrating all usecases of ashmem to memfd so that we can possibly
>> remove the ashmem driver in the future from staging while also
>> benefiting from using memfd and contributing to it. Note staging drivers
>> are also not ABI and generally can be removed at anytime.
>> 
>> One of the main usecases Android has is the ability to create a region
>> and mmap it as writeable, then add protection against making any
>> "future" writes while keeping the existing already mmap'ed
>> writeable-region active.  This allows us to implement a usecase where
>> receivers of the shared memory buffer can get a read-only view, while
>> the sender continues to write to the buffer.
>> See CursorWindow documentation in Android for more details:
>> https://developer.android.com/reference/android/database/CursorWindow
>> 
>> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
>> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
>> which prevents any future mmap and write syscalls from succeeding while
>> keeping the existing mmap active.
> 
> Please CC linux-api@ on patches like this. If you had done that, I
> might have criticized your v1 patch instead of your v3 patch...
> 
>> The following program shows the seal
>> working in action:
> [...]
>> Cc: [e-mail]
>> Cc: [e-mail]
>> Cc: [e-mail]
>> Cc: [e-mail]
>> Cc: [e-mail]
>> Reviewed-by: John Stultz <[e-mail]>
>> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
>> ---
> [...]
>> diff --git a/mm/memfd.c b/mm/memfd.c
>> index 2bb5e257080e..5ba9804e9515 100644
>> --- a/mm/memfd.c
>> +++ b/mm/memfd.c
> [...]
>> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
>>                }
>>        }
>> 
>> +       if ((seals & F_SEAL_FUTURE_WRITE) &&
>> +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
>> +               /*
>> +                * The FUTURE_WRITE seal also prevents growing and shrinking
>> +                * so we need them to be already set, or requested now.
>> +                */
>> +               int test_seals = (seals | *file_seals) &
>> +                                (F_SEAL_GROW | F_SEAL_SHRINK);
>> +
>> +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
>> +                       error = -EINVAL;
>> +                       goto unlock;
>> +               }
>> +
>> +               spin_lock(&file->f_lock);
>> +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
>> +               spin_unlock(&file->f_lock);
>> +       }
> 
> So you're fiddling around with the file, but not the inode? How are
> you preventing code like the following from re-opening the file as
> writable?
> 
> $ cat memfd.c
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <printf.h>
> #include <fcntl.h>
> #include <err.h>
> #include <stdio.h>
> 
> int main(void) {
>  int fd = syscall(__NR_memfd_create, "testfd", 0);
>  if (fd == -1) err(1, "memfd");
>  char path[100];
>  sprintf(path, "/proc/self/fd/%d", fd);
>  int fd2 = open(path, O_RDWR);
>  if (fd2 == -1) err(1, "reopen");
>  printf("reopen successful: %d\n", fd2);
> }
> $ gcc -o memfd memfd.c
> $ ./memfd
> reopen successful: 4
> $
> 
> That aside: I wonder whether a better API would be something that
> allows you to create a new readonly file descriptor, instead of
> fiddling with the writability of an existing fd.

Every now and then I try to write a patch to prevent using proc to reopen a file with greater permission than the original open.

I like your idea to have a clean way to reopen a a memfd with reduced permissions. But I would make it a syscall instead and maybe make it only work for memfd at first.  And the proc issue would need to be fixed, too.

Daniel Colascione Nov. 9, 2018, 10:20 p.m. UTC | #7

On Fri, Nov 9, 2018 at 1:06 PM, Jann Horn <[e-mail]> wrote:
>
> +linux-api for API addition
> +hughd as FYI since this is somewhat related to mm/shmem
>
> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> <[e-mail]> wrote:
> > Android uses ashmem for sharing memory regions. We are looking forward
> > to migrating all usecases of ashmem to memfd so that we can possibly
> > remove the ashmem driver in the future from staging while also
> > benefiting from using memfd and contributing to it. Note staging drivers
> > are also not ABI and generally can be removed at anytime.
> >
> > One of the main usecases Android has is the ability to create a region
> > and mmap it as writeable, then add protection against making any
> > "future" writes while keeping the existing already mmap'ed
> > writeable-region active.  This allows us to implement a usecase where
> > receivers of the shared memory buffer can get a read-only view, while
> > the sender continues to write to the buffer.
> > See CursorWindow documentation in Android for more details:
> > https://developer.android.com/reference/android/database/CursorWindow
> >
> > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> > which prevents any future mmap and write syscalls from succeeding while
> > keeping the existing mmap active.
>
> Please CC linux-api@ on patches like this. If you had done that, I
> might have criticized your v1 patch instead of your v3 patch...
>
> > The following program shows the seal
> > working in action:
> [...]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Reviewed-by: John Stultz <[e-mail]>
> > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> > ---
> [...]
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 2bb5e257080e..5ba9804e9515 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> [...]
> > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> >                 }
> >         }
> >
> > +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> > +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> > +               /*
> > +                * The FUTURE_WRITE seal also prevents growing and shrinking
> > +                * so we need them to be already set, or requested now.
> > +                */
> > +               int test_seals = (seals | *file_seals) &
> > +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> > +
> > +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> > +                       error = -EINVAL;
> > +                       goto unlock;
> > +               }
> > +
> > +               spin_lock(&file->f_lock);
> > +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > +               spin_unlock(&file->f_lock);
> > +       }
>
> So you're fiddling around with the file, but not the inode? How are
> you preventing code like the following from re-opening the file as
> writable?

Good catch. That's fixable too though, isn't it, just by fiddling with
the inode, right?

Another, more general fix might be to prevent /proc/pid/fd/N opens
from "upgrading" access modes. But that'd be a bigger ABI break.

> That aside: I wonder whether a better API would be something that
> allows you to create a new readonly file descriptor, instead of
> fiddling with the writability of an existing fd.

That doesn't work, unfortunately. The ashmem API we're replacing with
memfd requires file descriptor continuity. I also looked into opening
a new FD and dup2(2)ing atop the old one, but this approach doesn't
work in the case that the old FD has already leaked to some other
context (e.g., another dup, SCM_RIGHTS). See
https://developer.android.com/ndk/reference/group/memory. We can't
break ASharedMemory_setProt.

Andy Lutomirski Nov. 9, 2018, 10:37 p.m. UTC | #8

> On Nov 9, 2018, at 2:20 PM, Daniel Colascione <[e-mail]> wrote:
> 
>> On Fri, Nov 9, 2018 at 1:06 PM, Jann Horn <[e-mail]> wrote:
>> 
>> +linux-api for API addition
>> +hughd as FYI since this is somewhat related to mm/shmem
>> 
>> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
>> <[e-mail]> wrote:
>>> Android uses ashmem for sharing memory regions. We are looking forward
>>> to migrating all usecases of ashmem to memfd so that we can possibly
>>> remove the ashmem driver in the future from staging while also
>>> benefiting from using memfd and contributing to it. Note staging drivers
>>> are also not ABI and generally can be removed at anytime.
>>> 
>>> One of the main usecases Android has is the ability to create a region
>>> and mmap it as writeable, then add protection against making any
>>> "future" writes while keeping the existing already mmap'ed
>>> writeable-region active.  This allows us to implement a usecase where
>>> receivers of the shared memory buffer can get a read-only view, while
>>> the sender continues to write to the buffer.
>>> See CursorWindow documentation in Android for more details:
>>> https://developer.android.com/reference/android/database/CursorWindow
>>> 
>>> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
>>> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
>>> which prevents any future mmap and write syscalls from succeeding while
>>> keeping the existing mmap active.
>> 
>> Please CC linux-api@ on patches like this. If you had done that, I
>> might have criticized your v1 patch instead of your v3 patch...
>> 
>>> The following program shows the seal
>>> working in action:
>> [...]
>>> Cc: [e-mail]
>>> Cc: [e-mail]
>>> Cc: [e-mail]
>>> Cc: [e-mail]
>>> Cc: [e-mail]
>>> Reviewed-by: John Stultz <[e-mail]>
>>> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
>>> ---
>> [...]
>>> diff --git a/mm/memfd.c b/mm/memfd.c
>>> index 2bb5e257080e..5ba9804e9515 100644
>>> --- a/mm/memfd.c
>>> +++ b/mm/memfd.c
>> [...]
>>> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
>>>                }
>>>        }
>>> 
>>> +       if ((seals & F_SEAL_FUTURE_WRITE) &&
>>> +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
>>> +               /*
>>> +                * The FUTURE_WRITE seal also prevents growing and shrinking
>>> +                * so we need them to be already set, or requested now.
>>> +                */
>>> +               int test_seals = (seals | *file_seals) &
>>> +                                (F_SEAL_GROW | F_SEAL_SHRINK);
>>> +
>>> +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
>>> +                       error = -EINVAL;
>>> +                       goto unlock;
>>> +               }
>>> +
>>> +               spin_lock(&file->f_lock);
>>> +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
>>> +               spin_unlock(&file->f_lock);
>>> +       }
>> 
>> So you're fiddling around with the file, but not the inode? How are
>> you preventing code like the following from re-opening the file as
>> writable?
> 
> Good catch. That's fixable too though, isn't it, just by fiddling with
> the inode, right?

True.

> 
> Another, more general fix might be to prevent /proc/pid/fd/N opens
> from "upgrading" access modes. But that'd be a bigger ABI break.

I think we should fix that, too.  I consider it a bug fix, not an ABI break, personally.

> 
>> That aside: I wonder whether a better API would be something that
>> allows you to create a new readonly file descriptor, instead of
>> fiddling with the writability of an existing fd.
> 
> That doesn't work, unfortunately. The ashmem API we're replacing with
> memfd requires file descriptor continuity. I also looked into opening
> a new FD and dup2(2)ing atop the old one, but this approach doesn't
> work in the case that the old FD has already leaked to some other
> context (e.g., another dup, SCM_RIGHTS). See
> https://developer.android.com/ndk/reference/group/memory. We can't
> break ASharedMemory_setProt.


Hmm.  If we fix the general reopen bug, a way to drop write access from an existing struct file would do what Android needs, right?  I don’t know if there are general VFS issues with that.

Daniel Colascione Nov. 9, 2018, 10:42 p.m. UTC | #9

On Fri, Nov 9, 2018 at 2:37 PM, Andy Lutomirski <[e-mail]> wrote:
>> Another, more general fix might be to prevent /proc/pid/fd/N opens
>> from "upgrading" access modes. But that'd be a bigger ABI break.
>
> I think we should fix that, too.  I consider it a bug fix, not an ABI break, personally.

Someone, somewhere is probably relying on it though, and that means
that we probably can't change it unless it's actually causing
problems.

<mumble>spacebar heating</mumble>

>>> That aside: I wonder whether a better API would be something that
>>> allows you to create a new readonly file descriptor, instead of
>>> fiddling with the writability of an existing fd.
>>
>> That doesn't work, unfortunately. The ashmem API we're replacing with
>> memfd requires file descriptor continuity. I also looked into opening
>> a new FD and dup2(2)ing atop the old one, but this approach doesn't
>> work in the case that the old FD has already leaked to some other
>> context (e.g., another dup, SCM_RIGHTS). See
>> https://developer.android.com/ndk/reference/group/memory. We can't
>> break ASharedMemory_setProt.
>
>
> Hmm.  If we fix the general reopen bug, a way to drop write access from an existing struct file would do what Android needs, right?  I don’t know if there are general VFS issues with that.

I also proposed that. :-) Maybe it'd work best as a special case of
the perennial revoke(2) that people keep proposing. You'd be able to
selectively revoke all access or just write access.

Andy Lutomirski Nov. 9, 2018, 11:14 p.m. UTC | #10

> On Nov 9, 2018, at 2:42 PM, Daniel Colascione <[e-mail]> wrote:
> 
> On Fri, Nov 9, 2018 at 2:37 PM, Andy Lutomirski <[e-mail]> wrote:
>>> Another, more general fix might be to prevent /proc/pid/fd/N opens
>>> from "upgrading" access modes. But that'd be a bigger ABI break.
>> 
>> I think we should fix that, too.  I consider it a bug fix, not an ABI break, personally.
> 
> Someone, somewhere is probably relying on it though, and that means
> that we probably can't change it unless it's actually causing
> problems.
> 
> <mumble>spacebar heating</mumble>

I think it has caused problems in the past. It’s certainly extremely surprising behavior.  I’d say it should be fixed and, if needed, a sysctl to unfix it might be okay.

> 
>>>> That aside: I wonder whether a better API would be something that
>>>> allows you to create a new readonly file descriptor, instead of
>>>> fiddling with the writability of an existing fd.
>>> 
>>> That doesn't work, unfortunately. The ashmem API we're replacing with
>>> memfd requires file descriptor continuity. I also looked into opening
>>> a new FD and dup2(2)ing atop the old one, but this approach doesn't
>>> work in the case that the old FD has already leaked to some other
>>> context (e.g., another dup, SCM_RIGHTS). See
>>> https://developer.android.com/ndk/reference/group/memory. We can't
>>> break ASharedMemory_setProt.
>> 
>> 
>> Hmm.  If we fix the general reopen bug, a way to drop write access from an existing struct file would do what Android needs, right?  I don’t know if there are general VFS issues with that.
> 
> I also proposed that. :-) Maybe it'd work best as a special case of
> the perennial revoke(2) that people keep proposing. You'd be able to
> selectively revoke all access or just write access.

Sounds good to me, modulo possible races, but that shouldn’t be too hard to deal with.

Joel Fernandes Nov. 9, 2018, 11:46 p.m. UTC | #11

On Fri, Nov 09, 2018 at 10:06:31PM +0100, Jann Horn wrote:
> +linux-api for API addition
> +hughd as FYI since this is somewhat related to mm/shmem
> 
> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> <[e-mail]> wrote:
> > Android uses ashmem for sharing memory regions. We are looking forward
> > to migrating all usecases of ashmem to memfd so that we can possibly
> > remove the ashmem driver in the future from staging while also
> > benefiting from using memfd and contributing to it. Note staging drivers
> > are also not ABI and generally can be removed at anytime.
> >
> > One of the main usecases Android has is the ability to create a region
> > and mmap it as writeable, then add protection against making any
> > "future" writes while keeping the existing already mmap'ed
> > writeable-region active.  This allows us to implement a usecase where
> > receivers of the shared memory buffer can get a read-only view, while
> > the sender continues to write to the buffer.
> > See CursorWindow documentation in Android for more details:
> > https://developer.android.com/reference/android/database/CursorWindow
> >
> > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> > which prevents any future mmap and write syscalls from succeeding while
> > keeping the existing mmap active.
> 
> Please CC linux-api@ on patches like this. If you had done that, I
> might have criticized your v1 patch instead of your v3 patch...

Ok, will do from next time.

> > The following program shows the seal
> > working in action:
> [...]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Reviewed-by: John Stultz <[e-mail]>
> > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> > ---
> [...]
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 2bb5e257080e..5ba9804e9515 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> [...]
> > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> >                 }
> >         }
> >
> > +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> > +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> > +               /*
> > +                * The FUTURE_WRITE seal also prevents growing and shrinking
> > +                * so we need them to be already set, or requested now.
> > +                */
> > +               int test_seals = (seals | *file_seals) &
> > +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> > +
> > +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> > +                       error = -EINVAL;
> > +                       goto unlock;
> > +               }
> > +
> > +               spin_lock(&file->f_lock);
> > +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > +               spin_unlock(&file->f_lock);
> > +       }
> 
> So you're fiddling around with the file, but not the inode? How are
> you preventing code like the following from re-opening the file as
> writable?
> 
> $ cat memfd.c
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <printf.h>
> #include <fcntl.h>
> #include <err.h>
> #include <stdio.h>
> 
> int main(void) {
>   int fd = syscall(__NR_memfd_create, "testfd", 0);
>   if (fd == -1) err(1, "memfd");
>   char path[100];
>   sprintf(path, "/proc/self/fd/%d", fd);
>   int fd2 = open(path, O_RDWR);
>   if (fd2 == -1) err(1, "reopen");
>   printf("reopen successful: %d\n", fd2);
> }
> $ gcc -o memfd memfd.c
> $ ./memfd
> reopen successful: 4

Great catch and this is indeed an issue :-(. I verified it too.

> That aside: I wonder whether a better API would be something that
> allows you to create a new readonly file descriptor, instead of
> fiddling with the writability of an existing fd.

Android usecases cannot deal with a new fd number because it breaks the
continuity of having the same old fd, as Dan also pointed out.

Also such API will have the same issues you brought up?

thanks,

 - Joel

Joel Fernandes Nov. 10, 2018, 1:36 a.m. UTC | #12

On Fri, Nov 09, 2018 at 03:14:02PM -0800, Andy Lutomirski wrote:
> >>>> That aside: I wonder whether a better API would be something that
> >>>> allows you to create a new readonly file descriptor, instead of
> >>>> fiddling with the writability of an existing fd.
> >>> 
> >>> That doesn't work, unfortunately. The ashmem API we're replacing with
> >>> memfd requires file descriptor continuity. I also looked into opening
> >>> a new FD and dup2(2)ing atop the old one, but this approach doesn't
> >>> work in the case that the old FD has already leaked to some other
> >>> context (e.g., another dup, SCM_RIGHTS). See
> >>> https://developer.android.com/ndk/reference/group/memory. We can't
> >>> break ASharedMemory_setProt.
> >> 
> >> 
> >> Hmm.  If we fix the general reopen bug, a way to drop write access from
> >> an existing struct file would do what Android needs, right?  I don’t
> >> know if there are general VFS issues with that.
> > 

I don't think there is a way to fix this in /proc/pid/fd. At the proc
level, the /proc/pid/fd/N files are just soft symlinks that follow through to
the actual file. The open is actually done on that inode/file. I think
changing it the way being discussed here means changing the way symlinks work
in Linux.

I think the right way to fix this is at the memfd inode level. I am working
on a follow up patch on top of this patch, and will send that out in a few
days (along with the man page updates).

thanks!

 - Joel

Joel Fernandes Nov. 10, 2018, 1:49 a.m. UTC | #13

On Fri, Nov 09, 2018 at 08:02:14PM +0000, Michael Tirado wrote:
[...]
> > > That aside: I wonder whether a better API would be something that
> > > allows you to create a new readonly file descriptor, instead of
> > > fiddling with the writability of an existing fd.
> >
> > Every now and then I try to write a patch to prevent using proc to reopen
> > a file with greater permission than the original open.
> >
> > I like your idea to have a clean way to reopen a a memfd with reduced
> > permissions. But I would make it a syscall instead and maybe make it only
> > work for memfd at first.  And the proc issue would need to be fixed, too.
> 
> IMO the best solution would handle the issue at memfd creation time by
> removing the race condition.

I agree, this is another idea I'm exploring. We could add a new .open
callback to shmem_file_operations and check for seals there.

thanks,

 - Joel

Joel Fernandes Nov. 10, 2018, 3:20 a.m. UTC | #14

On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
> On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <[e-mail]> wrote:
> > On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> > <[e-mail]> wrote:
> > > Android uses ashmem for sharing memory regions. We are looking forward
> > > to migrating all usecases of ashmem to memfd so that we can possibly
> > > remove the ashmem driver in the future from staging while also
> > > benefiting from using memfd and contributing to it. Note staging drivers
> > > are also not ABI and generally can be removed at anytime.
> > >
> > > One of the main usecases Android has is the ability to create a region
> > > and mmap it as writeable, then add protection against making any
> > > "future" writes while keeping the existing already mmap'ed
> > > writeable-region active.  This allows us to implement a usecase where
> > > receivers of the shared memory buffer can get a read-only view, while
> > > the sender continues to write to the buffer.
> > > See CursorWindow documentation in Android for more details:
> > > https://developer.android.com/reference/android/database/CursorWindow
> > >
> > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> > > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> > > which prevents any future mmap and write syscalls from succeeding while
> > > keeping the existing mmap active.
> >
> > Please CC linux-api@ on patches like this. If you had done that, I
> > might have criticized your v1 patch instead of your v3 patch...
> >
> > > The following program shows the seal
> > > working in action:
> > [...]
> > > Cc: [e-mail]
> > > Cc: [e-mail]
> > > Cc: [e-mail]
> > > Cc: [e-mail]
> > > Cc: [e-mail]
> > > Reviewed-by: John Stultz <[e-mail]>
> > > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> > > ---
> > [...]
> > > diff --git a/mm/memfd.c b/mm/memfd.c
> > > index 2bb5e257080e..5ba9804e9515 100644
> > > --- a/mm/memfd.c
> > > +++ b/mm/memfd.c
> > [...]
> > > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> > >                 }
> > >         }
> > >
> > > +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> > > +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> > > +               /*
> > > +                * The FUTURE_WRITE seal also prevents growing and shrinking
> > > +                * so we need them to be already set, or requested now.
> > > +                */
> > > +               int test_seals = (seals | *file_seals) &
> > > +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> > > +
> > > +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> > > +                       error = -EINVAL;
> > > +                       goto unlock;
> > > +               }
> > > +
> > > +               spin_lock(&file->f_lock);
> > > +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > > +               spin_unlock(&file->f_lock);
> > > +       }
> >
> > So you're fiddling around with the file, but not the inode? How are
> > you preventing code like the following from re-opening the file as
> > writable?
> >
> > $ cat memfd.c
> > #define _GNU_SOURCE
> > #include <unistd.h>
> > #include <sys/syscall.h>
> > #include <printf.h>
> > #include <fcntl.h>
> > #include <err.h>
> > #include <stdio.h>
> >
> > int main(void) {
> >   int fd = syscall(__NR_memfd_create, "testfd", 0);
> >   if (fd == -1) err(1, "memfd");
> >   char path[100];
> >   sprintf(path, "/proc/self/fd/%d", fd);
> >   int fd2 = open(path, O_RDWR);
> >   if (fd2 == -1) err(1, "reopen");
> >   printf("reopen successful: %d\n", fd2);
> > }
> > $ gcc -o memfd memfd.c
> > $ ./memfd
> > reopen successful: 4
> > $
> >
> > That aside: I wonder whether a better API would be something that
> > allows you to create a new readonly file descriptor, instead of
> > fiddling with the writability of an existing fd.
> 
> My favorite approach would be to forbid open() on memfds, hope that
> nobody notices the tiny API break, and then add an ioctl for "reopen
> this memfd with reduced permissions" - but that's just my personal
> opinion.

I did something along these lines and it fixes the issue, but I forbid open
of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its not
an ABI break because this is a brand new seal. That seems the least intrusive
solution and it works. Do you mind testing it and I'll add your and Tested-by
to the new fix? The patch is based on top of this series.

---8<-----------
From: "Joel Fernandes (Google)" <[e-mail]>
Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed memfd

Jann Horn found that reopening an F_SEAL_FUTURE_WRITE sealed memfd
through /proc/self/fd/N symlink as writeable succeeds. The simplest fix
without causing ABI breakages and ugly VFS hacks is to simply deny all
opens on F_SEAL_FUTURE_WRITE sealed fds.

Reported-by: Jann Horn <[e-mail]>
Signed-off-by: Joel Fernandes (Google) <[e-mail]>
---
 mm/shmem.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 446942677cd4..5b378c486b8f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3611,7 +3611,25 @@ static const struct address_space_operations shmem_aops = {
 	.error_remove_page = generic_error_remove_page,
 };
 
+/* Could arrive here for memfds opened through /proc/ */
+int shmem_open(struct inode *inode, struct file *file)
+{
+	struct shmem_inode_info *info = SHMEM_I(inode);
+
+	/*
+	 * memfds for which future writes have been prevented
+	 * should not be reopened, say, through /proc/pid/fd/N
+	 * symlinks otherwise it can cause a sealed memfd to be
+	 * promoted to writable.
+	 */
+	if (info->seals & F_SEAL_FUTURE_WRITE)
+		return -EACCES;
+
+	return 0;
+}
+
 static const struct file_operations shmem_file_operations = {
+	.open		= shmem_open,
 	.mmap		= shmem_mmap,
 	.get_unmapped_area = shmem_get_unmapped_area,
 #ifdef CONFIG_TMPFS

Joel Fernandes Nov. 10, 2018, 3:54 a.m. UTC | #15

On Fri, Nov 09, 2018 at 12:36:34PM -0800, Andrew Morton wrote:
> On Wed,  7 Nov 2018 20:15:36 -0800 "Joel Fernandes (Google)" <[e-mail]> wrote:
> 
> > Android uses ashmem for sharing memory regions. We are looking forward
> > to migrating all usecases of ashmem to memfd so that we can possibly
> > remove the ashmem driver in the future from staging while also
> > benefiting from using memfd and contributing to it. Note staging drivers
> > are also not ABI and generally can be removed at anytime.
> > 
> > One of the main usecases Android has is the ability to create a region
> > and mmap it as writeable, then add protection against making any
> > "future" writes while keeping the existing already mmap'ed
> > writeable-region active.  This allows us to implement a usecase where
> > receivers of the shared memory buffer can get a read-only view, while
> > the sender continues to write to the buffer.
> > See CursorWindow documentation in Android for more details:
> > https://developer.android.com/reference/android/database/CursorWindow
> 
> It appears that the memfd_create and fcntl manpages will require
> updating.  Please attend to this at the appropriate time?

Yes, I am planning to send those out shortly. I finished working on them.

Also just to let you know, I posted a fix for the security issue Jann Horn
reported and requested him to test it:
https://lore.kernel.org/lkml/[e-mail]/T/#m8d9d185e6480d095f0ab8f84bcb103892181f77d

This fix along with the 2 other patches I posted in v3 are all that's needed. thanks!

- Joel

Andy Lutomirski Nov. 10, 2018, 6:05 a.m. UTC | #16

> On Nov 9, 2018, at 7:20 PM, Joel Fernandes <[e-mail]> wrote:
> 
>> On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
>>> On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <[e-mail]> wrote:
>>> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
>>> <[e-mail]> wrote:
>>>> Android uses ashmem for sharing memory regions. We are looking forward
>>>> to migrating all usecases of ashmem to memfd so that we can possibly
>>>> remove the ashmem driver in the future from staging while also
>>>> benefiting from using memfd and contributing to it. Note staging drivers
>>>> are also not ABI and generally can be removed at anytime.
>>>> 
>>>> One of the main usecases Android has is the ability to create a region
>>>> and mmap it as writeable, then add protection against making any
>>>> "future" writes while keeping the existing already mmap'ed
>>>> writeable-region active.  This allows us to implement a usecase where
>>>> receivers of the shared memory buffer can get a read-only view, while
>>>> the sender continues to write to the buffer.
>>>> See CursorWindow documentation in Android for more details:
>>>> https://developer.android.com/reference/android/database/CursorWindow
>>>> 
>>>> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
>>>> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
>>>> which prevents any future mmap and write syscalls from succeeding while
>>>> keeping the existing mmap active.
>>> 
>>> Please CC linux-api@ on patches like this. If you had done that, I
>>> might have criticized your v1 patch instead of your v3 patch...
>>> 
>>>> The following program shows the seal
>>>> working in action:
>>> [...]
>>>> Cc: [e-mail]
>>>> Cc: [e-mail]
>>>> Cc: [e-mail]
>>>> Cc: [e-mail]
>>>> Cc: [e-mail]
>>>> Reviewed-by: John Stultz <[e-mail]>
>>>> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
>>>> ---
>>> [...]
>>>> diff --git a/mm/memfd.c b/mm/memfd.c
>>>> index 2bb5e257080e..5ba9804e9515 100644
>>>> --- a/mm/memfd.c
>>>> +++ b/mm/memfd.c
>>> [...]
>>>> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
>>>>                }
>>>>        }
>>>> 
>>>> +       if ((seals & F_SEAL_FUTURE_WRITE) &&
>>>> +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
>>>> +               /*
>>>> +                * The FUTURE_WRITE seal also prevents growing and shrinking
>>>> +                * so we need them to be already set, or requested now.
>>>> +                */
>>>> +               int test_seals = (seals | *file_seals) &
>>>> +                                (F_SEAL_GROW | F_SEAL_SHRINK);
>>>> +
>>>> +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
>>>> +                       error = -EINVAL;
>>>> +                       goto unlock;
>>>> +               }
>>>> +
>>>> +               spin_lock(&file->f_lock);
>>>> +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
>>>> +               spin_unlock(&file->f_lock);
>>>> +       }
>>> 
>>> So you're fiddling around with the file, but not the inode? How are
>>> you preventing code like the following from re-opening the file as
>>> writable?
>>> 
>>> $ cat memfd.c
>>> #define _GNU_SOURCE
>>> #include <unistd.h>
>>> #include <sys/syscall.h>
>>> #include <printf.h>
>>> #include <fcntl.h>
>>> #include <err.h>
>>> #include <stdio.h>
>>> 
>>> int main(void) {
>>>  int fd = syscall(__NR_memfd_create, "testfd", 0);
>>>  if (fd == -1) err(1, "memfd");
>>>  char path[100];
>>>  sprintf(path, "/proc/self/fd/%d", fd);
>>>  int fd2 = open(path, O_RDWR);
>>>  if (fd2 == -1) err(1, "reopen");
>>>  printf("reopen successful: %d\n", fd2);
>>> }
>>> $ gcc -o memfd memfd.c
>>> $ ./memfd
>>> reopen successful: 4
>>> $
>>> 
>>> That aside: I wonder whether a better API would be something that
>>> allows you to create a new readonly file descriptor, instead of
>>> fiddling with the writability of an existing fd.
>> 
>> My favorite approach would be to forbid open() on memfds, hope that
>> nobody notices the tiny API break, and then add an ioctl for "reopen
>> this memfd with reduced permissions" - but that's just my personal
>> opinion.
> 
> I did something along these lines and it fixes the issue, but I forbid open
> of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its not
> an ABI break because this is a brand new seal. That seems the least intrusive
> solution and it works. Do you mind testing it and I'll add your and Tested-by
> to the new fix? The patch is based on top of this series.
> 
> ---8<-----------
> From: "Joel Fernandes (Google)" <[e-mail]>
> Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed memfd
> 
> Jann Horn found that reopening an F_SEAL_FUTURE_WRITE sealed memfd
> through /proc/self/fd/N symlink as writeable succeeds. The simplest fix
> without causing ABI breakages and ugly VFS hacks is to simply deny all
> opens on F_SEAL_FUTURE_WRITE sealed fds.
> 
> Reported-by: Jann Horn <[e-mail]>
> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> ---
> mm/shmem.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 446942677cd4..5b378c486b8f 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3611,7 +3611,25 @@ static const struct address_space_operations shmem_aops = {
>    .error_remove_page = generic_error_remove_page,
> };
> 
> +/* Could arrive here for memfds opened through /proc/ */
> +int shmem_open(struct inode *inode, struct file *file)
> +{
> +    struct shmem_inode_info *info = SHMEM_I(inode);
> +
> +    /*
> +     * memfds for which future writes have been prevented
> +     * should not be reopened, say, through /proc/pid/fd/N
> +     * symlinks otherwise it can cause a sealed memfd to be
> +     * promoted to writable.
> +     */
> +    if (info->seals & F_SEAL_FUTURE_WRITE)
> +        return -EACCES;
> +
> +    return 0;
> +}

The result of this series is very warty. We have a concept of seals, and they all work similarly, except the new future write seal. That one:

- causes a probably-observable effect in the file mode in F_GETFL.

- causes reopen to fail.

- does *not* affect other struct files that may already exist on the same inode.

- mysteriously malfunctions if you try to set it again on another struct file that already exists

- probably is insecure when used on hugetlbfs.

I see two reasonable solutions:

1. Don’t fiddle with the struct file at all. Instead make the inode flag work by itself.

2. Don’t call it a “seal”.  Instead fix the /proc hole and add an API to drop write access on an existing struct file.

I personally prefer #2.

> +
> static const struct file_operations shmem_file_operations = {
> +    .open        = shmem_open,
>    .mmap        = shmem_mmap,
>    .get_unmapped_area = shmem_get_unmapped_area,
> #ifdef CONFIG_TMPFS
> -- 
> 2.19.1.930.g4563a0d9d0-goog
>

Joel Fernandes Nov. 10, 2018, 5:10 p.m. UTC | #17

On Sat, Nov 10, 2018 at 04:26:46AM -0800, Daniel Colascione wrote:
> On Friday, November 9, 2018, Joel Fernandes <[e-mail]> wrote:
> 
> > On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
> > > On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <[e-mail]> wrote:
> > > > On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> > > > <[e-mail]> wrote:
> > > > > Android uses ashmem for sharing memory regions. We are looking
> > forward
> > > > > to migrating all usecases of ashmem to memfd so that we can possibly
> > > > > remove the ashmem driver in the future from staging while also
> > > > > benefiting from using memfd and contributing to it. Note staging
> > drivers
> > > > > are also not ABI and generally can be removed at anytime.
> > > > >
> > > > > One of the main usecases Android has is the ability to create a
> > region
> > > > > and mmap it as writeable, then add protection against making any
> > > > > "future" writes while keeping the existing already mmap'ed
> > > > > writeable-region active.  This allows us to implement a usecase where
> > > > > receivers of the shared memory buffer can get a read-only view, while
> > > > > the sender continues to write to the buffer.
> > > > > See CursorWindow documentation in Android for more details:
> > > > > https://developer.android.com/reference/android/database/
> > CursorWindow
> > > > >
> > > > > This usecase cannot be implemented with the existing F_SEAL_WRITE
> > seal.
> > > > > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE
> > seal
> > > > > which prevents any future mmap and write syscalls from succeeding
> > while
> > > > > keeping the existing mmap active.
> > > >
> > > > Please CC linux-api@ on patches like this. If you had done that, I
> > > > might have criticized your v1 patch instead of your v3 patch...
> > > >
> > > > > The following program shows the seal
> > > > > working in action:
> > > > [...]
> > > > > Cc: [e-mail]
> > > > > Cc: [e-mail]
> > > > > Cc: [e-mail]
> > > > > Cc: [e-mail]
> > > > > Cc: [e-mail]
> > > > > Reviewed-by: John Stultz <[e-mail]>
> > > > > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> > > > > ---
> > > > [...]
> > > > > diff --git a/mm/memfd.c b/mm/memfd.c
> > > > > index 2bb5e257080e..5ba9804e9515 100644
> > > > > --- a/mm/memfd.c
> > > > > +++ b/mm/memfd.c
> > > > [...]
> > > > > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file,
> > unsigned int seals)
> > > > >                 }
> > > > >         }
> > > > >
> > > > > +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> > > > > +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> > > > > +               /*
> > > > > +                * The FUTURE_WRITE seal also prevents growing and
> > shrinking
> > > > > +                * so we need them to be already set, or requested
> > now.
> > > > > +                */
> > > > > +               int test_seals = (seals | *file_seals) &
> > > > > +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> > > > > +
> > > > > +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> > > > > +                       error = -EINVAL;
> > > > > +                       goto unlock;
> > > > > +               }
> > > > > +
> > > > > +               spin_lock(&file->f_lock);
> > > > > +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > > > > +               spin_unlock(&file->f_lock);
> > > > > +       }
> > > >
> > > > So you're fiddling around with the file, but not the inode? How are
> > > > you preventing code like the following from re-opening the file as
> > > > writable?
> > > >
> > > > $ cat memfd.c
> > > > #define _GNU_SOURCE
> > > > #include <unistd.h>
> > > > #include <sys/syscall.h>
> > > > #include <printf.h>
> > > > #include <fcntl.h>
> > > > #include <err.h>
> > > > #include <stdio.h>
> > > >
> > > > int main(void) {
> > > >   int fd = syscall(__NR_memfd_create, "testfd", 0);
> > > >   if (fd == -1) err(1, "memfd");
> > > >   char path[100];
> > > >   sprintf(path, "/proc/self/fd/%d", fd);
> > > >   int fd2 = open(path, O_RDWR);
> > > >   if (fd2 == -1) err(1, "reopen");
> > > >   printf("reopen successful: %d\n", fd2);
> > > > }
> > > > $ gcc -o memfd memfd.c
> > > > $ ./memfd
> > > > reopen successful: 4
> > > > $
> > > >
> > > > That aside: I wonder whether a better API would be something that
> > > > allows you to create a new readonly file descriptor, instead of
> > > > fiddling with the writability of an existing fd.
> > >
> > > My favorite approach would be to forbid open() on memfds, hope that
> > > nobody notices the tiny API break, and then add an ioctl for "reopen
> > > this memfd with reduced permissions" - but that's just my personal
> > > opinion.
> >
> > I did something along these lines and it fixes the issue, but I forbid open
> > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its
> > not
> > an ABI break because this is a brand new seal. That seems the least
> > intrusive
> > solution and it works. Do you mind testing it and I'll add your and
> > Tested-by
> > to the new fix? The patch is based on top of this series.
> >
> 
> Please don't forbid reopens entirely. You're taking a feature that works
> generally (reopens) and breaking it in one specific case (memfd write
> sealed files). The open modes are available in .open in the struct file:
> you can deny *only* opens for write instead of denying reopens generally.

Yes, as we discussed over chat already, I will implement it that way.

Also lets continue to discuss Andy's concerns he raised on the other thread.

thanks,

 - Joel

Joel Fernandes Nov. 10, 2018, 6:24 p.m. UTC | #18

Thanks Andy for your thoughts, my comments below:

On Fri, Nov 09, 2018 at 10:05:14PM -0800, Andy Lutomirski wrote:
> 
> 
> > On Nov 9, 2018, at 7:20 PM, Joel Fernandes <[e-mail]> wrote:
> > 
> >> On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
> >>> On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <[e-mail]> wrote:
> >>> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
> >>> <[e-mail]> wrote:
> >>>> Android uses ashmem for sharing memory regions. We are looking forward
> >>>> to migrating all usecases of ashmem to memfd so that we can possibly
> >>>> remove the ashmem driver in the future from staging while also
> >>>> benefiting from using memfd and contributing to it. Note staging drivers
> >>>> are also not ABI and generally can be removed at anytime.
> >>>> 
> >>>> One of the main usecases Android has is the ability to create a region
> >>>> and mmap it as writeable, then add protection against making any
> >>>> "future" writes while keeping the existing already mmap'ed
> >>>> writeable-region active.  This allows us to implement a usecase where
> >>>> receivers of the shared memory buffer can get a read-only view, while
> >>>> the sender continues to write to the buffer.
> >>>> See CursorWindow documentation in Android for more details:
> >>>> https://developer.android.com/reference/android/database/CursorWindow
> >>>> 
> >>>> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> >>>> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> >>>> which prevents any future mmap and write syscalls from succeeding while
> >>>> keeping the existing mmap active.
> >>> 
> >>> Please CC linux-api@ on patches like this. If you had done that, I
> >>> might have criticized your v1 patch instead of your v3 patch...
> >>> 
> >>>> The following program shows the seal
> >>>> working in action:
> >>> [...]
> >>>> Cc: [e-mail]
> >>>> Cc: [e-mail]
> >>>> Cc: [e-mail]
> >>>> Cc: [e-mail]
> >>>> Cc: [e-mail]
> >>>> Reviewed-by: John Stultz <[e-mail]>
> >>>> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> >>>> ---
> >>> [...]
> >>>> diff --git a/mm/memfd.c b/mm/memfd.c
> >>>> index 2bb5e257080e..5ba9804e9515 100644
> >>>> --- a/mm/memfd.c
> >>>> +++ b/mm/memfd.c
> >>> [...]
> >>>> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> >>>>                }
> >>>>        }
> >>>> 
> >>>> +       if ((seals & F_SEAL_FUTURE_WRITE) &&
> >>>> +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
> >>>> +               /*
> >>>> +                * The FUTURE_WRITE seal also prevents growing and shrinking
> >>>> +                * so we need them to be already set, or requested now.
> >>>> +                */
> >>>> +               int test_seals = (seals | *file_seals) &
> >>>> +                                (F_SEAL_GROW | F_SEAL_SHRINK);
> >>>> +
> >>>> +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
> >>>> +                       error = -EINVAL;
> >>>> +                       goto unlock;
> >>>> +               }
> >>>> +
> >>>> +               spin_lock(&file->f_lock);
> >>>> +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> >>>> +               spin_unlock(&file->f_lock);
> >>>> +       }
> >>> 
> >>> So you're fiddling around with the file, but not the inode? How are
> >>> you preventing code like the following from re-opening the file as
> >>> writable?
> >>> 
> >>> $ cat memfd.c
> >>> #define _GNU_SOURCE
> >>> #include <unistd.h>
> >>> #include <sys/syscall.h>
> >>> #include <printf.h>
> >>> #include <fcntl.h>
> >>> #include <err.h>
> >>> #include <stdio.h>
> >>> 
> >>> int main(void) {
> >>>  int fd = syscall(__NR_memfd_create, "testfd", 0);
> >>>  if (fd == -1) err(1, "memfd");
> >>>  char path[100];
> >>>  sprintf(path, "/proc/self/fd/%d", fd);
> >>>  int fd2 = open(path, O_RDWR);
> >>>  if (fd2 == -1) err(1, "reopen");
> >>>  printf("reopen successful: %d\n", fd2);
> >>> }
> >>> $ gcc -o memfd memfd.c
> >>> $ ./memfd
> >>> reopen successful: 4
> >>> $
> >>> 
> >>> That aside: I wonder whether a better API would be something that
> >>> allows you to create a new readonly file descriptor, instead of
> >>> fiddling with the writability of an existing fd.
> >> 
> >> My favorite approach would be to forbid open() on memfds, hope that
> >> nobody notices the tiny API break, and then add an ioctl for "reopen
> >> this memfd with reduced permissions" - but that's just my personal
> >> opinion.
> > 
> > I did something along these lines and it fixes the issue, but I forbid open
> > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its not
> > an ABI break because this is a brand new seal. That seems the least intrusive
> > solution and it works. Do you mind testing it and I'll add your and Tested-by
> > to the new fix? The patch is based on top of this series.
> > 
> > ---8<-----------
> > From: "Joel Fernandes (Google)" <[e-mail]>
> > Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed memfd
> > 
> > Jann Horn found that reopening an F_SEAL_FUTURE_WRITE sealed memfd
> > through /proc/self/fd/N symlink as writeable succeeds. The simplest fix
> > without causing ABI breakages and ugly VFS hacks is to simply deny all
> > opens on F_SEAL_FUTURE_WRITE sealed fds.
> > 
> > Reported-by: Jann Horn <[e-mail]>
> > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> > ---
> > mm/shmem.c | 18 ++++++++++++++++++
> > 1 file changed, 18 insertions(+)
> > 
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 446942677cd4..5b378c486b8f 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -3611,7 +3611,25 @@ static const struct address_space_operations shmem_aops = {
> >    .error_remove_page = generic_error_remove_page,
> > };
> > 
> > +/* Could arrive here for memfds opened through /proc/ */
> > +int shmem_open(struct inode *inode, struct file *file)
> > +{
> > +    struct shmem_inode_info *info = SHMEM_I(inode);
> > +
> > +    /*
> > +     * memfds for which future writes have been prevented
> > +     * should not be reopened, say, through /proc/pid/fd/N
> > +     * symlinks otherwise it can cause a sealed memfd to be
> > +     * promoted to writable.
> > +     */
> > +    if (info->seals & F_SEAL_FUTURE_WRITE)
> > +        return -EACCES;
> > +
> > +    return 0;
> > +}
> 
> The result of this series is very warty. We have a concept of seals, and they all work similarly, except the new future write seal. That one:
> 

I don't see it as warty, different seals will work differently. It works
quite well for our usecase, and since Linux is all about solving real
problems in the real work, it would be useful to have it.

> - causes a probably-observable effect in the file mode in F_GETFL.

Wouldn't that be the right thing to observe anyway?

> - causes reopen to fail.

So this concern isn't true anymore if we make reopen fail only for WRITE
opens as Daniel suggested. I will make this change so that the security fix
is a clean one.

> - does *not* affect other struct files that may already exist on the same inode.

TBH if you really want to block all writes to the file, then you want
F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
to another process and we want to prevent any new writes in the receiver
side. There is no way this other receiving process can have an existing fd
unless it was already sent one without the seal applied.  The proposed seal
could be renamed to F_SEAL_FD_WRITE if that is preferred.

> - mysteriously malfunctions if you try to set it again on another struct
> file that already exists
> 

I didn't follow this, could you explain more?

> - probably is insecure when used on hugetlbfs.

The usecase is not expected to prevent all writes, indeed the usecase
requires existing mmaps to continue to be able to write into the memory map.
So would you call that a security issue too? The use of the seal wants to
allow existing mmap regions to be continue to be written into (I mentioned
more details in the cover letter).

> I see two reasonable solutions:
> 
> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
> work by itself.

Currently, the various VFS paths check only the struct file's f_mode to deny
writes of already opened files. This would mean more checking in all those
paths (and modification of all those paths).

Anyway going with that idea, we could
1. call deny_write_access(file) from the memfd's seal path which decrements
the inode::i_writecount.
2. call get_write_access(inode) in the various VFS paths in addition to
checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)

That will prevent both reopens, and writes from succeeding. However I worry a
bit about 2 not being too familiar with VFS internals, about what the
consequences of doing that may be.

> 2. Don’t call it a “seal”.  Instead fix the /proc hole and add an API to
> drop write access on an existing struct file.
> 
> I personally prefer #2.

So I don't think proc has a hole looking at the code yesterday. As I was
saying in other thread, its just how symlinks work. If we were to apply this
API to files in general, and we had symlinks to files and we tried to reopen the
file, we would run into the same issue - that's why I believe it has nothing
to do with proc, and the fix has to be in the underlying inode being pointed
to. Does that make sense or did I miss something?

thanks,

 - Joel

Daniel Colascione Nov. 10, 2018, 6:45 p.m. UTC | #19

On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
> Thanks Andy for your thoughts, my comments below:
>
> On Fri, Nov 09, 2018 at 10:05:14PM -0800, Andy Lutomirski wrote:
>>
>>
>> > On Nov 9, 2018, at 7:20 PM, Joel Fernandes <[e-mail]> wrote:
>> >
>> >> On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote:
>> >>> On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <[e-mail]> wrote:
>> >>> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google)
>> >>> <[e-mail]> wrote:
>> >>>> Android uses ashmem for sharing memory regions. We are looking forward
>> >>>> to migrating all usecases of ashmem to memfd so that we can possibly
>> >>>> remove the ashmem driver in the future from staging while also
>> >>>> benefiting from using memfd and contributing to it. Note staging drivers
>> >>>> are also not ABI and generally can be removed at anytime.
>> >>>>
>> >>>> One of the main usecases Android has is the ability to create a region
>> >>>> and mmap it as writeable, then add protection against making any
>> >>>> "future" writes while keeping the existing already mmap'ed
>> >>>> writeable-region active.  This allows us to implement a usecase where
>> >>>> receivers of the shared memory buffer can get a read-only view, while
>> >>>> the sender continues to write to the buffer.
>> >>>> See CursorWindow documentation in Android for more details:
>> >>>> https://developer.android.com/reference/android/database/CursorWindow
>> >>>>
>> >>>> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
>> >>>> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
>> >>>> which prevents any future mmap and write syscalls from succeeding while
>> >>>> keeping the existing mmap active.
>> >>>
>> >>> Please CC linux-api@ on patches like this. If you had done that, I
>> >>> might have criticized your v1 patch instead of your v3 patch...
>> >>>
>> >>>> The following program shows the seal
>> >>>> working in action:
>> >>> [...]
>> >>>> Cc: [e-mail]
>> >>>> Cc: [e-mail]
>> >>>> Cc: [e-mail]
>> >>>> Cc: [e-mail]
>> >>>> Cc: [e-mail]
>> >>>> Reviewed-by: John Stultz <[e-mail]>
>> >>>> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
>> >>>> ---
>> >>> [...]
>> >>>> diff --git a/mm/memfd.c b/mm/memfd.c
>> >>>> index 2bb5e257080e..5ba9804e9515 100644
>> >>>> --- a/mm/memfd.c
>> >>>> +++ b/mm/memfd.c
>> >>> [...]
>> >>>> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
>> >>>>                }
>> >>>>        }
>> >>>>
>> >>>> +       if ((seals & F_SEAL_FUTURE_WRITE) &&
>> >>>> +           !(*file_seals & F_SEAL_FUTURE_WRITE)) {
>> >>>> +               /*
>> >>>> +                * The FUTURE_WRITE seal also prevents growing and shrinking
>> >>>> +                * so we need them to be already set, or requested now.
>> >>>> +                */
>> >>>> +               int test_seals = (seals | *file_seals) &
>> >>>> +                                (F_SEAL_GROW | F_SEAL_SHRINK);
>> >>>> +
>> >>>> +               if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
>> >>>> +                       error = -EINVAL;
>> >>>> +                       goto unlock;
>> >>>> +               }
>> >>>> +
>> >>>> +               spin_lock(&file->f_lock);
>> >>>> +               file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
>> >>>> +               spin_unlock(&file->f_lock);
>> >>>> +       }
>> >>>
>> >>> So you're fiddling around with the file, but not the inode? How are
>> >>> you preventing code like the following from re-opening the file as
>> >>> writable?
>> >>>
>> >>> $ cat memfd.c
>> >>> #define _GNU_SOURCE
>> >>> #include <unistd.h>
>> >>> #include <sys/syscall.h>
>> >>> #include <printf.h>
>> >>> #include <fcntl.h>
>> >>> #include <err.h>
>> >>> #include <stdio.h>
>> >>>
>> >>> int main(void) {
>> >>>  int fd = syscall(__NR_memfd_create, "testfd", 0);
>> >>>  if (fd == -1) err(1, "memfd");
>> >>>  char path[100];
>> >>>  sprintf(path, "/proc/self/fd/%d", fd);
>> >>>  int fd2 = open(path, O_RDWR);
>> >>>  if (fd2 == -1) err(1, "reopen");
>> >>>  printf("reopen successful: %d\n", fd2);
>> >>> }
>> >>> $ gcc -o memfd memfd.c
>> >>> $ ./memfd
>> >>> reopen successful: 4
>> >>> $
>> >>>
>> >>> That aside: I wonder whether a better API would be something that
>> >>> allows you to create a new readonly file descriptor, instead of
>> >>> fiddling with the writability of an existing fd.
>> >>
>> >> My favorite approach would be to forbid open() on memfds, hope that
>> >> nobody notices the tiny API break, and then add an ioctl for "reopen
>> >> this memfd with reduced permissions" - but that's just my personal
>> >> opinion.
>> >
>> > I did something along these lines and it fixes the issue, but I forbid open
>> > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its not
>> > an ABI break because this is a brand new seal. That seems the least intrusive
>> > solution and it works. Do you mind testing it and I'll add your and Tested-by
>> > to the new fix? The patch is based on top of this series.
>> >
>> > ---8<-----------
>> > From: "Joel Fernandes (Google)" <[e-mail]>
>> > Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed memfd
>> >
>> > Jann Horn found that reopening an F_SEAL_FUTURE_WRITE sealed memfd
>> > through /proc/self/fd/N symlink as writeable succeeds. The simplest fix
>> > without causing ABI breakages and ugly VFS hacks is to simply deny all
>> > opens on F_SEAL_FUTURE_WRITE sealed fds.
>> >
>> > Reported-by: Jann Horn <[e-mail]>
>> > Signed-off-by: Joel Fernandes (Google) <[e-mail]>
>> > ---
>> > mm/shmem.c | 18 ++++++++++++++++++
>> > 1 file changed, 18 insertions(+)
>> >
>> > diff --git a/mm/shmem.c b/mm/shmem.c
>> > index 446942677cd4..5b378c486b8f 100644
>> > --- a/mm/shmem.c
>> > +++ b/mm/shmem.c
>> > @@ -3611,7 +3611,25 @@ static const struct address_space_operations shmem_aops = {
>> >    .error_remove_page = generic_error_remove_page,
>> > };
>> >
>> > +/* Could arrive here for memfds opened through /proc/ */
>> > +int shmem_open(struct inode *inode, struct file *file)
>> > +{
>> > +    struct shmem_inode_info *info = SHMEM_I(inode);
>> > +
>> > +    /*
>> > +     * memfds for which future writes have been prevented
>> > +     * should not be reopened, say, through /proc/pid/fd/N
>> > +     * symlinks otherwise it can cause a sealed memfd to be
>> > +     * promoted to writable.
>> > +     */
>> > +    if (info->seals & F_SEAL_FUTURE_WRITE)
>> > +        return -EACCES;
>> > +
>> > +    return 0;
>> > +}
>>
>> The result of this series is very warty. We have a concept of seals, and they all work similarly, except the new future write seal. That one:
>>
>
> I don't see it as warty, different seals will work differently. It works
> quite well for our usecase, and since Linux is all about solving real
> problems in the real work, it would be useful to have it.
>
>> - causes a probably-observable effect in the file mode in F_GETFL.
>
> Wouldn't that be the right thing to observe anyway?
>
>> - causes reopen to fail.
>
> So this concern isn't true anymore if we make reopen fail only for WRITE
> opens as Daniel suggested. I will make this change so that the security fix
> is a clean one.
>
>> - does *not* affect other struct files that may already exist on the same inode.
>
> TBH if you really want to block all writes to the file, then you want
> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
> to another process and we want to prevent any new writes in the receiver
> side. There is no way this other receiving process can have an existing fd
> unless it was already sent one without the seal applied.  The proposed seal
> could be renamed to F_SEAL_FD_WRITE if that is preferred.
>
>> - mysteriously malfunctions if you try to set it again on another struct
>> file that already exists
>>
>
> I didn't follow this, could you explain more?
>
>> - probably is insecure when used on hugetlbfs.
>
> The usecase is not expected to prevent all writes, indeed the usecase
> requires existing mmaps to continue to be able to write into the memory map.
> So would you call that a security issue too? The use of the seal wants to
> allow existing mmap regions to be continue to be written into (I mentioned
> more details in the cover letter).
>
>> I see two reasonable solutions:
>>
>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>> work by itself.
>
> Currently, the various VFS paths check only the struct file's f_mode to deny
> writes of already opened files. This would mean more checking in all those
> paths (and modification of all those paths).
>
> Anyway going with that idea, we could
> 1. call deny_write_access(file) from the memfd's seal path which decrements
> the inode::i_writecount.
> 2. call get_write_access(inode) in the various VFS paths in addition to
> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>
> That will prevent both reopens, and writes from succeeding. However I worry a
> bit about 2 not being too familiar with VFS internals, about what the
> consequences of doing that may be.

IMHO, modifying both the inode and the struct file separately is fine,
since they mean different things. In regular filesystems, it's fine to
have a read-write open file description for a file whose inode grants
write permission to nobody. Speaking of which: is fchmod enough to
prevent this attack?

>> 2. Don’t call it a “seal”.  Instead fix the /proc hole and add an API to
>> drop write access on an existing struct file.
>>
>> I personally prefer #2.
>
> So I don't think proc has a hole looking at the code yesterday. As I was
> saying in other thread, its just how symlinks work. If we were to apply this
> API to files in general, and we had symlinks to files and we tried to reopen the
> file, we would run into the same issue - that's why I believe it has nothing
> to do with proc, and the fix has to be in the underlying inode being pointed
> to. Does that make sense or did I miss something?

+1. Another point to consider is that even if we did somehow fix the
"proc hole", any other means (perhaps in the future) of obtaining an
inode reference would then be insecure, perhaps silently. For example
--- is there a way to use open_by_handle_at(2) to open a memfd file?

Daniel Colascione Nov. 10, 2018, 7:11 p.m. UTC | #20

On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
>> Thanks Andy for your thoughts, my comments below:
[snip]
>> I don't see it as warty, different seals will work differently. It works
>> quite well for our usecase, and since Linux is all about solving real
>> problems in the real work, it would be useful to have it.
>>
>>> - causes a probably-observable effect in the file mode in F_GETFL.
>>
>> Wouldn't that be the right thing to observe anyway?
>>
>>> - causes reopen to fail.
>>
>> So this concern isn't true anymore if we make reopen fail only for WRITE
>> opens as Daniel suggested. I will make this change so that the security fix
>> is a clean one.
>>
>>> - does *not* affect other struct files that may already exist on the same inode.
>>
>> TBH if you really want to block all writes to the file, then you want
>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
>> to another process and we want to prevent any new writes in the receiver
>> side. There is no way this other receiving process can have an existing fd
>> unless it was already sent one without the seal applied.  The proposed seal
>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
>>
>>> - mysteriously malfunctions if you try to set it again on another struct
>>> file that already exists
>>>
>>
>> I didn't follow this, could you explain more?
>>
>>> - probably is insecure when used on hugetlbfs.
>>
>> The usecase is not expected to prevent all writes, indeed the usecase
>> requires existing mmaps to continue to be able to write into the memory map.
>> So would you call that a security issue too? The use of the seal wants to
>> allow existing mmap regions to be continue to be written into (I mentioned
>> more details in the cover letter).
>>
>>> I see two reasonable solutions:
>>>
>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>>> work by itself.
>>
>> Currently, the various VFS paths check only the struct file's f_mode to deny
>> writes of already opened files. This would mean more checking in all those
>> paths (and modification of all those paths).
>>
>> Anyway going with that idea, we could
>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>> the inode::i_writecount.
>> 2. call get_write_access(inode) in the various VFS paths in addition to
>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>>
>> That will prevent both reopens, and writes from succeeding. However I worry a
>> bit about 2 not being too familiar with VFS internals, about what the
>> consequences of doing that may be.
>
> IMHO, modifying both the inode and the struct file separately is fine,
> since they mean different things. In regular filesystems, it's fine to
> have a read-write open file description for a file whose inode grants
> write permission to nobody. Speaking of which: is fchmod enough to
> prevent this attack?

Well, yes and no. fchmod does prevent reopening the file RW, but
anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
isn't sufficient by itself. While it might be good enough for Android
(in the sense that it'll prevent RW-reopens from other security
contexts to which we send an open memfd file), it's still conceptually
ugly, IMHO. Let's go with the original approach of just tweaking the
inode so that open-for-write is permanently blocked.

Andy Lutomirski Nov. 10, 2018, 7:55 p.m. UTC | #21

> On Nov 10, 2018, at 11:11 AM, Daniel Colascione <[e-mail]> wrote:
> 
>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
>>> Thanks Andy for your thoughts, my comments below:
> [snip]
>>> I don't see it as warty, different seals will work differently. It works
>>> quite well for our usecase, and since Linux is all about solving real
>>> problems in the real work, it would be useful to have it.
>>> 
>>>> - causes a probably-observable effect in the file mode in F_GETFL.
>>> 
>>> Wouldn't that be the right thing to observe anyway?
>>> 
>>>> - causes reopen to fail.
>>> 
>>> So this concern isn't true anymore if we make reopen fail only for WRITE
>>> opens as Daniel suggested. I will make this change so that the security fix
>>> is a clean one.
>>> 
>>>> - does *not* affect other struct files that may already exist on the same inode.
>>> 
>>> TBH if you really want to block all writes to the file, then you want
>>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
>>> to another process and we want to prevent any new writes in the receiver
>>> side. There is no way this other receiving process can have an existing fd
>>> unless it was already sent one without the seal applied.  The proposed seal
>>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
>>> 
>>>> - mysteriously malfunctions if you try to set it again on another struct
>>>> file that already exists
>>>> 
>>> 
>>> I didn't follow this, could you explain more?
>>> 
>>>> - probably is insecure when used on hugetlbfs.
>>> 
>>> The usecase is not expected to prevent all writes, indeed the usecase
>>> requires existing mmaps to continue to be able to write into the memory map.
>>> So would you call that a security issue too? The use of the seal wants to
>>> allow existing mmap regions to be continue to be written into (I mentioned
>>> more details in the cover letter).
>>> 
>>>> I see two reasonable solutions:
>>>> 
>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>>>> work by itself.
>>> 
>>> Currently, the various VFS paths check only the struct file's f_mode to deny
>>> writes of already opened files. This would mean more checking in all those
>>> paths (and modification of all those paths).
>>> 
>>> Anyway going with that idea, we could
>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>>> the inode::i_writecount.
>>> 2. call get_write_access(inode) in the various VFS paths in addition to
>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>>> 
>>> That will prevent both reopens, and writes from succeeding. However I worry a
>>> bit about 2 not being too familiar with VFS internals, about what the
>>> consequences of doing that may be.
>> 
>> IMHO, modifying both the inode and the struct file separately is fine,
>> since they mean different things. In regular filesystems, it's fine to
>> have a read-write open file description for a file whose inode grants
>> write permission to nobody. Speaking of which: is fchmod enough to
>> prevent this attack?
> 
> Well, yes and no. fchmod does prevent reopening the file RW, but
> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
> isn't sufficient by itself. While it might be good enough for Android
> (in the sense that it'll prevent RW-reopens from other security
> contexts to which we send an open memfd file), it's still conceptually
> ugly, IMHO. Let's go with the original approach of just tweaking the
> inode so that open-for-write is permanently blocked.

This should be straightforward. Just add a new seal type and wire it up. It should be considerably simpler than SEAL_WRITE.

Joel Fernandes Nov. 10, 2018, 10:09 p.m. UTC | #22

On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote:
> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
> > On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
> >> Thanks Andy for your thoughts, my comments below:
> [snip]
> >> I don't see it as warty, different seals will work differently. It works
> >> quite well for our usecase, and since Linux is all about solving real
> >> problems in the real work, it would be useful to have it.
> >>
> >>> - causes a probably-observable effect in the file mode in F_GETFL.
> >>
> >> Wouldn't that be the right thing to observe anyway?
> >>
> >>> - causes reopen to fail.
> >>
> >> So this concern isn't true anymore if we make reopen fail only for WRITE
> >> opens as Daniel suggested. I will make this change so that the security fix
> >> is a clean one.
> >>
> >>> - does *not* affect other struct files that may already exist on the same inode.
> >>
> >> TBH if you really want to block all writes to the file, then you want
> >> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
> >> to another process and we want to prevent any new writes in the receiver
> >> side. There is no way this other receiving process can have an existing fd
> >> unless it was already sent one without the seal applied.  The proposed seal
> >> could be renamed to F_SEAL_FD_WRITE if that is preferred.
> >>
> >>> - mysteriously malfunctions if you try to set it again on another struct
> >>> file that already exists
> >>>
> >>
> >> I didn't follow this, could you explain more?
> >>
> >>> - probably is insecure when used on hugetlbfs.
> >>
> >> The usecase is not expected to prevent all writes, indeed the usecase
> >> requires existing mmaps to continue to be able to write into the memory map.
> >> So would you call that a security issue too? The use of the seal wants to
> >> allow existing mmap regions to be continue to be written into (I mentioned
> >> more details in the cover letter).
> >>
> >>> I see two reasonable solutions:
> >>>
> >>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
> >>> work by itself.
> >>
> >> Currently, the various VFS paths check only the struct file's f_mode to deny
> >> writes of already opened files. This would mean more checking in all those
> >> paths (and modification of all those paths).
> >>
> >> Anyway going with that idea, we could
> >> 1. call deny_write_access(file) from the memfd's seal path which decrements
> >> the inode::i_writecount.
> >> 2. call get_write_access(inode) in the various VFS paths in addition to
> >> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
> >>
> >> That will prevent both reopens, and writes from succeeding. However I worry a
> >> bit about 2 not being too familiar with VFS internals, about what the
> >> consequences of doing that may be.
> >
> > IMHO, modifying both the inode and the struct file separately is fine,
> > since they mean different things. In regular filesystems, it's fine to
> > have a read-write open file description for a file whose inode grants
> > write permission to nobody. Speaking of which: is fchmod enough to
> > prevent this attack?
> 
> Well, yes and no. fchmod does prevent reopening the file RW, but
> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
> isn't sufficient by itself. While it might be good enough for Android
> (in the sense that it'll prevent RW-reopens from other security
> contexts to which we send an open memfd file), it's still conceptually
> ugly, IMHO. Let's go with the original approach of just tweaking the
> inode so that open-for-write is permanently blocked.

Agreed with the idea of modifying both file and inode flags. I was thinking
modifying i_mode may do the trick but as you pointed it probably could be
reverted by chmod or some other attribute setting calls.

OTOH, I don't think deny_write_access(file) can be reverted from any
user-facing path so we could do that from the seal to prevent the future
opens in write mode. I'll double check and test that out tomorrow.

thanks,

 - Joel

Andy Lutomirski Nov. 10, 2018, 10:18 p.m. UTC | #23

> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <[e-mail]> wrote:
> 
>> On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote:
>>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
>>>> Thanks Andy for your thoughts, my comments below:
>> [snip]
>>>> I don't see it as warty, different seals will work differently. It works
>>>> quite well for our usecase, and since Linux is all about solving real
>>>> problems in the real work, it would be useful to have it.
>>>> 
>>>>> - causes a probably-observable effect in the file mode in F_GETFL.
>>>> 
>>>> Wouldn't that be the right thing to observe anyway?
>>>> 
>>>>> - causes reopen to fail.
>>>> 
>>>> So this concern isn't true anymore if we make reopen fail only for WRITE
>>>> opens as Daniel suggested. I will make this change so that the security fix
>>>> is a clean one.
>>>> 
>>>>> - does *not* affect other struct files that may already exist on the same inode.
>>>> 
>>>> TBH if you really want to block all writes to the file, then you want
>>>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
>>>> to another process and we want to prevent any new writes in the receiver
>>>> side. There is no way this other receiving process can have an existing fd
>>>> unless it was already sent one without the seal applied.  The proposed seal
>>>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
>>>> 
>>>>> - mysteriously malfunctions if you try to set it again on another struct
>>>>> file that already exists
>>>>> 
>>>> 
>>>> I didn't follow this, could you explain more?
>>>> 
>>>>> - probably is insecure when used on hugetlbfs.
>>>> 
>>>> The usecase is not expected to prevent all writes, indeed the usecase
>>>> requires existing mmaps to continue to be able to write into the memory map.
>>>> So would you call that a security issue too? The use of the seal wants to
>>>> allow existing mmap regions to be continue to be written into (I mentioned
>>>> more details in the cover letter).
>>>> 
>>>>> I see two reasonable solutions:
>>>>> 
>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>>>>> work by itself.
>>>> 
>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
>>>> writes of already opened files. This would mean more checking in all those
>>>> paths (and modification of all those paths).
>>>> 
>>>> Anyway going with that idea, we could
>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>>>> the inode::i_writecount.
>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>>>> 
>>>> That will prevent both reopens, and writes from succeeding. However I worry a
>>>> bit about 2 not being too familiar with VFS internals, about what the
>>>> consequences of doing that may be.
>>> 
>>> IMHO, modifying both the inode and the struct file separately is fine,
>>> since they mean different things. In regular filesystems, it's fine to
>>> have a read-write open file description for a file whose inode grants
>>> write permission to nobody. Speaking of which: is fchmod enough to
>>> prevent this attack?
>> 
>> Well, yes and no. fchmod does prevent reopening the file RW, but
>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
>> isn't sufficient by itself. While it might be good enough for Android
>> (in the sense that it'll prevent RW-reopens from other security
>> contexts to which we send an open memfd file), it's still conceptually
>> ugly, IMHO. Let's go with the original approach of just tweaking the
>> inode so that open-for-write is permanently blocked.
> 
> Agreed with the idea of modifying both file and inode flags. I was thinking
> modifying i_mode may do the trick but as you pointed it probably could be
> reverted by chmod or some other attribute setting calls.
> 
> OTOH, I don't think deny_write_access(file) can be reverted from any
> user-facing path so we could do that from the seal to prevent the future
> opens in write mode. I'll double check and test that out tomorrow.
> 
> 

This seems considerably more complicated and more fragile than needed. Just add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE variant work exactly like it with two exceptions:

- shmem_mmap and maybe its hugetlbfs equivalent should check for it and act accordingly.

- add_seals won’t need the wait_for_pins and mapping_deny_write logic.

That really should be all that’s needed.

Joel Fernandes Nov. 11, 2018, 2:38 a.m. UTC | #24

On Sat, Nov 10, 2018 at 02:18:23PM -0800, Andy Lutomirski wrote:
> 
> > On Nov 10, 2018, at 2:09 PM, Joel Fernandes <[e-mail]> wrote:
> > 
> >> On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote:
> >>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
> >>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
> >>>> Thanks Andy for your thoughts, my comments below:
> >> [snip]
> >>>> I don't see it as warty, different seals will work differently. It works
> >>>> quite well for our usecase, and since Linux is all about solving real
> >>>> problems in the real work, it would be useful to have it.
> >>>> 
> >>>>> - causes a probably-observable effect in the file mode in F_GETFL.
> >>>> 
> >>>> Wouldn't that be the right thing to observe anyway?
> >>>> 
> >>>>> - causes reopen to fail.
> >>>> 
> >>>> So this concern isn't true anymore if we make reopen fail only for WRITE
> >>>> opens as Daniel suggested. I will make this change so that the security fix
> >>>> is a clean one.
> >>>> 
> >>>>> - does *not* affect other struct files that may already exist on the same inode.
> >>>> 
> >>>> TBH if you really want to block all writes to the file, then you want
> >>>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
> >>>> to another process and we want to prevent any new writes in the receiver
> >>>> side. There is no way this other receiving process can have an existing fd
> >>>> unless it was already sent one without the seal applied.  The proposed seal
> >>>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
> >>>> 
> >>>>> - mysteriously malfunctions if you try to set it again on another struct
> >>>>> file that already exists
> >>>>> 
> >>>> 
> >>>> I didn't follow this, could you explain more?
> >>>> 
> >>>>> - probably is insecure when used on hugetlbfs.
> >>>> 
> >>>> The usecase is not expected to prevent all writes, indeed the usecase
> >>>> requires existing mmaps to continue to be able to write into the memory map.
> >>>> So would you call that a security issue too? The use of the seal wants to
> >>>> allow existing mmap regions to be continue to be written into (I mentioned
> >>>> more details in the cover letter).
> >>>> 
> >>>>> I see two reasonable solutions:
> >>>>> 
> >>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
> >>>>> work by itself.
> >>>> 
> >>>> Currently, the various VFS paths check only the struct file's f_mode to deny
> >>>> writes of already opened files. This would mean more checking in all those
> >>>> paths (and modification of all those paths).
> >>>> 
> >>>> Anyway going with that idea, we could
> >>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
> >>>> the inode::i_writecount.
> >>>> 2. call get_write_access(inode) in the various VFS paths in addition to
> >>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
> >>>> 
> >>>> That will prevent both reopens, and writes from succeeding. However I worry a
> >>>> bit about 2 not being too familiar with VFS internals, about what the
> >>>> consequences of doing that may be.
> >>> 
> >>> IMHO, modifying both the inode and the struct file separately is fine,
> >>> since they mean different things. In regular filesystems, it's fine to
> >>> have a read-write open file description for a file whose inode grants
> >>> write permission to nobody. Speaking of which: is fchmod enough to
> >>> prevent this attack?
> >> 
> >> Well, yes and no. fchmod does prevent reopening the file RW, but
> >> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
> >> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
> >> isn't sufficient by itself. While it might be good enough for Android
> >> (in the sense that it'll prevent RW-reopens from other security
> >> contexts to which we send an open memfd file), it's still conceptually
> >> ugly, IMHO. Let's go with the original approach of just tweaking the
> >> inode so that open-for-write is permanently blocked.
> > 
> > Agreed with the idea of modifying both file and inode flags. I was thinking
> > modifying i_mode may do the trick but as you pointed it probably could be
> > reverted by chmod or some other attribute setting calls.
> > 
> > OTOH, I don't think deny_write_access(file) can be reverted from any
> > user-facing path so we could do that from the seal to prevent the future
> > opens in write mode. I'll double check and test that out tomorrow.
> > 
> > 
> 
> This seems considerably more complicated and more fragile than needed. Just
> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
> variant work exactly like it with two exceptions:
> 
> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
> accordingly.

There's more to it than that, we also need to block future writes through
write syscall, so we have to hook into the write path too once the seal is
set, not just the mmap. That means we have to add code in mm/shmem.c to do
that in all those handlers, to check for the seal (and hope we didn't miss a
file_operations handler). Is that what you are proposing?

Also, it means we have to keep CONFIG_TMPFS enabled so that the
shmem_file_operations write handlers like write_iter are hooked up. Currently
memfd works even with !CONFIG_TMPFS.

> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
> 
> That really should be all that’s needed.

It seems a fair idea what you're saying. But I don't see how its less
complex.. IMO its far more simple to have VFS do the denial of the operations
based on the flags of its datastructures.. and if it works (which I will test
to be sure it will), then we should be good.

Btw by any chance, are you also coming by LPC conference next week?

thanks!

 - Joel

Andy Lutomirski Nov. 11, 2018, 3:40 a.m. UTC | #25

> On Nov 10, 2018, at 6:38 PM, Joel Fernandes <[e-mail]> wrote:
> 
>> On Sat, Nov 10, 2018 at 02:18:23PM -0800, Andy Lutomirski wrote:
>> 
>>>> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <[e-mail]> wrote:
>>>> 
>>>>> On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote:
>>>>>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
>>>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
>>>>>> Thanks Andy for your thoughts, my comments below:
>>>> [snip]
>>>>>> I don't see it as warty, different seals will work differently. It works
>>>>>> quite well for our usecase, and since Linux is all about solving real
>>>>>> problems in the real work, it would be useful to have it.
>>>>>> 
>>>>>>> - causes a probably-observable effect in the file mode in F_GETFL.
>>>>>> 
>>>>>> Wouldn't that be the right thing to observe anyway?
>>>>>> 
>>>>>>> - causes reopen to fail.
>>>>>> 
>>>>>> So this concern isn't true anymore if we make reopen fail only for WRITE
>>>>>> opens as Daniel suggested. I will make this change so that the security fix
>>>>>> is a clean one.
>>>>>> 
>>>>>>> - does *not* affect other struct files that may already exist on the same inode.
>>>>>> 
>>>>>> TBH if you really want to block all writes to the file, then you want
>>>>>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
>>>>>> to another process and we want to prevent any new writes in the receiver
>>>>>> side. There is no way this other receiving process can have an existing fd
>>>>>> unless it was already sent one without the seal applied.  The proposed seal
>>>>>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
>>>>>> 
>>>>>>> - mysteriously malfunctions if you try to set it again on another struct
>>>>>>> file that already exists
>>>>>>> 
>>>>>> 
>>>>>> I didn't follow this, could you explain more?
>>>>>> 
>>>>>>> - probably is insecure when used on hugetlbfs.
>>>>>> 
>>>>>> The usecase is not expected to prevent all writes, indeed the usecase
>>>>>> requires existing mmaps to continue to be able to write into the memory map.
>>>>>> So would you call that a security issue too? The use of the seal wants to
>>>>>> allow existing mmap regions to be continue to be written into (I mentioned
>>>>>> more details in the cover letter).
>>>>>> 
>>>>>>> I see two reasonable solutions:
>>>>>>> 
>>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>>>>>>> work by itself.
>>>>>> 
>>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
>>>>>> writes of already opened files. This would mean more checking in all those
>>>>>> paths (and modification of all those paths).
>>>>>> 
>>>>>> Anyway going with that idea, we could
>>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>>>>>> the inode::i_writecount.
>>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
>>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>>>>>> 
>>>>>> That will prevent both reopens, and writes from succeeding. However I worry a
>>>>>> bit about 2 not being too familiar with VFS internals, about what the
>>>>>> consequences of doing that may be.
>>>>> 
>>>>> IMHO, modifying both the inode and the struct file separately is fine,
>>>>> since they mean different things. In regular filesystems, it's fine to
>>>>> have a read-write open file description for a file whose inode grants
>>>>> write permission to nobody. Speaking of which: is fchmod enough to
>>>>> prevent this attack?
>>>> 
>>>> Well, yes and no. fchmod does prevent reopening the file RW, but
>>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
>>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
>>>> isn't sufficient by itself. While it might be good enough for Android
>>>> (in the sense that it'll prevent RW-reopens from other security
>>>> contexts to which we send an open memfd file), it's still conceptually
>>>> ugly, IMHO. Let's go with the original approach of just tweaking the
>>>> inode so that open-for-write is permanently blocked.
>>> 
>>> Agreed with the idea of modifying both file and inode flags. I was thinking
>>> modifying i_mode may do the trick but as you pointed it probably could be
>>> reverted by chmod or some other attribute setting calls.
>>> 
>>> OTOH, I don't think deny_write_access(file) can be reverted from any
>>> user-facing path so we could do that from the seal to prevent the future
>>> opens in write mode. I'll double check and test that out tomorrow.
>>> 
>>> 
>> 
>> This seems considerably more complicated and more fragile than needed. Just
>> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
>> variant work exactly like it with two exceptions:
>> 
>> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
>> accordingly.
> 
> There's more to it than that, we also need to block future writes through
> write syscall, so we have to hook into the write path too once the seal is
> set, not just the mmap. That means we have to add code in mm/shmem.c to do
> that in all those handlers, to check for the seal (and hope we didn't miss a
> file_operations handler). Is that what you are proposing?

The existing code already does this. That’s why I suggested grepping :)

> 
> Also, it means we have to keep CONFIG_TMPFS enabled so that the
> shmem_file_operations write handlers like write_iter are hooked up. Currently
> memfd works even with !CONFIG_TMPFS.

If so, that sounds like it may already be a bug.

> 
>> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
>> 
>> That really should be all that’s needed.
> 
> It seems a fair idea what you're saying. But I don't see how its less
> complex.. IMO its far more simple to have VFS do the denial of the operations
> based on the flags of its datastructures.. and if it works (which I will test
> to be sure it will), then we should be good.

I agree it’s complicated, but the code is already written.  You should just need to adjust some masks.

> 
> Btw by any chance, are you also coming by LPC conference next week?
> 

No.  I’d like to, but I can’t make the trip this year.

> thanks!
> 
> - Joel
>

Joel Fernandes Nov. 11, 2018, 4:01 a.m. UTC | #26

On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
> 
> 
> > On Nov 10, 2018, at 6:38 PM, Joel Fernandes <[e-mail]> wrote:
> > 
> >> On Sat, Nov 10, 2018 at 02:18:23PM -0800, Andy Lutomirski wrote:
> >> 
> >>>> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <[e-mail]> wrote:
> >>>> 
> >>>>> On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote:
> >>>>>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <[e-mail]> wrote:
> >>>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <[e-mail]> wrote:
> >>>>>> Thanks Andy for your thoughts, my comments below:
> >>>> [snip]
> >>>>>> I don't see it as warty, different seals will work differently. It works
> >>>>>> quite well for our usecase, and since Linux is all about solving real
> >>>>>> problems in the real work, it would be useful to have it.
> >>>>>> 
> >>>>>>> - causes a probably-observable effect in the file mode in F_GETFL.
> >>>>>> 
> >>>>>> Wouldn't that be the right thing to observe anyway?
> >>>>>> 
> >>>>>>> - causes reopen to fail.
> >>>>>> 
> >>>>>> So this concern isn't true anymore if we make reopen fail only for WRITE
> >>>>>> opens as Daniel suggested. I will make this change so that the security fix
> >>>>>> is a clean one.
> >>>>>> 
> >>>>>>> - does *not* affect other struct files that may already exist on the same inode.
> >>>>>> 
> >>>>>> TBH if you really want to block all writes to the file, then you want
> >>>>>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC
> >>>>>> to another process and we want to prevent any new writes in the receiver
> >>>>>> side. There is no way this other receiving process can have an existing fd
> >>>>>> unless it was already sent one without the seal applied.  The proposed seal
> >>>>>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
> >>>>>> 
> >>>>>>> - mysteriously malfunctions if you try to set it again on another struct
> >>>>>>> file that already exists
> >>>>>>> 
> >>>>>> 
> >>>>>> I didn't follow this, could you explain more?
> >>>>>> 
> >>>>>>> - probably is insecure when used on hugetlbfs.
> >>>>>> 
> >>>>>> The usecase is not expected to prevent all writes, indeed the usecase
> >>>>>> requires existing mmaps to continue to be able to write into the memory map.
> >>>>>> So would you call that a security issue too? The use of the seal wants to
> >>>>>> allow existing mmap regions to be continue to be written into (I mentioned
> >>>>>> more details in the cover letter).
> >>>>>> 
> >>>>>>> I see two reasonable solutions:
> >>>>>>> 
> >>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
> >>>>>>> work by itself.
> >>>>>> 
> >>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
> >>>>>> writes of already opened files. This would mean more checking in all those
> >>>>>> paths (and modification of all those paths).
> >>>>>> 
> >>>>>> Anyway going with that idea, we could
> >>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
> >>>>>> the inode::i_writecount.
> >>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
> >>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
> >>>>>> 
> >>>>>> That will prevent both reopens, and writes from succeeding. However I worry a
> >>>>>> bit about 2 not being too familiar with VFS internals, about what the
> >>>>>> consequences of doing that may be.
> >>>>> 
> >>>>> IMHO, modifying both the inode and the struct file separately is fine,
> >>>>> since they mean different things. In regular filesystems, it's fine to
> >>>>> have a read-write open file description for a file whose inode grants
> >>>>> write permission to nobody. Speaking of which: is fchmod enough to
> >>>>> prevent this attack?
> >>>> 
> >>>> Well, yes and no. fchmod does prevent reopening the file RW, but
> >>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
> >>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
> >>>> isn't sufficient by itself. While it might be good enough for Android
> >>>> (in the sense that it'll prevent RW-reopens from other security
> >>>> contexts to which we send an open memfd file), it's still conceptually
> >>>> ugly, IMHO. Let's go with the original approach of just tweaking the
> >>>> inode so that open-for-write is permanently blocked.
> >>> 
> >>> Agreed with the idea of modifying both file and inode flags. I was thinking
> >>> modifying i_mode may do the trick but as you pointed it probably could be
> >>> reverted by chmod or some other attribute setting calls.
> >>> 
> >>> OTOH, I don't think deny_write_access(file) can be reverted from any
> >>> user-facing path so we could do that from the seal to prevent the future
> >>> opens in write mode. I'll double check and test that out tomorrow.
> >>> 
> >>> 
> >> 
> >> This seems considerably more complicated and more fragile than needed. Just
> >> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
> >> variant work exactly like it with two exceptions:
> >> 
> >> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
> >> accordingly.
> > 
> > There's more to it than that, we also need to block future writes through
> > write syscall, so we have to hook into the write path too once the seal is
> > set, not just the mmap. That means we have to add code in mm/shmem.c to do
> > that in all those handlers, to check for the seal (and hope we didn't miss a
> > file_operations handler). Is that what you are proposing?
> 
> The existing code already does this. That’s why I suggested grepping :)

Ahh sorry I see your point now. Ok let me try this approach. Thank you!
Probably we can make this work this way and it is sufficient.

> > 
> > Also, it means we have to keep CONFIG_TMPFS enabled so that the
> > shmem_file_operations write handlers like write_iter are hooked up. Currently
> > memfd works even with !CONFIG_TMPFS.
> 
> If so, that sounds like it may already be a bug.

Actually, its not a bug. If CONFIG_TMPFS is disabled, then IIRC write syscall
will be prevented anyway and then the mmap is the only way. I'll double check
that once I work on this idea.

> > 
> >> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
> >> 
> >> That really should be all that’s needed.
> > 
> > It seems a fair idea what you're saying. But I don't see how its less
> > complex.. IMO its far more simple to have VFS do the denial of the operations
> > based on the flags of its datastructures.. and if it works (which I will test
> > to be sure it will), then we should be good.
> 
> I agree it’s complicated, but the code is already written.  You should just
> need to adjust some masks.
> 

Right.

> > 
> > Btw by any chance, are you also coming by LPC conference next week?
> > 
> 
> No.  I’d like to, but I can’t make the trip this year.

Ok, no worries.

thanks,

- Joel

Joel Fernandes Nov. 11, 2018, 8:09 a.m. UTC | #27

On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
[...]
> >>>>>>> I see two reasonable solutions:
> >>>>>>> 
> >>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
> >>>>>>> work by itself.
> >>>>>> 
> >>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
> >>>>>> writes of already opened files. This would mean more checking in all those
> >>>>>> paths (and modification of all those paths).
> >>>>>> 
> >>>>>> Anyway going with that idea, we could
> >>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
> >>>>>> the inode::i_writecount.
> >>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
> >>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
> >>>>>> 
> >>>>>> That will prevent both reopens, and writes from succeeding. However I worry a
> >>>>>> bit about 2 not being too familiar with VFS internals, about what the
> >>>>>> consequences of doing that may be.
> >>>>> 
> >>>>> IMHO, modifying both the inode and the struct file separately is fine,
> >>>>> since they mean different things. In regular filesystems, it's fine to
> >>>>> have a read-write open file description for a file whose inode grants
> >>>>> write permission to nobody. Speaking of which: is fchmod enough to
> >>>>> prevent this attack?
> >>>> 
> >>>> Well, yes and no. fchmod does prevent reopening the file RW, but
> >>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
> >>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
> >>>> isn't sufficient by itself. While it might be good enough for Android
> >>>> (in the sense that it'll prevent RW-reopens from other security
> >>>> contexts to which we send an open memfd file), it's still conceptually
> >>>> ugly, IMHO. Let's go with the original approach of just tweaking the
> >>>> inode so that open-for-write is permanently blocked.
> >>> 
> >>> Agreed with the idea of modifying both file and inode flags. I was thinking
> >>> modifying i_mode may do the trick but as you pointed it probably could be
> >>> reverted by chmod or some other attribute setting calls.
> >>> 
> >>> OTOH, I don't think deny_write_access(file) can be reverted from any
> >>> user-facing path so we could do that from the seal to prevent the future
> >>> opens in write mode. I'll double check and test that out tomorrow.
> >>> 
> >>> 
> >> 
> >> This seems considerably more complicated and more fragile than needed. Just
> >> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
> >> variant work exactly like it with two exceptions:
> >> 
> >> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
> >> accordingly.
> > 
> > There's more to it than that, we also need to block future writes through
> > write syscall, so we have to hook into the write path too once the seal is
> > set, not just the mmap. That means we have to add code in mm/shmem.c to do
> > that in all those handlers, to check for the seal (and hope we didn't miss a
> > file_operations handler). Is that what you are proposing?
> 
> The existing code already does this. That’s why I suggested grepping :)
> 
> > 
> > Also, it means we have to keep CONFIG_TMPFS enabled so that the
> > shmem_file_operations write handlers like write_iter are hooked up. Currently
> > memfd works even with !CONFIG_TMPFS.
> 
> If so, that sounds like it may already be a bug.
> 
> > 
> >> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
> >> 
> >> That really should be all that’s needed.
> > 
> > It seems a fair idea what you're saying. But I don't see how its less
> > complex.. IMO its far more simple to have VFS do the denial of the operations
> > based on the flags of its datastructures.. and if it works (which I will test
> > to be sure it will), then we should be good.
> 
> I agree it’s complicated, but the code is already written.  You should just
> need to adjust some masks.
> 

Its actually not that bad and a great idea, I did something like the
following and it works pretty well. I would say its cleaner than the old
approach for sure (and I also added a /proc/pid/fd/N reopen test to the
selftest and made sure that issue goes away).

Side note: One subtelty I discovered from the existing selftests is once the
F_SEAL_WRITE are active, an mmap of PROT_READ and MAP_SHARED region is
expected to fail. This is also evident from this code in mmap_region:
		if (vm_flags & VM_SHARED) {
			error = mapping_map_writable(file->f_mapping);
			if (error)
				goto allow_write_and_free_vma;
		}

---8<-----------------------

From: "Joel Fernandes (Google)" <[e-mail]>
Subject: [PATCH] mm/memfd: implement future write seal using shmem ops

Signed-off-by: Joel Fernandes (Google) <[e-mail]>
---
 fs/hugetlbfs/inode.c |  2 +-
 mm/memfd.c           | 19 -------------------
 mm/shmem.c           | 13 ++++++++++---
 3 files changed, 11 insertions(+), 23 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 32920a10100e..1978581abfdf 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 		inode_lock(inode);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			inode_unlock(inode);
 			return -EPERM;
 		}
diff --git a/mm/memfd.c b/mm/memfd.c
index 5ba9804e9515..a9ece5fab439 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -220,25 +220,6 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	if ((seals & F_SEAL_FUTURE_WRITE) &&
-	    !(*file_seals & F_SEAL_FUTURE_WRITE)) {
-		/*
-		 * The FUTURE_WRITE seal also prevents growing and shrinking
-		 * so we need them to be already set, or requested now.
-		 */
-		int test_seals = (seals | *file_seals) &
-				 (F_SEAL_GROW | F_SEAL_SHRINK);
-
-		if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
-			error = -EINVAL;
-			goto unlock;
-		}
-
-		spin_lock(&file->f_lock);
-		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
-		spin_unlock(&file->f_lock);
-	}
-
 	*file_seals |= seals;
 	error = 0;
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 446942677cd4..7dad7efd8b99 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2163,6 +2163,12 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user)
 
 static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct shmem_inode_info *info = SHMEM_I(file_inode(file));
+
+	/* New shared mmaps are not allowed when "future write" seal active. */
+	if ((vma->vm_flags & VM_SHARED) && (info->seals & F_SEAL_FUTURE_WRITE))
+		return -EPERM;
+
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2391,8 +2397,9 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
-		if (info->seals & F_SEAL_WRITE)
+	if (unlikely(info->seals & (F_SEAL_GROW |
+				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
@@ -2657,7 +2664,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & F_SEAL_WRITE || info->seals & F_SEAL_FUTURE_WRITE) {
 			error = -EPERM;
 			goto out;
 		}

Daniel Colascione Nov. 11, 2018, 8:30 a.m. UTC | #28

On Sun, Nov 11, 2018 at 12:09 AM, Joel Fernandes <[e-mail]> wrote:
> On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
> [...]
>> >>>>>>> I see two reasonable solutions:
>> >>>>>>>
>> >>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>> >>>>>>> work by itself.
>> >>>>>>
>> >>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
>> >>>>>> writes of already opened files. This would mean more checking in all those
>> >>>>>> paths (and modification of all those paths).
>> >>>>>>
>> >>>>>> Anyway going with that idea, we could
>> >>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>> >>>>>> the inode::i_writecount.
>> >>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
>> >>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>> >>>>>>
>> >>>>>> That will prevent both reopens, and writes from succeeding. However I worry a
>> >>>>>> bit about 2 not being too familiar with VFS internals, about what the
>> >>>>>> consequences of doing that may be.
>> >>>>>
>> >>>>> IMHO, modifying both the inode and the struct file separately is fine,
>> >>>>> since they mean different things. In regular filesystems, it's fine to
>> >>>>> have a read-write open file description for a file whose inode grants
>> >>>>> write permission to nobody. Speaking of which: is fchmod enough to
>> >>>>> prevent this attack?
>> >>>>
>> >>>> Well, yes and no. fchmod does prevent reopening the file RW, but
>> >>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
>> >>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
>> >>>> isn't sufficient by itself. While it might be good enough for Android
>> >>>> (in the sense that it'll prevent RW-reopens from other security
>> >>>> contexts to which we send an open memfd file), it's still conceptually
>> >>>> ugly, IMHO. Let's go with the original approach of just tweaking the
>> >>>> inode so that open-for-write is permanently blocked.
>> >>>
>> >>> Agreed with the idea of modifying both file and inode flags. I was thinking
>> >>> modifying i_mode may do the trick but as you pointed it probably could be
>> >>> reverted by chmod or some other attribute setting calls.
>> >>>
>> >>> OTOH, I don't think deny_write_access(file) can be reverted from any
>> >>> user-facing path so we could do that from the seal to prevent the future
>> >>> opens in write mode. I'll double check and test that out tomorrow.
>> >>>
>> >>>
>> >>
>> >> This seems considerably more complicated and more fragile than needed. Just
>> >> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
>> >> variant work exactly like it with two exceptions:
>> >>
>> >> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
>> >> accordingly.
>> >
>> > There's more to it than that, we also need to block future writes through
>> > write syscall, so we have to hook into the write path too once the seal is
>> > set, not just the mmap. That means we have to add code in mm/shmem.c to do
>> > that in all those handlers, to check for the seal (and hope we didn't miss a
>> > file_operations handler). Is that what you are proposing?
>>
>> The existing code already does this. That’s why I suggested grepping :)
>>
>> >
>> > Also, it means we have to keep CONFIG_TMPFS enabled so that the
>> > shmem_file_operations write handlers like write_iter are hooked up. Currently
>> > memfd works even with !CONFIG_TMPFS.
>>
>> If so, that sounds like it may already be a bug.

Why shouldn't memfd work independently of CONFIG_TMPFS? In particular,
write(2) on tmpfs FDs shouldn't work differently. If it does, that's a
kernel implementation detail leaking out into userspace.

>> >> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
>> >>
>> >> That really should be all that’s needed.
>> >
>> > It seems a fair idea what you're saying. But I don't see how its less
>> > complex.. IMO its far more simple to have VFS do the denial of the operations
>> > based on the flags of its datastructures.. and if it works (which I will test
>> > to be sure it will), then we should be good.
>>
>> I agree it’s complicated, but the code is already written.  You should just
>> need to adjust some masks.
>>
>
> Its actually not that bad and a great idea, I did something like the
> following and it works pretty well. I would say its cleaner than the old
> approach for sure (and I also added a /proc/pid/fd/N reopen test to the
> selftest and made sure that issue goes away).
>
> Side note: One subtelty I discovered from the existing selftests is once the
> F_SEAL_WRITE are active, an mmap of PROT_READ and MAP_SHARED region is
> expected to fail. This is also evident from this code in mmap_region:
>                 if (vm_flags & VM_SHARED) {
>                         error = mapping_map_writable(file->f_mapping);
>                         if (error)
>                                 goto allow_write_and_free_vma;
>                 }
>

This behavior seems like a bug. Why should MAP_SHARED writes be denied
here? There's no semantic incompatibility between shared mappings and
the seal. And I think this change would represent an ABI break using
memfd seals for ashmem, since ashmem currently allows MAP_SHARED
mappings after changing prot_mask.

> ---8<-----------------------
>
> From: "Joel Fernandes (Google)" <[e-mail]>
> Subject: [PATCH] mm/memfd: implement future write seal using shmem ops
>
> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> ---
>  fs/hugetlbfs/inode.c |  2 +-
>  mm/memfd.c           | 19 -------------------
>  mm/shmem.c           | 13 ++++++++++---
>  3 files changed, 11 insertions(+), 23 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 32920a10100e..1978581abfdf 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>                 inode_lock(inode);
>
>                 /* protected by i_mutex */
> -               if (info->seals & F_SEAL_WRITE) {
> +               if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
>                         inode_unlock(inode);
>                         return -EPERM;
>                 }

Maybe we can always set F_SEAL_FUTURE_WRITE when F_SEAL_WRITE so we
can just test one bit except where the F_SEAL_WRITE behavior differs
from F_SEAL_FUTURE_WRITE.

Andy Lutomirski Nov. 11, 2018, 3:14 p.m. UTC | #29

> On Nov 11, 2018, at 12:30 AM, Daniel Colascione <[e-mail]> wrote:
> 
>> On Sun, Nov 11, 2018 at 12:09 AM, Joel Fernandes <[e-mail]> wrote:
>> On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
>> [...]
>>>>>>>>>> I see two reasonable solutions:
>>>>>>>>>> 
>>>>>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>>>>>>>>>> work by itself.
>>>>>>>>> 
>>>>>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
>>>>>>>>> writes of already opened files. This would mean more checking in all those
>>>>>>>>> paths (and modification of all those paths).
>>>>>>>>> 
>>>>>>>>> Anyway going with that idea, we could
>>>>>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>>>>>>>>> the inode::i_writecount.
>>>>>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
>>>>>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
>>>>>>>>> 
>>>>>>>>> That will prevent both reopens, and writes from succeeding. However I worry a
>>>>>>>>> bit about 2 not being too familiar with VFS internals, about what the
>>>>>>>>> consequences of doing that may be.
>>>>>>>> 
>>>>>>>> IMHO, modifying both the inode and the struct file separately is fine,
>>>>>>>> since they mean different things. In regular filesystems, it's fine to
>>>>>>>> have a read-write open file description for a file whose inode grants
>>>>>>>> write permission to nobody. Speaking of which: is fchmod enough to
>>>>>>>> prevent this attack?
>>>>>>> 
>>>>>>> Well, yes and no. fchmod does prevent reopening the file RW, but
>>>>>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
>>>>>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
>>>>>>> isn't sufficient by itself. While it might be good enough for Android
>>>>>>> (in the sense that it'll prevent RW-reopens from other security
>>>>>>> contexts to which we send an open memfd file), it's still conceptually
>>>>>>> ugly, IMHO. Let's go with the original approach of just tweaking the
>>>>>>> inode so that open-for-write is permanently blocked.
>>>>>> 
>>>>>> Agreed with the idea of modifying both file and inode flags. I was thinking
>>>>>> modifying i_mode may do the trick but as you pointed it probably could be
>>>>>> reverted by chmod or some other attribute setting calls.
>>>>>> 
>>>>>> OTOH, I don't think deny_write_access(file) can be reverted from any
>>>>>> user-facing path so we could do that from the seal to prevent the future
>>>>>> opens in write mode. I'll double check and test that out tomorrow.
>>>>>> 
>>>>>> 
>>>>> 
>>>>> This seems considerably more complicated and more fragile than needed. Just
>>>>> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
>>>>> variant work exactly like it with two exceptions:
>>>>> 
>>>>> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
>>>>> accordingly.
>>>> 
>>>> There's more to it than that, we also need to block future writes through
>>>> write syscall, so we have to hook into the write path too once the seal is
>>>> set, not just the mmap. That means we have to add code in mm/shmem.c to do
>>>> that in all those handlers, to check for the seal (and hope we didn't miss a
>>>> file_operations handler). Is that what you are proposing?
>>> 
>>> The existing code already does this. That’s why I suggested grepping :)
>>> 
>>>> 
>>>> Also, it means we have to keep CONFIG_TMPFS enabled so that the
>>>> shmem_file_operations write handlers like write_iter are hooked up. Currently
>>>> memfd works even with !CONFIG_TMPFS.
>>> 
>>> If so, that sounds like it may already be a bug.
> 
> Why shouldn't memfd work independently of CONFIG_TMPFS? In particular,
> write(2) on tmpfs FDs shouldn't work differently. If it does, that's a
> kernel implementation detail leaking out into userspace.
> 
>>>>> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
>>>>> 
>>>>> That really should be all that’s needed.
>>>> 
>>>> It seems a fair idea what you're saying. But I don't see how its less
>>>> complex.. IMO its far more simple to have VFS do the denial of the operations
>>>> based on the flags of its datastructures.. and if it works (which I will test
>>>> to be sure it will), then we should be good.
>>> 
>>> I agree it’s complicated, but the code is already written.  You should just
>>> need to adjust some masks.
>>> 
>> 
>> Its actually not that bad and a great idea, I did something like the
>> following and it works pretty well. I would say its cleaner than the old
>> approach for sure (and I also added a /proc/pid/fd/N reopen test to the
>> selftest and made sure that issue goes away).
>> 
>> Side note: One subtelty I discovered from the existing selftests is once the
>> F_SEAL_WRITE are active, an mmap of PROT_READ and MAP_SHARED region is
>> expected to fail. This is also evident from this code in mmap_region:
>>                if (vm_flags & VM_SHARED) {
>>                        error = mapping_map_writable(file->f_mapping);
>>                        if (error)
>>                                goto allow_write_and_free_vma;
>>                }
>> 
> 
> This behavior seems like a bug. Why should MAP_SHARED writes be denied
> here? There's no semantic incompatibility between shared mappings and
> the seal. And I think this change would represent an ABI break using
> memfd seals for ashmem, since ashmem currently allows MAP_SHARED
> mappings after changing prot_mask.

Hmm. I’m guessing the intent is that the mmap count should track writable mappings in addition to mappings that could be made writable using mprotect.  I think you could address this for SEAL_FUTURE in two ways:

1. In shmem_mmap, mask off VM_MAYWRITE if SEAL_FUTURE is set, or

2. Add a new vm operation that allows a vma to reject an mprotect attempt, like security_file_mprotect but per vma.  Then give it reasonable semantics for shmem.

(1) probably gives the semantics you want for SEAL_FUTURE: old maps can be mprotected, but new maps can’t.

> 
>> ---8<-----------------------
>> 
>> From: "Joel Fernandes (Google)" <[e-mail]>
>> Subject: [PATCH] mm/memfd: implement future write seal using shmem ops
>> 
>> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
>> ---
>> fs/hugetlbfs/inode.c |  2 +-
>> mm/memfd.c           | 19 -------------------
>> mm/shmem.c           | 13 ++++++++++---
>> 3 files changed, 11 insertions(+), 23 deletions(-)
>> 
>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>> index 32920a10100e..1978581abfdf 100644
>> --- a/fs/hugetlbfs/inode.c
>> +++ b/fs/hugetlbfs/inode.c
>> @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>>                inode_lock(inode);
>> 
>>                /* protected by i_mutex */
>> -               if (info->seals & F_SEAL_WRITE) {
>> +               if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
>>                        inode_unlock(inode);
>>                        return -EPERM;
>>                }
> 
> Maybe we can always set F_SEAL_FUTURE_WRITE when F_SEAL_WRITE so we
> can just test one bit except where the F_SEAL_WRITE behavior differs
> from F_SEAL_FUTURE_WRITE.

This could plausibly confuse existing users that read the seal mask.

Joel Fernandes Nov. 11, 2018, 5:36 p.m. UTC | #30

On Sun, Nov 11, 2018 at 07:14:33AM -0800, Andy Lutomirski wrote:
[...]
> >>>>>>>>>> I see two reasonable solutions:
> >>>>>>>>>> 
> >>>>>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
> >>>>>>>>>> work by itself.
> >>>>>>>>> 
> >>>>>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny
> >>>>>>>>> writes of already opened files. This would mean more checking in all those
> >>>>>>>>> paths (and modification of all those paths).
> >>>>>>>>> 
> >>>>>>>>> Anyway going with that idea, we could
> >>>>>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
> >>>>>>>>> the inode::i_writecount.
> >>>>>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to
> >>>>>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative)
> >>>>>>>>> 
> >>>>>>>>> That will prevent both reopens, and writes from succeeding. However I worry a
> >>>>>>>>> bit about 2 not being too familiar with VFS internals, about what the
> >>>>>>>>> consequences of doing that may be.
> >>>>>>>> 
> >>>>>>>> IMHO, modifying both the inode and the struct file separately is fine,
> >>>>>>>> since they mean different things. In regular filesystems, it's fine to
> >>>>>>>> have a read-write open file description for a file whose inode grants
> >>>>>>>> write permission to nobody. Speaking of which: is fchmod enough to
> >>>>>>>> prevent this attack?
> >>>>>>> 
> >>>>>>> Well, yes and no. fchmod does prevent reopening the file RW, but
> >>>>>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A
> >>>>>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably
> >>>>>>> isn't sufficient by itself. While it might be good enough for Android
> >>>>>>> (in the sense that it'll prevent RW-reopens from other security
> >>>>>>> contexts to which we send an open memfd file), it's still conceptually
> >>>>>>> ugly, IMHO. Let's go with the original approach of just tweaking the
> >>>>>>> inode so that open-for-write is permanently blocked.
> >>>>>> 
> >>>>>> Agreed with the idea of modifying both file and inode flags. I was thinking
> >>>>>> modifying i_mode may do the trick but as you pointed it probably could be
> >>>>>> reverted by chmod or some other attribute setting calls.
> >>>>>> 
> >>>>>> OTOH, I don't think deny_write_access(file) can be reverted from any
> >>>>>> user-facing path so we could do that from the seal to prevent the future
> >>>>>> opens in write mode. I'll double check and test that out tomorrow.
> >>>>>> 
> >>>>>> 
> >>>>> 
> >>>>> This seems considerably more complicated and more fragile than needed. Just
> >>>>> add a new F_SEAL_WRITE_FUTURE.  Grep for F_SEAL_WRITE and make the _FUTURE
> >>>>> variant work exactly like it with two exceptions:
> >>>>> 
> >>>>> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act
> >>>>> accordingly.
> >>>> 
> >>>> There's more to it than that, we also need to block future writes through
> >>>> write syscall, so we have to hook into the write path too once the seal is
> >>>> set, not just the mmap. That means we have to add code in mm/shmem.c to do
> >>>> that in all those handlers, to check for the seal (and hope we didn't miss a
> >>>> file_operations handler). Is that what you are proposing?
> >>> 
> >>> The existing code already does this. That’s why I suggested grepping :)
> >>> 
> >>>> 
> >>>> Also, it means we have to keep CONFIG_TMPFS enabled so that the
> >>>> shmem_file_operations write handlers like write_iter are hooked up. Currently
> >>>> memfd works even with !CONFIG_TMPFS.
> >>> 
> >>> If so, that sounds like it may already be a bug.
> > 
> > Why shouldn't memfd work independently of CONFIG_TMPFS? In particular,
> > write(2) on tmpfs FDs shouldn't work differently. If it does, that's a
> > kernel implementation detail leaking out into userspace.
> > 
> >>>>> - add_seals won’t need the wait_for_pins and mapping_deny_write logic.
> >>>>> 
> >>>>> That really should be all that’s needed.
> >>>> 
> >>>> It seems a fair idea what you're saying. But I don't see how its less
> >>>> complex.. IMO its far more simple to have VFS do the denial of the operations
> >>>> based on the flags of its datastructures.. and if it works (which I will test
> >>>> to be sure it will), then we should be good.
> >>> 
> >>> I agree it’s complicated, but the code is already written.  You should just
> >>> need to adjust some masks.
> >>> 
> >> 
> >> Its actually not that bad and a great idea, I did something like the
> >> following and it works pretty well. I would say its cleaner than the old
> >> approach for sure (and I also added a /proc/pid/fd/N reopen test to the
> >> selftest and made sure that issue goes away).
> >> 
> >> Side note: One subtelty I discovered from the existing selftests is once the
> >> F_SEAL_WRITE are active, an mmap of PROT_READ and MAP_SHARED region is
> >> expected to fail. This is also evident from this code in mmap_region:
> >>                if (vm_flags & VM_SHARED) {
> >>                        error = mapping_map_writable(file->f_mapping);
> >>                        if (error)
> >>                                goto allow_write_and_free_vma;
> >>                }
> >> 
> > 
> > This behavior seems like a bug. Why should MAP_SHARED writes be denied
> > here? There's no semantic incompatibility between shared mappings and
> > the seal. And I think this change would represent an ABI break using
> > memfd seals for ashmem, since ashmem currently allows MAP_SHARED
> > mappings after changing prot_mask.
> 
> Hmm. I’m guessing the intent is that the mmap count should track writable
> mappings in addition to mappings that could be made writable using
> mprotect.  I think you could address this for SEAL_FUTURE in two ways:
> 
> 1. In shmem_mmap, mask off VM_MAYWRITE if SEAL_FUTURE is set, or
> 
> 2. Add a new vm operation that allows a vma to reject an mprotect attempt,
> like security_file_mprotect but per vma.  Then give it reasonable semantics
> for shmem.
> 
> (1) probably gives the semantics you want for SEAL_FUTURE: old maps can be
> mprotected, but new maps can’t.

Thanks Andy and Daniel! This occured to me too and I like the solution in (1).
I tested that now PROT_READ + MAP_SHARED works and the mrprotect is not able
to revert the protection. In fact (1) is exactly what we do in the ashmem
driver.

The updated patch now looks like the following:

---8<-----------------------

From: "Joel Fernandes" <[e-mail]>
Subject: [PATCH] mm/memfd: implement future write seal using shmem ops

Signed-off-by: Joel Fernandes <[e-mail]>
---
 fs/hugetlbfs/inode.c |  2 +-
 mm/memfd.c           | 19 -------------------
 mm/shmem.c           | 24 +++++++++++++++++++++---
 3 files changed, 22 insertions(+), 23 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 32920a10100e..1978581abfdf 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 		inode_lock(inode);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			inode_unlock(inode);
 			return -EPERM;
 		}
diff --git a/mm/memfd.c b/mm/memfd.c
index 5ba9804e9515..a9ece5fab439 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -220,25 +220,6 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	if ((seals & F_SEAL_FUTURE_WRITE) &&
-	    !(*file_seals & F_SEAL_FUTURE_WRITE)) {
-		/*
-		 * The FUTURE_WRITE seal also prevents growing and shrinking
-		 * so we need them to be already set, or requested now.
-		 */
-		int test_seals = (seals | *file_seals) &
-				 (F_SEAL_GROW | F_SEAL_SHRINK);
-
-		if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
-			error = -EINVAL;
-			goto unlock;
-		}
-
-		spin_lock(&file->f_lock);
-		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
-		spin_unlock(&file->f_lock);
-	}
-
 	*file_seals |= seals;
 	error = 0;
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 446942677cd4..ef6039f1ea2a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2163,6 +2163,23 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user)
 
 static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct shmem_inode_info *info = SHMEM_I(file_inode(file));
+
+	/*
+	 * New PROT_READ and MAP_SHARED mmaps are not allowed when "future
+	 * write" seal active.
+	 */
+	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE) &&
+	    (info->seals & F_SEAL_FUTURE_WRITE))
+		return -EPERM;
+
+	/*
+	 * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED read-only
+	 * mapping, take care to not allow mprotect to revert protections.
+	 */
+	if (info->seals & F_SEAL_FUTURE_WRITE)
+		vma->vm_flags &= ~(VM_MAYWRITE);
+
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2391,8 +2408,9 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
-		if (info->seals & F_SEAL_WRITE)
+	if (unlikely(info->seals & (F_SEAL_GROW |
+				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
@@ -2657,7 +2675,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & F_SEAL_WRITE || info->seals & F_SEAL_FUTURE_WRITE) {
 			error = -EPERM;
 			goto out;
 		}

Patch

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 6448cdd9a350..a2f8658f1c55 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -41,6 +41,7 @@ 
 #define F_SEAL_SHRINK	0x0002	/* prevent file from shrinking */
 #define F_SEAL_GROW	0x0004	/* prevent file from growing */
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
+#define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/memfd.c b/mm/memfd.c
index 2bb5e257080e..5ba9804e9515 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -150,7 +150,8 @@  static unsigned int *memfd_file_seals_ptr(struct file *file)
 #define F_ALL_SEALS (F_SEAL_SEAL | \
 		     F_SEAL_SHRINK | \
 		     F_SEAL_GROW | \
-		     F_SEAL_WRITE)
+		     F_SEAL_WRITE | \
+		     F_SEAL_FUTURE_WRITE)
 
 static int memfd_add_seals(struct file *file, unsigned int seals)
 {
@@ -219,6 +220,25 @@  static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
+	if ((seals & F_SEAL_FUTURE_WRITE) &&
+	    !(*file_seals & F_SEAL_FUTURE_WRITE)) {
+		/*
+		 * The FUTURE_WRITE seal also prevents growing and shrinking
+		 * so we need them to be already set, or requested now.
+		 */
+		int test_seals = (seals | *file_seals) &
+				 (F_SEAL_GROW | F_SEAL_SHRINK);
+
+		if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
+			error = -EINVAL;
+			goto unlock;
+		}
+
+		spin_lock(&file->f_lock);
+		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
+		spin_unlock(&file->f_lock);
+	}
+
 	*file_seals |= seals;
 	error = 0;