[v2] tomoyo: Don't check open/getattr permission on sockets.

Commit Message

Tetsuo Handa June 22, 2019, 4:45 a.m. UTC

On 2019/06/19 5:49, Al Viro wrote:
> On Sun, Jun 16, 2019 at 03:49:00PM +0900, Tetsuo Handa wrote:
>> Hello, Al.
>>
>> Q1: Do you agree that we should fix TOMOYO side rather than SOCKET_I()->sk
>>     management.
> 
> You do realize that sockets are not unique in that respect, right?
> All kinds of interesting stuff can be accessed via /proc/*/fd/*, and
> it _can_ be closed under you.  So I'd suggest checking how your code
> copes with similar for pipes, FIFOs, epoll, etc., accessed that way...

I know all kinds of interesting stuff can be accessed via /proc/*/fd/*,
and it _can_ be closed under me.

Regarding sockets, I was accessing "struct socket" memory and
"struct sock" memory which are outside of "struct inode" memory.

But regarding other objects, I am accessing "struct dentry" memory,
"struct super_block" memory and "struct inode" memory. I'm expecting
that these memory can't be kfree()d as long as "struct path" holds
a reference. Is my expectation correct?

> 
> We are _not_ going to be checking that in fs/open.c - the stuff found
> via /proc/*/fd/* can have the associated file closed by the time
> we get to calling ->open() and we won't know that until said call.

OK. Then, fixing TOMOYO side is the correct way.

> 
>> Q2: Do you see any problem with using f->f_path.dentry->d_inode ?
>>     Do we need to use d_backing_inode() or d_inode() ?
> 
> Huh?  What's wrong with file_inode(f), in the first place?  And
> just when can that be NULL, while we are at it?

Oh, I was not aware of file_inode(). Thanks.

> 
>>>  static int tomoyo_inode_getattr(const struct path *path)
>>>  {
>>> +	/* It is not safe to call tomoyo_get_socket_name(). */
>>> +	if (path->dentry->d_inode && S_ISSOCK(path->dentry->d_inode->i_mode))
>>> +		return 0;
> 
> Can that be called for a negative?
> 

I check for NULL when I'm not sure it is guaranteed to hold a valid pointer.
You meant "we are sure that path->dentry->d_inode is valid", don't you?

By the way, "negative" associates with IS_ERR() range. I guess that
"NULL" is the better name...

Anyway, here is V2 patch.

From c63c4074300921d6d1c33c3b8dc9c84ebfededf5 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <[e-mail]>
Date: Sat, 22 Jun 2019 13:14:26 +0900
Subject: [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.

syzbot is reporting that use of SOCKET_I()->sk from open() can result in
use after free problem [1], for socket's inode is still reachable via
/proc/pid/fd/n despite destruction of SOCKET_I()->sk already completed.

But there is no point with calling security_file_open() on sockets
because open("/proc/pid/fd/n", !O_PATH) on sockets fails with -ENXIO.

There is some point with calling security_inode_getattr() on sockets
because stat("/proc/pid/fd/n") and fstat(open("/proc/pid/fd/n", O_PATH))
are valid. If we want to access "struct sock"->sk_{family,type,protocol}
fields, we will need to use security_socket_post_create() hook and
security_inode_free() hook in order to remember these fields because
security_sk_free() hook is called before the inode is destructed. But
since information which can be protected by checking
security_inode_getattr() on sockets is trivial, let's not be bothered by
"struct inode"->i_security management.

There is point with calling security_file_ioctl() on sockets. Since
ioctl(open("/proc/pid/fd/n", O_PATH)) is invalid, security_file_ioctl()
on sockets should remain safe.

[1] https://syzkaller.appspot.com/bug?id=73d590010454403d55164cca23bd0565b1eb3b74

Signed-off-by: Tetsuo Handa <[e-mail]>
Reported-by: syzbot <[e-mail]>
---
 security/tomoyo/tomoyo.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Eric Biggers Aug. 22, 2019, 6:30 a.m. UTC | #1

Hi Tetsuo,

On Sat, Jun 22, 2019 at 01:45:30PM +0900, Tetsuo Handa wrote:
> On 2019/06/19 5:49, Al Viro wrote:
> > On Sun, Jun 16, 2019 at 03:49:00PM +0900, Tetsuo Handa wrote:
> >> Hello, Al.
> >>
> >> Q1: Do you agree that we should fix TOMOYO side rather than SOCKET_I()->sk
> >>     management.
> > 
> > You do realize that sockets are not unique in that respect, right?
> > All kinds of interesting stuff can be accessed via /proc/*/fd/*, and
> > it _can_ be closed under you.  So I'd suggest checking how your code
> > copes with similar for pipes, FIFOs, epoll, etc., accessed that way...
> 
> I know all kinds of interesting stuff can be accessed via /proc/*/fd/*,
> and it _can_ be closed under me.
> 
> Regarding sockets, I was accessing "struct socket" memory and
> "struct sock" memory which are outside of "struct inode" memory.
> 
> But regarding other objects, I am accessing "struct dentry" memory,
> "struct super_block" memory and "struct inode" memory. I'm expecting
> that these memory can't be kfree()d as long as "struct path" holds
> a reference. Is my expectation correct?
> 
> > 
> > We are _not_ going to be checking that in fs/open.c - the stuff found
> > via /proc/*/fd/* can have the associated file closed by the time
> > we get to calling ->open() and we won't know that until said call.
> 
> OK. Then, fixing TOMOYO side is the correct way.
> 
> > 
> >> Q2: Do you see any problem with using f->f_path.dentry->d_inode ?
> >>     Do we need to use d_backing_inode() or d_inode() ?
> > 
> > Huh?  What's wrong with file_inode(f), in the first place?  And
> > just when can that be NULL, while we are at it?
> 
> Oh, I was not aware of file_inode(). Thanks.
> 
> > 
> >>>  static int tomoyo_inode_getattr(const struct path *path)
> >>>  {
> >>> +	/* It is not safe to call tomoyo_get_socket_name(). */
> >>> +	if (path->dentry->d_inode && S_ISSOCK(path->dentry->d_inode->i_mode))
> >>> +		return 0;
> > 
> > Can that be called for a negative?
> > 
> 
> I check for NULL when I'm not sure it is guaranteed to hold a valid pointer.
> You meant "we are sure that path->dentry->d_inode is valid", don't you?
> 
> By the way, "negative" associates with IS_ERR() range. I guess that
> "NULL" is the better name...
> 
> Anyway, here is V2 patch.
> 
> From c63c4074300921d6d1c33c3b8dc9c84ebfededf5 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <[e-mail]>
> Date: Sat, 22 Jun 2019 13:14:26 +0900
> Subject: [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.
> 
> syzbot is reporting that use of SOCKET_I()->sk from open() can result in
> use after free problem [1], for socket's inode is still reachable via
> /proc/pid/fd/n despite destruction of SOCKET_I()->sk already completed.
> 
> But there is no point with calling security_file_open() on sockets
> because open("/proc/pid/fd/n", !O_PATH) on sockets fails with -ENXIO.
> 
> There is some point with calling security_inode_getattr() on sockets
> because stat("/proc/pid/fd/n") and fstat(open("/proc/pid/fd/n", O_PATH))
> are valid. If we want to access "struct sock"->sk_{family,type,protocol}
> fields, we will need to use security_socket_post_create() hook and
> security_inode_free() hook in order to remember these fields because
> security_sk_free() hook is called before the inode is destructed. But
> since information which can be protected by checking
> security_inode_getattr() on sockets is trivial, let's not be bothered by
> "struct inode"->i_security management.
> 
> There is point with calling security_file_ioctl() on sockets. Since
> ioctl(open("/proc/pid/fd/n", O_PATH)) is invalid, security_file_ioctl()
> on sockets should remain safe.
> 
> [1] https://syzkaller.appspot.com/bug?id=73d590010454403d55164cca23bd0565b1eb3b74
> 
> Signed-off-by: Tetsuo Handa <[e-mail]>
> Reported-by: syzbot <[e-mail]>
> ---
>  security/tomoyo/tomoyo.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
> index 716c92e..8ea3f5d 100644
> --- a/security/tomoyo/tomoyo.c
> +++ b/security/tomoyo/tomoyo.c
> @@ -126,6 +126,9 @@ static int tomoyo_bprm_check_security(struct linux_binprm *bprm)
>   */
>  static int tomoyo_inode_getattr(const struct path *path)
>  {
> +	/* It is not safe to call tomoyo_get_socket_name(). */
> +	if (S_ISSOCK(d_inode(path->dentry)->i_mode))
> +		return 0;
>  	return tomoyo_path_perm(TOMOYO_TYPE_GETATTR, path, NULL);
>  }
>  
> @@ -316,6 +319,9 @@ static int tomoyo_file_open(struct file *f)
>  	/* Don't check read permission here if called from do_execve(). */
>  	if (current->in_execve)
>  		return 0;
> +	/* Sockets can't be opened by open(). */
> +	if (S_ISSOCK(file_inode(f)->i_mode))
> +		return 0;
>  	return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
>  					    f->f_flags);
>  }
> -- 

What happened to this patch?

Also, isn't the same bug in other places too?:

	- tomoyo_path_chmod()
	- tomoyo_path_chown()
	- smack_inode_getsecurity()
	- smack_inode_setsecurity()

- Eric

Tetsuo Handa Aug. 22, 2019, 6:55 a.m. UTC | #2

Eric Biggers wrote:
> What happened to this patch?

I have to learn how to manage a git tree for sending
pull requests, but I can't find time to try.

> 
> Also, isn't the same bug in other places too?:
> 
> 	- tomoyo_path_chmod()
> 	- tomoyo_path_chown()
> 	- smack_inode_getsecurity()
> 	- smack_inode_setsecurity()

What's the bug? The file descriptor returned by open(O_PATH) cannot be
passed to read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2) etc.

Eric Biggers Aug. 22, 2019, 7:01 a.m. UTC | #3

On Thu, Aug 22, 2019 at 03:55:31PM +0900, Tetsuo Handa wrote:
> Eric Biggers wrote:
> > What happened to this patch?
> 
> I have to learn how to manage a git tree for sending
> pull requests, but I can't find time to try.
> 
> > 
> > Also, isn't the same bug in other places too?:
> > 
> > 	- tomoyo_path_chmod()
> > 	- tomoyo_path_chown()
> > 	- smack_inode_getsecurity()
> > 	- smack_inode_setsecurity()
> 
> What's the bug? The file descriptor returned by open(O_PATH) cannot be
> passed to read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2) etc.
> 

chmod(2), chown(2), getxattr(2), and setxattr(2) take a path, not a fd.

- Eric

Tetsuo Handa Aug. 22, 2019, 7:42 a.m. UTC | #4

Eric Biggers wrote:
> On Thu, Aug 22, 2019 at 03:55:31PM +0900, Tetsuo Handa wrote:
> > > Also, isn't the same bug in other places too?:
> > > 
> > > 	- tomoyo_path_chmod()
> > > 	- tomoyo_path_chown()
> > > 	- smack_inode_getsecurity()
> > > 	- smack_inode_setsecurity()
> > 
> > What's the bug? The file descriptor returned by open(O_PATH) cannot be
> > passed to read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2) etc.
> > 
> 
> chmod(2), chown(2), getxattr(2), and setxattr(2) take a path, not a fd.
> 

OK. Then, is the correct fix

  inode_lock(inode);
  if (SOCKET_I(inode)->sk) {
    // Can access SOCKET_I(sock)->sk->*
  } else {
    // Already close()d. Don't touch.
  }
  inode_unlock(inode);

thanks to

  commit 6d8c50dcb029872b ("socket: close race condition between sock_close() and sockfs_setattr()")
  commit ff7b11aa481f682e ("net: socket: set sock->sk to NULL after calling proto_ops::release()")

changes?

Eric Biggers Aug. 22, 2019, 3:47 p.m. UTC | #5

On Thu, Aug 22, 2019 at 04:42:26PM +0900, Tetsuo Handa wrote:
> Eric Biggers wrote:
> > On Thu, Aug 22, 2019 at 03:55:31PM +0900, Tetsuo Handa wrote:
> > > > Also, isn't the same bug in other places too?:
> > > > 
> > > > 	- tomoyo_path_chmod()
> > > > 	- tomoyo_path_chown()
> > > > 	- smack_inode_getsecurity()
> > > > 	- smack_inode_setsecurity()
> > > 
> > > What's the bug? The file descriptor returned by open(O_PATH) cannot be
> > > passed to read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2) etc.
> > > 
> > 
> > chmod(2), chown(2), getxattr(2), and setxattr(2) take a path, not a fd.
> > 
> 
> OK. Then, is the correct fix
> 
>   inode_lock(inode);
>   if (SOCKET_I(inode)->sk) {
>     // Can access SOCKET_I(sock)->sk->*
>   } else {
>     // Already close()d. Don't touch.
>   }
>   inode_unlock(inode);
> 
> thanks to
> 
>   commit 6d8c50dcb029872b ("socket: close race condition between sock_close() and sockfs_setattr()")
>   commit ff7b11aa481f682e ("net: socket: set sock->sk to NULL after calling proto_ops::release()")
> 
> changes?

inode_lock() is already held during security_path_chmod(),
security_path_chown(), and security_inode_setxattr().
So you can't just take it again.

- Eric

Patch

diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index 716c92e..8ea3f5d 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c
@@ -126,6 +126,9 @@  static int tomoyo_bprm_check_security(struct linux_binprm *bprm)
  */
 static int tomoyo_inode_getattr(const struct path *path)
 {
+	/* It is not safe to call tomoyo_get_socket_name(). */
+	if (S_ISSOCK(d_inode(path->dentry)->i_mode))
+		return 0;
 	return tomoyo_path_perm(TOMOYO_TYPE_GETATTR, path, NULL);
 }
 
@@ -316,6 +319,9 @@  static int tomoyo_file_open(struct file *f)
 	/* Don't check read permission here if called from do_execve(). */
 	if (current->in_execve)
 		return 0;
+	/* Sockets can't be opened by open(). */
+	if (S_ISSOCK(file_inode(f)->i_mode))
+		return 0;
 	return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
 					    f->f_flags);
 }