diff mbox series

selftests/seccomp: fix check of fds being assigned

Message ID 20211115165227.101124-1-andrea.righi@canonical.com (mailing list archive)
State New
Headers show
Series selftests/seccomp: fix check of fds being assigned | expand

Commit Message

Andrea Righi Nov. 15, 2021, 4:52 p.m. UTC 
There might be an arbitrary free open fd slot when we run the addfd
sub-test, so checking for progressive numbers of file descriptors
starting from memfd is not always a reliable check and we could get the
following failure:

  #  RUN           global.user_notification_addfd ...
  # seccomp_bpf.c:3989:user_notification_addfd:Expected listener (18) == nextfd++ (9)
  # user_notification_addfd: Test terminated by assertion

Simply check if memfd and listener are valid file descriptors and start
counting for progressive file checking with the listener fd.

Fixes: 93e720d710df ("selftests/seccomp: More closely track fds being assigned")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

Comments

Christian Brauner Nov. 16, 2021, 9:33 a.m. UTC | #1 
On Mon, Nov 15, 2021 at 05:52:27PM +0100, Andrea Righi wrote:
> There might be an arbitrary free open fd slot when we run the addfd
> sub-test, so checking for progressive numbers of file descriptors
> starting from memfd is not always a reliable check and we could get the
> following failure:
> 
>   #  RUN           global.user_notification_addfd ...
>   # seccomp_bpf.c:3989:user_notification_addfd:Expected listener (18) == nextfd++ (9)
>   # user_notification_addfd: Test terminated by assertion
> 
> Simply check if memfd and listener are valid file descriptors and start
> counting for progressive file checking with the listener fd.
> 
> Fixes: 93e720d710df ("selftests/seccomp: More closely track fds being assigned")
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> ---

Thanks!
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Kees Cook Nov. 18, 2021, 5:37 p.m. UTC | #2 
On Mon, Nov 15, 2021 at 05:52:27PM +0100, Andrea Righi wrote:
> There might be an arbitrary free open fd slot when we run the addfd
> sub-test, so checking for progressive numbers of file descriptors
> starting from memfd is not always a reliable check and we could get the
> following failure:
> 
>   #  RUN           global.user_notification_addfd ...
>   # seccomp_bpf.c:3989:user_notification_addfd:Expected listener (18) == nextfd++ (9)

What injected 9 extra fds into this test?

>   # user_notification_addfd: Test terminated by assertion
> 
> Simply check if memfd and listener are valid file descriptors and start
> counting for progressive file checking with the listener fd.

Hm, so I attempted to fix this once already:
93e720d710df ("selftests/seccomp: More closely track fds being assigned")
so I'm not sure the proposed patch really improves it in the general
case.

> Fixes: 93e720d710df ("selftests/seccomp: More closely track fds being assigned")
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> ---
>  tools/testing/selftests/seccomp/seccomp_bpf.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index d425688cf59c..4f37153378a1 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -3975,18 +3975,17 @@ TEST(user_notification_addfd)
>  	/* There may be arbitrary already-open fds at test start. */
>  	memfd = memfd_create("test", 0);
>  	ASSERT_GE(memfd, 0);
> -	nextfd = memfd + 1;
>  
>  	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
>  	ASSERT_EQ(0, ret) {
>  		TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
>  	}
>  
> -	/* fd: 4 */
>  	/* Check that the basic notification machinery works */
>  	listener = user_notif_syscall(__NR_getppid,
>  				      SECCOMP_FILTER_FLAG_NEW_LISTENER);
> -	ASSERT_EQ(listener, nextfd++);
> +	ASSERT_GE(listener, 0);
> +	nextfd = listener + 1;

e.g. if there was a hole in the fd map for memfd, why not listener too?

Should the test dup2 memfd up to fd 100 and start counting there or
something? What is the condition that fills the fds for this process?

-Kees

>  
>  	pid = fork();
>  	ASSERT_GE(pid, 0);
> -- 
> 2.32.0
>
Andrea Righi Nov. 26, 2021, 9:01 a.m. UTC | #3 
On Thu, Nov 18, 2021 at 09:37:03AM -0800, Kees Cook wrote:
> On Mon, Nov 15, 2021 at 05:52:27PM +0100, Andrea Righi wrote:
> > There might be an arbitrary free open fd slot when we run the addfd
> > sub-test, so checking for progressive numbers of file descriptors
> > starting from memfd is not always a reliable check and we could get the
> > following failure:
> > 
> >   #  RUN           global.user_notification_addfd ...
> >   # seccomp_bpf.c:3989:user_notification_addfd:Expected listener (18) == nextfd++ (9)
> 
> What injected 9 extra fds into this test?
> 

We run the kselftest inside a framework (bash/python scripts basically)
and this is what I see (I added a simple `ls -l /proc/pid/fd` in
seccomp_bpf.c after memfd is created):

11/26 08:50:08 DEBUG|     utils:0153| [stdout] # #  RUN           global.user_notification_addfd ...
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # total 0
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # lrwx------ 1 root root 64 Nov 26 08:50 0 -> /dev/pts/0
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 1 -> pipe:[28844]
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # lrwx------ 1 root root 64 Nov 26 08:50 10 -> /dev/pts/0
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # lrwx------ 1 root root 64 Nov 26 08:50 11 -> /dev/pts/0
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 12 -> /home/ubuntu/autotest/client/results/default/ubuntu_kernel_selftests.seccomp:seccomp_bpf/debug/ubuntu_kernel_selftests.seccomp:seccomp_bpf.DEBUG
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 13 -> /home/ubuntu/autotest/client/results/default/ubuntu_kernel_selftests.seccomp:seccomp_bpf/debug/ubuntu_kernel_selftests.seccomp:seccomp_bpf.INFO
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 14 -> /home/ubuntu/autotest/client/results/default/ubuntu_kernel_selftests.seccomp:seccomp_bpf/debug/ubuntu_kernel_selftests.seccomp:seccomp_bpf.WARNING
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 15 -> /home/ubuntu/autotest/client/results/default/ubuntu_kernel_selftests.seccomp:seccomp_bpf/debug/ubuntu_kernel_selftests.seccomp:seccomp_bpf.ERROR
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 16 -> pipe:[27608]
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 17 -> pipe:[27609]
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 2 -> pipe:[28844]
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 3 -> pipe:[27803]
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 4 -> pipe:[26387]
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 5 -> /home/ubuntu/autotest/client/results/default/debug/client.WARNING
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # l-wx------ 1 root root 64 Nov 26 08:50 6 -> /home/ubuntu/autotest/client/results/default/debug/client.ERROR
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # lrwx------ 1 root root 64 Nov 26 08:50 7 -> /dev/pts/0
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # lrwx------ 1 root root 64 Nov 26 08:50 8 -> /memfd:test (deleted)
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # lrwx------ 1 root root 64 Nov 26 08:50 9 -> /dev/pts/0
11/26 08:50:08 DEBUG|     utils:0153| [stdout] # # seccomp_bpf.c:3993:user_notification_addfd:Expected listener (18) == nextfd++ (9)
11/26 08:50:09 DEBUG|     utils:0153| [stdout] # # user_notification_addfd: Test terminated by assertion
11/26 08:50:09 DEBUG|     utils:0153| [stdout] # #          FAIL  global.user_notification_addfd

As we can see memfd has been allocated in a hole (fd=8) and listener
will get fd=18, so checking for sequential fd numbers is not working in
this case.

> >   # user_notification_addfd: Test terminated by assertion
> > 
> > Simply check if memfd and listener are valid file descriptors and start
> > counting for progressive file checking with the listener fd.
> 
> Hm, so I attempted to fix this once already:
> 93e720d710df ("selftests/seccomp: More closely track fds being assigned")
> so I'm not sure the proposed patch really improves it in the general
> case.

I agree that my patch doesn't fix 100% of the cases, we may still have
fd holes.

> 
> > Fixes: 93e720d710df ("selftests/seccomp: More closely track fds being assigned")
> > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> > ---
> >  tools/testing/selftests/seccomp/seccomp_bpf.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > index d425688cf59c..4f37153378a1 100644
> > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > @@ -3975,18 +3975,17 @@ TEST(user_notification_addfd)
> >  	/* There may be arbitrary already-open fds at test start. */
> >  	memfd = memfd_create("test", 0);
> >  	ASSERT_GE(memfd, 0);
> > -	nextfd = memfd + 1;
> >  
> >  	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> >  	ASSERT_EQ(0, ret) {
> >  		TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
> >  	}
> >  
> > -	/* fd: 4 */
> >  	/* Check that the basic notification machinery works */
> >  	listener = user_notif_syscall(__NR_getppid,
> >  				      SECCOMP_FILTER_FLAG_NEW_LISTENER);
> > -	ASSERT_EQ(listener, nextfd++);
> > +	ASSERT_GE(listener, 0);
> > +	nextfd = listener + 1;
> 
> e.g. if there was a hole in the fd map for memfd, why not listener too?
> 
> Should the test dup2 memfd up to fd 100 and start counting there or
> something? What is the condition that fills the fds for this process?

How about getting the allocated fd numbers from /proc/pid/fd and
figuring out the next fd number taking also the holes into account?

Thanks,
-Andrea
diff mbox series

Patch

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index d425688cf59c..4f37153378a1 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -3975,18 +3975,17 @@  TEST(user_notification_addfd)
 	/* There may be arbitrary already-open fds at test start. */
 	memfd = memfd_create("test", 0);
 	ASSERT_GE(memfd, 0);
-	nextfd = memfd + 1;
 
 	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
 	ASSERT_EQ(0, ret) {
 		TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
 	}
 
-	/* fd: 4 */
 	/* Check that the basic notification machinery works */
 	listener = user_notif_syscall(__NR_getppid,
 				      SECCOMP_FILTER_FLAG_NEW_LISTENER);
-	ASSERT_EQ(listener, nextfd++);
+	ASSERT_GE(listener, 0);
+	nextfd = listener + 1;
 
 	pid = fork();
 	ASSERT_GE(pid, 0);