Seccomp actions are not audited in the 4.8 kernel

[Tyler Hicks]

The following patch, released in v4.5, changed the auditing behavior of
seccomp:

  commit 96368701e1c89057bbf39222e965161c68a85b4b
  Author: Paul Moore <email address hidden>
  Date: Wed Jan 13 09:18:55 2016 -0500

      audit: force seccomp event logging to honor the audit_enabled flag

In Ubuntu, where the audit subsystem is not enabled by default, it means that
seccomp actions are not logged unless the user has installed auditd or added
the audit=1 kernel command line parameter.

This impacts snap confinement in Yakkety because seccomp actions are no longer
audited which means that snap authors cannot easily know which restricted
system calls they're using.

To test, build the attached program:

 $ sudo apt-get install libseccomp-dev
 ...
 $ gcc -o test test.c -lseccomp

Run the program. It should be killed when calling open().

 $ ./test
 Bad system call

Now look in the syslog. In 4.4 kernels, there will be an audit record showing that the test program was killed because it called open() (syscall 2):

  [666615.055437] audit: type=1326 audit(1474477027.391:261): auid=4294967295 uid=1000 gid=1000 ses=4294967295 pid=12546 comm="test" exe="/tmp/seccomp-log/test" sig=31 arch=c000003e syscall=2 compat=0 ip=0x7fde77e45790 code=0x0

This audit record is not present in 4.8 kernels.

[Tyler Hicks (tyhicks)]

I've tested and submitted a quick fix to the kernel team:

  https://lists.ubuntu.com/archives/kernel-team/2016-September/080066.html

[Jamie Strandboge (jdstrand)]

FYI, this will likely cause tests in (at least) snap-confine to fail.

[Tyler Hicks (tyhicks)]

I'm attaching the test program mentioned in the description.

[Tyler Hicks (tyhicks)]

It looks like this was first fixed in 4.8.0-15.16 and I've verified that it is fixed in 4.8.0-17.19.