Re: selftests: seccomp_bpf failure on 5.15

Kees Cook
2021-10-29T14:58:07Z

On Thu, Oct 28, 2021 at 05:06:53PM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
>
> > On Thu, Oct 28, 2021 at 12:26:26PM -0500, Eric W. Biederman wrote:
> >> Kees Cook <keescook@chromium.org> writes:
> >>
> >> > On Thu, Oct 28, 2021 at 06:21:12PM +0200, Andrea Righi wrote:
> >> >> The following sub-tests are failing in seccomp_bpf selftest:
> >> >>
> >> >> 18:56:54 DEBUG| [stdout] # selftests: seccomp: seccomp_bpf
> >> >> ...
> >> >> 18:56:57 DEBUG| [stdout] # # RUN TRACE_syscall.ptrace.kill_after ...
> >> >> 18:56:57 DEBUG| [stdout] # # seccomp_bpf.c:2023:kill_after:Expected entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY : PTRACE_EVENTMSG_SYSCALL_EXIT (1) == msg (0)
> >> >> 18:56:57 DEBUG| [stdout] # # seccomp_bpf.c:2023:kill_after:Expected entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY : PTRACE_EVENTMSG_SYSCALL_EXIT (2) == msg (1)
> >> >> 18:56:57 DEBUG| [stdout] # # seccomp_bpf.c:2023:kill_after:Expected entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY : PTRACE_EVENTMSG_SYSCALL_EXIT (1) == msg (2)
> >> >> 18:56:57 DEBUG| [stdout] # # kill_after: Test exited normally instead of by signal (code: 12)
> >> >> 18:56:57 DEBUG| [stdout] # # FAIL TRACE_syscall.ptrace.kill_after
> >> >> ...
> >> >> 18:56:57 DEBUG| [stdout] # # RUN TRACE_syscall.seccomp.kill_after ...
> >> >> 18:56:57 DEBUG| [stdout] # # seccomp_bpf.c:1547:kill_after:Expected !ptrace_syscall (1) == IS_SECCOMP_EVENT(status) (0)
> >> >> 18:56:57 DEBUG| [stdout] # # kill_after: Test exited normally instead of by signal (code: 0)
> >> >> 18:56:57 DEBUG| [stdout] # # FAIL TRACE_syscall.seccomp.kill_after
> >> >> 18:56:57 DEBUG| [stdout] # not ok 80 TRACE_syscall.seccomp.kill_after
> >> >> ...
> >> >> 18:56:57 DEBUG| [stdout] # # FAILED: 85 / 87 tests passed.
> >> >> 18:56:57 DEBUG| [stdout] # # Totals: pass:85 fail:2 xfail:0 xpass:0 skip:0 error:0
> >> >> 18:56:57 DEBUG| [stdout] not ok 1 selftests: seccomp: seccomp_bpf # exit=1
> >> >>
> >> >> I did some bisecting and found that the failures started to happen with:
> >> >>
> >> >> 307d522f5eb8 ("signal/seccomp: Refactor seccomp signal and coredump generation")
> >> >>
> >> >> Not sure if the test needs to be fixed after this commit, or if the
> >> >> commit is actually introducing an issue. I'll investigate more, unless
> >> >> someone knows already what's going on.
> >> >
> >> > Ah thanks for noticing; I will investigate...
> >>
> >>
> >> I just did a quick read through of the test and while
> >> I don't understand everything having a failure seems
> >> very weird.
> >>
> >> I don't understand the comment:
> >> /* Tracer will redirect getpid to getppid, and we should die. */
> >>
> >> As I think what happens is it the bpf programs loads the signal
> >> number. Tests to see if the signal number if GETPPID and allows
> >> that system call and causes any other system call to be terminated.
> >
> > The test suite runs a series of seccomp filter vs syscalls under tracing,
> > either with ptrace or with seccomp SECCOMP_RET_TRACE, to validate the
> > expected behavioral states. It seems that what's happened is that the
> > SIGSYS has suddenly become non-killing:
> >
> > # RUN TRACE_syscall.ptrace.kill_after ...
> > # seccomp_bpf.c:1555:kill_after:Expected WSTOPSIG(status) & 0x80 (0) == 0x80 (128)
> > # seccomp_bpf.c:1556:kill_after:WSTOPSIG: 31
> > # kill_after: Test exited normally instead of by signal (code: 12)
> > # FAIL TRACE_syscall.ptrace.kill_after
> >
> > i.e. the ptracer no longer sees a dead tracee, which would pass through
> > here:
> >
> > if (WIFSIGNALED(status) || WIFEXITED(status))
> > /* Child is dead. Time to go. */
> > return;
> >
> > So the above saw a SIG_TRAP|SIGSYS rather than a killing SIGSYS. i.e.
> > instead of WIFSIGNALED(stauts) being true, it instead catches a
> > PTRACE_EVENT_STOP for SIGSYS, which should be impossible (the process
> > should be getting killed).
>
> Oh. This is being ptraced as part of the test?
>
> Yes. The signal started being delivered. As far as that goes that
> sounds correct.
>
> Ptrace is allowed to intercept even fatal signals. Everything except
> SIGKILL.
>
> Is this a condition we don't want even ptrace to be able to catch?
>
> I think we can arrange it so that even ptrace can't intercept this
> signal. I need to sit this problem on the back burner for a few
> minutes. It is an angle I had not considered.
>
> Is it a problem that the debugger can see the signal if the process does
> not?

Right, I'm trying to understand that too. However, my neighbor just lost
power. :|

What I was in the middle of checking was what ptrace "sees" going
through a fatal SIGSYS; my initial debugging attempts were weird.

-Kees

--
Kees Cook