Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers

From: Kees Cook
Date: Wed Nov 17 2021 - 16:54:19 EST

On Wed, Nov 17, 2021 at 03:04:31PM -0600, Eric W. Biederman wrote:
> Kyle Huey <me@xxxxxxxxxxxx> writes:
> > On Wed, Nov 17, 2021 at 11:05 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
> >>
> >> On Wed, Nov 17, 2021 at 10:51 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> >
> >> > On Wed, Nov 17, 2021 at 10:47:13AM -0800, Kyle Huey wrote:
> >> > > rr, a userspace record and replay debugger[0], is completely broken on
> >> > > 5.16rc1. I bisected this to 00b06da29cf9dc633cdba87acd3f57f4df3fd5c7.
> >> > >
> >> > > That patch makes two changes, it blocks sigaction from changing signal
> >> > > handlers once the kernel has decided to force the program to take a
> >> > > signal and it also stops notifying ptracers of the signal in the same
> >> > > circumstances. The latter behavior is just wrong. There's no reason
> >> > > that ptrace should not be able to observe and even change
> >> > > (non-SIGKILL) forced signals. It should be reverted.
> >> > >
> >> > > This behavior change is also observable in gdb. If you take a program
> >> > > that sets SIGSYS to SIG_IGN and then raises a SIGSYS via
> >> > > SECCOMP_RET_TRAP and run it under gdb on a good kernel gdb will stop
> >> > > when the SIGSYS is raised, let you inspect program state, etc. After
> >> > > the SA_IMMUTABLE change gdb won't stop until the program has already
> >> > > died of SIGSYS.
> >> >
> >> > Ah, hm, this was trying to fix the case where a program trips
> >> > SECCOMP_RET_KILL (which is a "fatal SIGSYS"), and had been unobservable
> >> > before. I guess the fix was too broad...
> >>
> >> Perhaps I don't understand precisely what you mean by this, but gdb's
> >> behavior for a program that is SECCOMP_RET_KILLed was not changed by
> >> this patch (the SIGSYS is not observed until after program exit before
> >> or after this change).

The SA_IMMUTABLE change was to deal with failures seen in the seccomp
test suite after the recent fatal signal refactoring. Mainly that a
process that should have effectively performed do_exit() was suddenly
visible to the tracer.

> > Ah, maybe that behavior changed in 5.15 (my "before" here is a 5.14
> > kernel). I would argue that the debugger seeing the SIGSYS for
> > SECCOMP_RET_KILL is desirable though ...
> This is definitely worth discussing, and probably in need of fixing (aka
> something in rr seems to have broken).
> We definitely need protection against the race with sigaction.
> The fundamental question becomes does it make sense and is it safe
> to allow a debugger to stop at, and possibly change these signals.

I have no problem with a debugger getting notified about a fatal
(SECCOMP_RET_KILL*-originated) SIGSYS. But whatever happens, the kernel
needs to make sure the process does not continue. (i.e. signal can't be

> Stopping at something SA_IMMUTABLE as long as the signal is allowed to
> continue and kill the process when PTRACE_CONT happens seems harmless.
> Allowing the debugger to change the signal, or change it's handling
> I don't know.

Right -- I'm fine with a visibility change (the seccomp test suite is
just checking for various expected state machine changes across the
various signal/death cases: as long as it _dies_, that's what we want.
If a extra notification appears before it dies, that's okay, it just
needs the test suite to change).

> [...]
> Kees I am back to asking the question I had before I figured out
> SA_IMMUTABLE. Are there security concerns with debuggers intercepting

I see no problem with allowing a tracer to observe the signal, but the
signalled process must have no way to continue running. If we end up in
such a state, then a seccomp process with access to clone() and
ptrace() can escape the seccomp sandbox. This is why seccomp had been
using the big do_exit() hammer -- I really want to absolutely never have
a bug manifest with a bypassed SECCOMP_RET_KILL: having a completely
unavoidable "dying" state is needed.

Kees Cook