Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers

[Kyle Huey]

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).

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 ...

- Kyle