Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers

From: Kyle Huey
Date: Wed Nov 17 2021 - 14:06:11 EST

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).

- Kyle