Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers

From: Kees Cook
Date: Wed Nov 17 2021 - 13:51:37 EST

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...

Kees Cook