Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

[Kees Cook <keescook@chromium.org>]

On Thu, Feb 10, 2022 at 12:17:50PM -0600, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > Hi,
> >
> > This fixes the signal refactoring to actually kill unkillable processes
> > when receiving a fatal SIGSYS from seccomp. Thanks to Robert for the
> > report and Eric for the fix! I've also tweaked seccomp internal a bit to
> > fail more safely. This was a partial seccomp bypass, in the sense that
> > SECCOMP_RET_KILL_* didn't kill the process, but it didn't bypass other
> > aspects of the filters. (i.e. the syscall was still blocked, etc.)
> 
> Any luck on figuring out how to suppress the extra event?

I haven't found a good single indicator of a process being in an "I am dying"
state, and even if I did, it seems every architecture's exit path would
need to add a new test.

The best approach seems to be clearing the TIF_*WORK* bits, but that's
still a bit arch-specific. And I'm not sure which layer would do that.
At what point have we decided the process will not continue? More
than seccomp was calling do_exit() in the middle of a syscall, but those
appear to have all been either SIGKILL or SIGSEGV?

-- 
Kees Cook