linux-hardening - Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL

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Message-ID: <202202101033.9C04563D9@keescook>
Date:   Thu, 10 Feb 2022 10:41:57 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Robert Święcki <robert@...ecki.net>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

On Thu, Feb 10, 2022 at 12:17:50PM -0600, Eric W. Biederman wrote:
> Kees Cook <keescook@...omium.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