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

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Date:   Thu, 10 Feb 2022 12:58:07 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Kees Cook <keescook@...omium.org>
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

Kees Cook <keescook@...omium.org> writes:

> 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 "I am dying" state for a task is fatal_signal_pending, at least
before get_signal is reached, for a process there is SIGNAL_GROUP_EXIT.
Something I am busily cleaning up and making more reliable at the
moment.

What is the event that is happening?  Is it
tracehook_report_syscall_exit or something else?

>From the bits I have seen it seems like something else.

> 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?

This is where I get confused what TIF_WORK bits matter?

I expect if anything else mattered we would need to change it to
HANDLER_EXIT.

I made a mistake conflating to cases and I want to make certain I
successfully separate those two cases at the end of the day.

Eric