linux-kernel - Re: [PATCH] seccomp: Fix tracer exit notifications during fatal signals

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Date:	Fri, 12 Aug 2016 15:21:19 +1200
From:	"Robert O'Callahan" <robert@...llahan.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
	Kyle Huey <khuey@...ehuey.com>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>
Subject: Re: [PATCH] seccomp: Fix tracer exit notifications during fatal signals

Thanks!

On Fri, Aug 12, 2016 at 3:12 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> > The bug happens because when __seccomp_filter() detects
> > fatal_signal_pending(), it calls do_exit() without dequeuing the fatal
> > signal. When do_exit() sends the PTRACE_EVENT_EXIT
>
> I _never_ understood what PTRACE_EVENT_EXIT should actually do. I mean,
> when it should actually stop. This was never defined.
>
> > notification and
> > that task is descheduled, __schedule() notices that there is a fatal
> > signal pending and changes its state from TASK_TRACED to TASK_RUNNING.
>
> And this can happen anyway, with or without this change, with or without
> seccomp. Because another fatal signal can be pending. So PTRACE_EVENT_EXIT
> actually depends on /dev/random.

True. But at least (as Kees alluded to later) this patch ensures
PTRACE_EVENT_EXIT delivery when exit is due to exit_group() and no
genuine fatal signals are involved.

> Perhaps we should finally define what it should do. Say, it should only
> stop if SIGKILL was sent "implicitely" by exit/exec. But as for exec,
> there are more (off-topic) complications, not sure we actually want this...

The ptrace man page currently says:
> A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before actual signal death.  This may be changed in the future; SIGKILL is meant to always immediately kill tasks even under ptrace.  Last confirmed on Linux 3.13.

In practice, a PTRACE_EVENT_EXIT is almost always observed after
SIGKILL. That's nice for rr, because it lets us observe the process's
final state. But it allows a process to stay alive indefinitely after
receiving SIGKILL, so I can see why you might want to change it.

Rob
-- 
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