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

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Message-ID: <20160811151250.GA21519@redhat.com>
Date:	Thu, 11 Aug 2016 17:12:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Kyle Huey <khuey@...ehuey.com>,
	"Robert O'Callahan" <robert@...llahan.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>
Subject: Re: [PATCH] seccomp: Fix tracer exit notifications during fatal
	signals

On 08/10, Kees Cook wrote:
>
> This fixes a ptrace vs fatal pending signals bug as manifested in seccomp
> now that ptrace was reordered to happen after ptrace. The short version is
> that seccomp should not attempt to call do_exit() while fatal signals are
> pending under a tracer. This was needlessly paranoid. Instead, the syscall
> can just be skipped and normal signal handling, tracer notification, and
> process death can happen.

ACK.

I think this change is fine in any case, but...

> 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.

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...

Nevermind, the main problem is that _any_ change in this area can break
something. This code is sooooooo old.

But let me repeat, I think this change is fine anyway.

Acked-by: Oleg Nesterov <oleg@...hat.com>