linux-kernel - Re: [PATCH] proc: fix a race in do_io

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Date:	Mon, 4 Jul 2011 00:01:06 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, rientjes@...gle.com, wilsons@...rt.ca,
	security@...nel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] proc: fix a race in do_io_accounting()

On Sun, Jul 03, 2011 at 12:24 -0700, Linus Torvalds wrote:
> On Sun, Jul 3, 2011 at 3:39 AM, Vasiliy Kulikov <segoon@...nwall.com> wrote:
> >
> > The order of locking is similar to the one inside of
> > ptrace_attach(): first goes cred_guard_mutex, then lock_task_sighand().
> 
> Hmm. mm_for_maps() uses mutex_lock_killable(), as does lock_trace.

Killable/interruptable here makes sense.


> And neither proc_pid_wchan() nor the fd following ones
> (proc_pid_follow_link etc) use anything at all.
> 
> So I'm not sure. And do we really even care about the theoretical
> race? Even if we do hit the race window and happen to get it just as a
> process turns setuid, it would seem to be totally harmless (we're not
> going to see any of the sensitive IO anyway).

I consider this as a theoretical race too unless there is a crazy bug in
scheduler/timer.  But IMO it's better to just fully remove the risk
(even purely theoretical) given the lock is simple and it doesn't cost
much.


Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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