linux-kernel - Re: [PATCH 1/2] posix-timers: fix posix_timer_event() vs dequeue

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Date:	Thu, 24 Jul 2008 16:23:03 +0100
From:	Mark McLoughlin <markmc@...hat.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Oliver Pinter <oliver.pntr@...il.com>,
	Roland McGrath <roland@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH 1/2] posix-timers: fix posix_timer_event() vs
	dequeue_signal() race

On Wed, 2008-07-23 at 20:52 +0400, Oleg Nesterov wrote:
> The bug was reported and analysed by Mark McLoughlin <markmc@...hat.com>,
> the patch is based on his and Roland's suggestions.
> 
> posix_timer_event() always rewrites the pre-allocated siginfo before sending
> the signal. Most of the written info is the same all the time, but memset(0)
> is very wrong. If ->sigq is queued we can race with collect_signal() which
> can fail to find this siginfo looking at .si_signo, or copy_siginfo() can
> copy the wrong .si_code/si_tid/etc.
> 
> In short, sys_timer_settime() can in fact stop the active timer, or the user
> can receive the siginfo with the wrong .si_xxx values.
> 
> Move "memset(->info, 0)" from posix_timer_event() to alloc_posix_timer(),
> change send_sigqueue() to set .si_overrun = 0 when ->sigq is not queued.
> It would be nice to move the whole sigq->info initialization from send to
> create path, but this is not easy to do without uglifying timer_create()
> further.
> 
> As Roland rightly pointed out, we need more cleanups/fixes here, see the
> "FIXME" comment in the patch. Hopefully this patch makes sense anyway, and
> it can mask the most bad implications.
> 
> Reported-by: Mark McLoughlin <markmc@...hat.com>

I've re-tested and can confirm that the patch fixes the test case at:

  http://markmc.fedorapeople.org/test-posix-timer-race.c

Cheers,
Mark.

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