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@xxxxxxxxxx>,
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@xxxxxxxxxx>

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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