The bug was reported and analysed by Mark McLoughlin <[email protected]>,
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 <[email protected]>
Signed-off-by: Oleg Nesterov <[email protected]>
posix-timers.c | 17 +++++++++++++----
signal.c | 1 +
2 files changed, 14 insertions(+), 4 deletions(-)
--- 26-rc2/kernel/posix-timers.c~1_PTE_QUEUED 2008-07-20 14:47:53.000000000 +0400
+++ 26-rc2/kernel/posix-timers.c 2008-07-23 15:04:18.000000000 +0400
@@ -296,14 +296,22 @@ void do_schedule_next_timer(struct sigin
unlock_timer(timr, flags);
}
-int posix_timer_event(struct k_itimer *timr,int si_private)
+int posix_timer_event(struct k_itimer *timr, int si_private)
{
- memset(&timr->sigq->info, 0, sizeof(siginfo_t));
+ /*
+ * FIXME: if ->sigq is queued we can race with
+ * dequeue_signal()->do_schedule_next_timer().
+ *
+ * If dequeue_signal() sees the "right" value of
+ * si_sys_private it calls do_schedule_next_timer().
+ * We re-queue ->sigq and drop ->it_lock().
+ * do_schedule_next_timer() locks the timer
+ * and re-schedules it while ->sigq is pending.
+ * Not really bad, but not that we want.
+ */
timr->sigq->info.si_sys_private = si_private;
- /* Send signal to the process that owns this timer.*/
timr->sigq->info.si_signo = timr->it_sigev_signo;
- timr->sigq->info.si_errno = 0;
timr->sigq->info.si_code = SI_TIMER;
timr->sigq->info.si_tid = timr->it_id;
timr->sigq->info.si_value = timr->it_sigev_value;
@@ -435,6 +443,7 @@ static struct k_itimer * alloc_posix_tim
kmem_cache_free(posix_timers_cache, tmr);
tmr = NULL;
}
+ memset(&tmr->sigq->info, 0, sizeof(siginfo_t));
return tmr;
}
--- 26-rc2/kernel/signal.c~1_PTE_QUEUED 2008-07-06 19:29:27.000000000 +0400
+++ 26-rc2/kernel/signal.c 2008-07-23 13:55:11.000000000 +0400
@@ -1310,6 +1310,7 @@ int send_sigqueue(struct sigqueue *q, st
q->info.si_overrun++;
goto out;
}
+ q->info.si_overrun = 0;
signalfd_notify(t, sig);
pending = group ? &t->signal->shared_pending : &t->pending;
On Wed, 2008-07-23 at 20:52 +0400, Oleg Nesterov wrote:
> The bug was reported and analysed by Mark McLoughlin <[email protected]>,
> 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 <[email protected]>
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.