posix-timers: fix posix_timer_event() vs dequeue_signal() race

The bug was reported and analysed by Mark McLoughlin <markmc@redhat.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@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Mark McLoughlin <markmc@redhat.com>
Cc: Oliver Pinter <oliver.pntr@gmail.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: stable@kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

 kernel/posix-timers.c |   17 +++++++++++++----
 kernel/signal.c       |    1 +
 2 files changed, 14 insertions(+), 4 deletions(-)

kernel/posix-timers.c

@@ -296,14 +296,22 @@ void do_schedule_next_timer(struct siginfo *info)
 		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_timer(void)
 		kmem_cache_free(posix_timers_cache, tmr);
 		tmr = NULL;
 	}
+	memset(&tmr->sigq->info, 0, sizeof(siginfo_t));
 	return tmr;
 }
 

kernel/signal.c

@@ -1280,6 +1280,7 @@ int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
 		q->info.si_overrun++;
 		goto out;
 	}
+	q->info.si_overrun = 0;
 
 	signalfd_notify(t, sig);
 	pending = group ? &t->signal->shared_pending : &t->pending;