[tip:timers/urgent] posix-timers: Fix division by zero bug

[tip-bot for Thomas Gleixner]

Commit-ID:  0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94
Gitweb:     https://git.kernel.org/tip/0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Mon, 17 Dec 2018 13:31:05 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 17 Dec 2018 17:35:45 +0100

posix-timers: Fix division by zero bug

The signal delivery path of posix-timers can try to rearm the timer even if
the interval is zero. That's handled for the common case (hrtimer) but not
for alarm timers. In that case the forwarding function raises a division by
zero exception.

The handling for hrtimer based posix timers is wrong because it marks the
timer as active despite the fact that it is stopped.

Move the check from common_hrtimer_rearm() to posixtimer_rearm() to cure
both issues.

Reported-by: syzbot+9d38bedac9cc77b8ad5e@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: sboyd@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: syzkaller-bugs@xxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1812171328050.1880@xxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/time/posix-timers.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index bd62b5eeb5a0..31f49ae80f43 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -289,9 +289,6 @@ static void common_hrtimer_rearm(struct k_itimer *timr)
 {
 	struct hrtimer *timer = &timr->it.real.timer;
 
-	if (!timr->it_interval)
-		return;
-
 	timr->it_overrun += hrtimer_forward(timer, timer->base->get_time(),
 					    timr->it_interval);
 	hrtimer_restart(timer);
@@ -317,7 +314,7 @@ void posixtimer_rearm(struct kernel_siginfo *info)
 	if (!timr)
 		return;
 
-	if (timr->it_requeue_pending == info->si_sys_private) {
+	if (timr->it_interval && timr->it_requeue_pending == info->si_sys_private) {
 		timr->kclock->timer_rearm(timr);
 
 		timr->it_active = 1;