[PATCH 5.13 190/223] posix-cpu-timers: Fix rearm racing against process tick

[Greg Kroah-Hartman]

From: Frederic Weisbecker <frederic@xxxxxxxxxx>

commit 1a3402d93c73bf6bb4df6d7c2aac35abfc3c50e2 upstream.

Since the process wide cputime counter is started locklessly from
posix_cpu_timer_rearm(), it can be concurrently stopped by operations
on other timers from the same thread group, such as in the following
unlucky scenario:

         CPU 0                                CPU 1
         -----                                -----
                                           timer_settime(TIMER B)
   posix_cpu_timer_rearm(TIMER A)
       cpu_clock_sample_group()
           (pct->timers_active already true)

                                           handle_posix_cpu_timers()
                                               check_process_timers()
                                                   stop_process_timers()
                                                       pct->timers_active = false
       arm_timer(TIMER A)

   tick -> run_posix_cpu_timers()
       // sees !pct->timers_active, ignore
       // our TIMER A

Fix this with simply locking process wide cputime counting start and
timer arm in the same block.

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/time/posix-cpu-timers.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -991,6 +991,11 @@ static void posix_cpu_timer_rearm(struct
 	if (!p)
 		goto out;
 
+	/* Protect timer list r/w in arm_timer() */
+	sighand = lock_task_sighand(p, &flags);
+	if (unlikely(sighand == NULL))
+		goto out;
+
 	/*
 	 * Fetch the current sample and update the timer's expiry time.
 	 */
@@ -1001,11 +1006,6 @@ static void posix_cpu_timer_rearm(struct
 
 	bump_cpu_timer(timer, now);
 
-	/* Protect timer list r/w in arm_timer() */
-	sighand = lock_task_sighand(p, &flags);
-	if (unlikely(sighand == NULL))
-		goto out;
-
 	/*
 	 * Now re-arm for the new expiry time.
 	 */