linux-kernel - timers: Move clearing of base::timer

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Date:   Sun, 06 Dec 2020 22:40:07 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: timers: Move clearing of base::timer_running under base::lock

syzbot reported KCSAN data races vs. timer_base::timer_running being set to
NULL without holding base::lock in expire_timers().

This looks innocent and most reads are clearly not problematic but for a
non-RT kernel it's completely irrelevant whether the store happens before
or after taking the lock. For an RT kernel moving the store under the lock
requires an extra unlock/lock pair in the case that there is a waiter for
the timer. But that's not the end of the world and definitely not worth the
trouble of adding boatloads of comments and annotations to the code. Famous
last words...

Reported-by: syzbot+aa7c2385d46c5eba0b89@...kaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/timer.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1263,8 +1263,10 @@ static inline void timer_base_unlock_exp
 static void timer_sync_wait_running(struct timer_base *base)
 {
 	if (atomic_read(&base->timer_waiters)) {
+		raw_spin_unlock_irq(&base->lock);
 		spin_unlock(&base->expiry_lock);
 		spin_lock(&base->expiry_lock);
+		raw_spin_lock_irq(&base->lock);
 	}
 }
 
@@ -1448,14 +1450,14 @@ static void expire_timers(struct timer_b
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
-			base->running_timer = NULL;
 			raw_spin_lock(&base->lock);
+			base->running_timer = NULL;
 		} else {
 			raw_spin_unlock_irq(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
+			raw_spin_lock_irq(&base->lock);
 			base->running_timer = NULL;
 			timer_sync_wait_running(base);
-			raw_spin_lock_irq(&base->lock);
 		}
 	}
 }