The following commit has been merged into the timers/urgent branch of tip:

Commit-ID: bb7262b295472eb6858b5c49893954794027cd84 Gitweb: https://git.kernel.org/tip/bb7262b295472eb6858b5c49893954794027cd84 Author: Thomas Gleixner tglx@linutronix.de AuthorDate: Sun, 06 Dec 2020 22:40:07 +01:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Tue, 27 Jul 2021 20:57:44 +02:00

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 Frederic identified an issue which is:

int data = 0;

void timer_func(struct timer_list *t) { data = 1; }

CPU 0 CPU 1 ------------------------------ -------------------------- base = lock_timer_base(timer, &flags); raw_spin_unlock(&base->lock); if (base->running_timer != timer) call_timer_fn(timer, fn, baseclk); ret = detach_if_pending(timer, base, true); base->running_timer = NULL; raw_spin_unlock_irqrestore(&base->lock, flags); raw_spin_lock(&base->lock);

x = data;

If the timer has previously executed on CPU 1 and then CPU 0 can observe base->running_timer == NULL and returns, assuming the timer has completed, but it's not guaranteed on all architectures. The comment for del_timer_sync() makes that guarantee. Moving the assignment under base->lock prevents this.

For non-RT kernel it's performance wise 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.

Reported-by: syzbot+aa7c2385d46c5eba0b89@syzkaller.appspotmail.com Reported-by: syzbot+abea4558531bae1ba9fe@syzkaller.appspotmail.com Fixes: 030dcdd197d7 ("timers: Prepare support for PREEMPT_RT") Signed-off-by: Thomas Gleixner tglx@linutronix.de Tested-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de Cc: stable@vger.kernel.org --- kernel/time/timer.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 9eb11c2..e3d2c23 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1265,8 +1265,10 @@ static inline void timer_base_unlock_expiry(struct timer_base *base) 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); } }

@@ -1457,14 +1459,14 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head) 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); } } }