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Message-Id: <20221103075548.6477-3-wagi@monom.org>
Date: Thu, 3 Nov 2022 08:55:46 +0100
From: Daniel Wagner <wagi@...om.org>
To: LKML <linux-kernel@...r.kernel.org>,
linux-rt-users <linux-rt-users@...r.kernel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Carsten Emde <C.Emde@...dl.org>,
John Kacur <jkacur@...hat.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Tom Zanussi <tom.zanussi@...ux.intel.com>,
Clark Williams <williams@...hat.com>,
Pavel Machek <pavel@...x.de>
Cc: syzbot+aa7c2385d46c5eba0b89@...kaller.appspotmail.com,
syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com,
stable@...r.kernel.org, Daniel Wagner <wagi@...om.org>
Subject: [PATCH RT 2/4] timers: Move clearing of base::timer_running under base:: Lock
From: Thomas Gleixner <tglx@...utronix.de>
v4.19.255-rt114-rc2 stable review patch.
If anyone has any objections, please let me know.
-----------
Upstream commit bb7262b295472eb6858b5c49893954794027cd84
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@...kaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@...kaller.appspotmail.com
Fixes: 030dcdd197d7 ("timers: Prepare support for PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Link: https://lore.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de
Cc: stable@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Daniel Wagner <wagi@...om.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 b859ecf6424b..603985720f54 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1282,8 +1282,10 @@ 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);
}
}
@@ -1458,14 +1460,14 @@ 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);
- 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);
+ raw_spin_lock_irq(&base->lock);
base->running_timer = NULL;
timer_sync_wait_running(base);
- raw_spin_lock_irq(&base->lock);
}
}
}
--
2.38.0
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