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Message-Id: <20250505223922.2682012-130-sashal@kernel.org>
Date: Mon, 5 May 2025 18:33:26 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
stable@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
Frederic Weisbecker <frederic@...nel.org>,
Sasha Levin <sashal@...nel.org>,
anna-maria@...utronix.de
Subject: [PATCH AUTOSEL 6.12 130/486] posix-timers: Ensure that timer initialization is fully visible
From: Thomas Gleixner <tglx@...utronix.de>
[ Upstream commit 2389c6efd3ad8edb3bcce0019b4edcc7d9c7de19 ]
Frederic pointed out that the memory operations to initialize the timer are
not guaranteed to be visible, when __lock_timer() observes timer::it_signal
valid under timer::it_lock:
T0 T1
--------- -----------
do_timer_create()
// A
new_timer->.... = ....
spin_lock(current->sighand)
// B
WRITE_ONCE(new_timer->it_signal, current->signal)
spin_unlock(current->sighand)
sys_timer_*()
t = __lock_timer()
spin_lock(&timr->it_lock)
// observes B
if (timr->it_signal == current->signal)
return timr;
if (!t)
return;
// Is not guaranteed to observe A
Protect the write of timer::it_signal, which makes the timer valid, with
timer::it_lock as well. This guarantees that T1 must observe the
initialization A completely, when it observes the valid signal pointer
under timer::it_lock. sighand::siglock must still be taken to protect the
signal::posix_timers list.
Reported-by: Frederic Weisbecker <frederic@...nel.org>
Suggested-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Link: https://lore.kernel.org/all/20250308155623.507944489@linutronix.de
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
kernel/time/posix-timers.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 896ff735034ce..c5d9bfbb082b8 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -514,14 +514,21 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
if (error)
goto out;
- spin_lock_irq(¤t->sighand->siglock);
- /* This makes the timer valid in the hash table */
- WRITE_ONCE(new_timer->it_signal, current->signal);
- hlist_add_head(&new_timer->list, ¤t->signal->posix_timers);
- spin_unlock_irq(¤t->sighand->siglock);
/*
- * After unlocking sighand::siglock @new_timer is subject to
- * concurrent removal and cannot be touched anymore
+ * timer::it_lock ensures that __lock_timer() observes a fully
+ * initialized timer when it observes a valid timer::it_signal.
+ *
+ * sighand::siglock is required to protect signal::posix_timers.
+ */
+ scoped_guard (spinlock_irq, &new_timer->it_lock) {
+ guard(spinlock)(¤t->sighand->siglock);
+ /* This makes the timer valid in the hash table */
+ WRITE_ONCE(new_timer->it_signal, current->signal);
+ hlist_add_head(&new_timer->list, ¤t->signal->posix_timers);
+ }
+ /*
+ * After unlocking @new_timer is subject to concurrent removal and
+ * cannot be touched anymore
*/
return 0;
out:
--
2.39.5
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