This is a note to let you know that I've just added the patch titled
posix-timers: Ensure timer ID search-loop limit is valid
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
posix-timers-ensure-timer-id-search-loop-limit-is-va.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6f9d9804623fde8829c8b0e2db90361f49fdc570
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Thu Jun 1 20:58:47 2023 +0200
posix-timers: Ensure timer ID search-loop limit is valid
[ Upstream commit 8ce8849dd1e78dadcee0ec9acbd259d239b7069f ]
posix_timer_add() tries to allocate a posix timer ID by starting from the
cached ID which was stored by the last successful allocation.
This is done in a loop searching the ID space for a free slot one by
one. The loop has to terminate when the search wrapped around to the
starting point.
But that's racy vs. establishing the starting point. That is read out
lockless, which leads to the following problem:
CPU0 CPU1
posix_timer_add()
start = sig->posix_timer_id;
lock(hash_lock);
... posix_timer_add()
if (++sig->posix_timer_id < 0)
start = sig->posix_timer_id;
sig->posix_timer_id = 0;
So CPU1 can observe a negative start value, i.e. -1, and the loop break
never happens because the condition can never be true:
if (sig->posix_timer_id == start)
break;
While this is unlikely to ever turn into an endless loop as the ID space is
huge (INT_MAX), the racy read of the start value caught the attention of
KCSAN and Dmitry unearthed that incorrectness.
Rewrite it so that all id operations are under the hash lock.
Reported-by: syzbot+5c54bd3eb218bb595aa9@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Link: https://lore.kernel.org/r/87bkhzdn6g.ffs@tglx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 20099268fa257..669e8cff40c74 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -135,7 +135,7 @@ struct signal_struct {
#ifdef CONFIG_POSIX_TIMERS
/* POSIX.1b Interval Timers */
- int posix_timer_id;
+ unsigned int next_posix_timer_id;
struct list_head posix_timers;
/* ITIMER_REAL timer for the process */
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index ed3c4a9543982..2d6cf93ca370a 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -140,25 +140,30 @@ static struct k_itimer *posix_timer_by_id(timer_t id)
static int posix_timer_add(struct k_itimer *timer)
{
struct signal_struct *sig = current->signal;
- int first_free_id = sig->posix_timer_id;
struct hlist_head *head;
- int ret = -ENOENT;
+ unsigned int cnt, id;
- do {
+ /*
+ * FIXME: Replace this by a per signal struct xarray once there is
+ * a plan to handle the resulting CRIU regression gracefully.
+ */
+ for (cnt = 0; cnt <= INT_MAX; cnt++) {
spin_lock(&hash_lock);
- head = &posix_timers_hashtable[hash(sig, sig->posix_timer_id)];
- if (!__posix_timers_find(head, sig, sig->posix_timer_id)) {
+ id = sig->next_posix_timer_id;
+
+ /* Write the next ID back. Clamp it to the positive space */
+ sig->next_posix_timer_id = (id + 1) & INT_MAX;
+
+ head = &posix_timers_hashtable[hash(sig, id)];
+ if (!__posix_timers_find(head, sig, id)) {
hlist_add_head_rcu(&timer->t_hash, head);
- ret = sig->posix_timer_id;
+ spin_unlock(&hash_lock);
+ return id;
}
- if (++sig->posix_timer_id < 0)
- sig->posix_timer_id = 0;
- if ((sig->posix_timer_id == first_free_id) && (ret == -ENOENT))
- /* Loop over all possible ids completed */
- ret = -EAGAIN;
spin_unlock(&hash_lock);
- } while (ret == -ENOENT);
- return ret;
+ }
+ /* POSIX return code when no timer ID could be allocated */
+ return -EAGAIN;
}
static inline void unlock_timer(struct k_itimer *timr, unsigned long flags)
