6.1-stable review patch. If anyone has any objections, please let me know.
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From: Sebastian Andrzej Siewior bigeasy@linutronix.de
[ Upstream commit b549113738e8c751b613118032a724b772aa83f2 ]
syzbot managed to trigger the following race:
T1 T2
futex_wait_requeue_pi() futex_do_wait() schedule() futex_requeue() futex_proxy_trylock_atomic() futex_requeue_pi_prepare() requeue_pi_wake_futex() futex_requeue_pi_complete() /* preempt */
* timeout/ signal wakes T1 *
futex_requeue_pi_wakeup_sync() // Q_REQUEUE_PI_LOCKED futex_hash_put() // back to userland, on stack futex_q is garbage
/* back */ wake_up_state(q->task, TASK_NORMAL);
In this scenario futex_wait_requeue_pi() is able to leave without using futex_q::lock_ptr for synchronization.
This can be prevented by reading futex_q::task before updating the futex_q::requeue_state. A reference on the task_struct is not needed because requeue_pi_wake_futex() is invoked with a spinlock_t held which implies a RCU read section.
Even if T1 terminates immediately after, the task_struct will remain valid during T2's wake_up_state(). A READ_ONCE on futex_q::task before futex_requeue_pi_complete() is enough because it ensures that the variable is read before the state is updated.
Read futex_q::task before updating the requeue state, use it for the following wakeup.
Fixes: 07d91ef510fb1 ("futex: Prevent requeue_pi() lock nesting issue on RT") Reported-by: syzbot+034246a838a10d181e78@syzkaller.appspotmail.com Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: Thomas Gleixner tglx@linutronix.de Closes: https://lore.kernel.org/all/68b75989.050a0220.3db4df.01dd.GAE@google.com/ Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/futex/requeue.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c index cba8b1a6a4cc2..7e43839ca7b05 100644 --- a/kernel/futex/requeue.c +++ b/kernel/futex/requeue.c @@ -223,18 +223,20 @@ static inline void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key, struct futex_hash_bucket *hb) { - q->key = *key; + struct task_struct *task;
+ q->key = *key; __futex_unqueue(q);
WARN_ON(!q->rt_waiter); q->rt_waiter = NULL;
q->lock_ptr = &hb->lock; + task = READ_ONCE(q->task);
/* Signal locked state to the waiter */ futex_requeue_pi_complete(q, 1); - wake_up_state(q->task, TASK_NORMAL); + wake_up_state(task, TASK_NORMAL); }
/**