From: Sebastian Andrzej Siewior <bigeasy@linutronix.de> stable inclusion from stable-v6.6.109 commit 348736955ed6ca6e99ca24b93b1d3fbfe352c181 category: bugfix bugzilla: https://gitee.com/src-openeuler/kernel/issues/ID229A CVE: CVE-2025-39977 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=... -------------------------------- [ 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> Signed-off-by: Yao Kai <yaokai34@huawei.com> --- 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 cba8b1a6a4cc..7e43839ca7b0 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); } /** -- 2.43.0