From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C36A9560AB8 for ; Tue, 8 Sep 2026 13:29:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788874191; cv=none; b=T7fSNfqxyv6nKlFUpvSPl50p3WFu4h6YMXERGHRZvV/kipG3XX2nsAAryJOsLVEercS9PxPDQRXi9M2lauIk6NBFrVFOABJ1C69+Xbp9ec/MaH84rrkBcPqE4SOeJgXp4GR2c+zftMZfXE0Y25QWZyzQvfUGEa8WAe/zc7jogsQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788874191; c=relaxed/simple; bh=QuwGrxrZQjIxm0I+5r7AKR224LwMuq3ERtpeX88Di3A=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=kuFsqtLHpkvSF8gjf2Z9t52ce+DggxWuKc6Nje7ql8kGT7x+etUSAIP87blYS1cf6alWZScaeHvgUbWL2G04R33jdQsEjvlM9gZztouna5QvwiwdzAiRmqxfLHDcdiIAxWlcSJ+gVA8iwaddFXROIqi/zBuCTHM+gJVN54w9XQ8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=qUN1+SwT; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="qUN1+SwT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B5CBD1F00A3D; Tue, 8 Sep 2026 13:29:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1788874178; bh=7Dj6seXRE+dbAaUzgbcXb7kP3lVDdBsMtI9SotfSUI8=; h=Subject:To:Cc:From:Date; b=qUN1+SwTkKwwRR4O0dTJzMt5EaD3YbW/pE9RoVnaLNsA2X5pZYSbYXX3I5kS4PCNy 5qWlq1qJmBnXNGMkWD1v1NNN4z/mDidhZ/Srg4pwPPNps41MFT3KEM7TBjP/KnBAHk ApBJWixQcN4VXDetEZuTym4xYZNX6Qe8kEXdEOas= Subject: FAILED: patch "[PATCH] futex: Prevent rcuwait use-after-free during requeue PI" failed to apply to 5.15-stable tree To: yaokai34@huawei.com,bigeasy@linutronix.de,tglx@kernel.org Cc: From: Date: Tue, 08 Sep 2026 15:18:26 +0200 Message-ID: <2026090826-jeep-tassel-64cd@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y git checkout FETCH_HEAD git cherry-pick -x a3b8d46fe401cba3a5c46dea610e6eb3dc15370e # git commit -s git send-email --to '' --in-reply-to '2026090826-jeep-tassel-64cd@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From a3b8d46fe401cba3a5c46dea610e6eb3dc15370e Mon Sep 17 00:00:00 2001 From: Yao Kai Date: Tue, 1 Sep 2026 15:54:52 +0200 Subject: [PATCH] futex: Prevent rcuwait use-after-free during requeue PI On PREEMPT_RT, FUTEX_CMP_REQUEUE_PI can trigger a KASAN report (slab-out-of-bounds) in futex_requeue_pi_complete() invocation of rcuwait_wake_up(). The futex_q used by futex_wait_requeue_pi() is allocated on the waiter's stack. An early wakeup can race with a PI requeue as follows: waiter requeue task ------ ------------ futex_wait_requeue_pi() futex_do_wait() schedule() futex_requeue futex_proxy_trylock_atomic() futex_requeue_pi_prepare() Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_IN_PROGRESS * timeout/ signal wakes waiter * futex_requeue_pi_wakeup_sync() Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_WAIT requeue_pi_wake_futex futex_requeue_pi_complete() cmpxchg Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_LOCKED rcuwait_wait_event() if (atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT) break /* no schedule() */ /* q.pi_state->owner == current */ futex_private_hash_put() /* return from syscall */ rcuwait_wake_up(&q->requeue_wait) /* q is gone */ futex_requeue_pi_complete() publishes Q_REQUEUE_PI_LOCKED before calling rcuwait_wake_up(). The waiter observes this state in rcuwait_wait_event() before invoking schedule() in rcuwait_wait_event(). Here, the waiter is free leave the syscall before requeue task can complete the wake. To address this race skip rcuwait_wake_up() in the Q_REQUEUE_PI_LOCKED case. This state is only published by requeue_pi_wake_futex(), which saves q->task before futex_requeue_pi_complete() and wakes the waiter via wake_up_state(). This wake is intended to wake the waiter from its futex_do_wait() sleep. If the waiter is still sleeping there, it can not get into the Q_REQUEUE_PI_WAIT state (and require this removed wake). Should the waiter be woken up from futex_do_wait() by other means (as in this example) and sleep in futex_requeue_pi_wakeup_sync() then the wake_up_state() from requeue_pi_wake_futex() will wake it, too. Should the waiter task terminate before wake_up_state() had a chance to wake the task then the task pointer does not become invalid because the futex_hash_bucket::lock is held and the task pointer is RCU protected. [bigeasy: Updated comment and commit message] Fixes: 07d91ef510fb1 ("futex: Prevent requeue_pi() lock nesting issue on RT") Signed-off-by: Yao Kai Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Reviewed-by: Sebastian Andrzej Siewior Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260901135453.3121948-3-bigeasy@linutronix.de diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c index 79823ad13683..b3f4a4bccb12 100644 --- a/kernel/futex/requeue.c +++ b/kernel/futex/requeue.c @@ -154,8 +154,16 @@ static inline void futex_requeue_pi_complete(struct futex_q *q, int locked) } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new)); #ifdef CONFIG_PREEMPT_RT - /* If the waiter interleaved with the requeue let it know */ - if (unlikely(old == Q_REQUEUE_PI_WAIT)) + /* + * The waiter in futex_requeue_pi_wakeup_sync() can interleave with the + * wake below: It will assign Q_REQUEUE_PI_IN_PROGRESS and here it will + * be updated to Q_REQUEUE_PI_LOCKED (locked = 1). The rcuwait_wait_event() + * will already read Q_REQUEUE_PI_LOCKED and skip the schedule() invocation, + * leading to an access of futex_q::requeue_wait after the waiter returned. + * In this case only we skip the wake here and rely on following wake in + * requeue_pi_wake_futex() to perform the wake if needed. + */ + if (unlikely(old == Q_REQUEUE_PI_WAIT) && new != Q_REQUEUE_PI_LOCKED) rcuwait_wake_up(&q->requeue_wait); #endif }