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 B3D2F583ACA; Wed, 9 Sep 2026 14:16:28 +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=1788963389; cv=none; b=DPnsbRYAW4wB0f1VpAh28ul8VTGEIsBZvPsdhUDA3ixVNtmMwdiQpqgd8O2OHXWCOS1kxrnYLgtL4JaS5Yp5KQ8U8cdEWmAXfHr7PEhNplUZEonWbkq/jcPmQ3Lt1NRMif1TYsWfF7SuIcwisJlW/FfX6pio9qnBrbaWtAbnEv0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788963389; c=relaxed/simple; bh=B+dzeZgVe2jPyTO9eKw2xbN8o1UniJTNg0troJpTlsg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=SIOXt0QbB9tlSdW+IzdPqVQafYaGvdvprMqibDMasp2GnqdBjQpH/6EKuKhnZ3dp4QooJAMaKcNPbgCsdey3b7WQANz3sY+iKw4J7jMbUvoVoK51DKo21jDpNKrEq6xraTSnfY9MGGa95gl1J/WJ+UIYVf/7yiu1UWM/8thtn0M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=Y7f0rfJi; 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="Y7f0rfJi" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 177EC1F00A3A; Wed, 9 Sep 2026 14:16:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1788963388; bh=xyn2+AuEw9Yjp+PAjMlt0uA6bmbmeUKMsqIfEddpkDQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Y7f0rfJizEDGts6G68kI/EDRatiuK0gHXXGI7NomXUf8M6ijNt4TS7mYQ6tM3P2Ar Lx/dSA96t1eSejxP87dx8jFOuzRk811kjMskYQAXJKZ972aPd3SjEiIRM3rY3X1KIN zQxZ0s7XepBZiZ7FD4iCb/3uR2El0e3BHtI8c/4g= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Yao Kai , Sebastian Andrzej Siewior , Thomas Gleixner Subject: [PATCH 6.18 062/583] futex: Prevent rcuwait use-after-free during requeue PI Date: Wed, 9 Sep 2026 15:35:47 +0200 Message-ID: <20260909134240.200896871@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260909134237.773280130@linuxfoundation.org> References: <20260909134237.773280130@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Yao Kai commit a3b8d46fe401cba3a5c46dea610e6eb3dc15370e upstream. 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 Signed-off-by: Greg Kroah-Hartman --- kernel/futex/requeue.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) --- a/kernel/futex/requeue.c +++ b/kernel/futex/requeue.c @@ -154,8 +154,16 @@ static inline void futex_requeue_pi_comp } 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 }