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 1CC0A4A091E for ; Thu, 10 Sep 2026 14:06:23 +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=1789049184; cv=none; b=VbkAHho/eWOVuPu7CB3f9m0SA6v8erhDP+HQ2NrBVMMam9Lv7IfWzhjdJt5+DC5uMIr0f/57rK7kxIoNX5wbWT+S/6LYRIDwUo3tSUtbZHXb58ZDpEs0hyhn/rI08Q6lwQbYGz8FsCUkL8WlA4novYszhv3U6vx+w/pmJlOd6Kk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789049184; c=relaxed/simple; bh=1lIT8qI+W4dDH3RWncPxu9QAMG/B9sx91f1OmWdQrhs=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ccRzYFhL+Ct30rEdmPQ+fKlLWeb5yUb2p8w4qi4/wruxKYfLI/GSChALZkkLWT1FXt6akMkU5l/dJYc6gaupiOQSXIGkGMKbyQkW6m0rEH54ZNaQTVijmIYh8NJNsv1Y9eXmreVosq0XWhsxW8MUQkRn6BPWJo3Z9GA36eAP/8M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RbdmIxH6; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="RbdmIxH6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 713041F00898; Thu, 10 Sep 2026 14:06:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789049183; bh=EmOw6tY9y9TQ3heCQ2Z3pMxGJB5bJX5MtNz+N3i70J8=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=RbdmIxH6lB3/ZEY1NWRkDbo2hClLoaxuUknjZsBjzOC08s9jvzqf4Ri8YMk81Wv5H v/URT8xD9+rVc0d9ADVTNa0s9v/GDKcOjAwmXT5nNnvpFGiVXbO+9pZ8CXdsLsRh6T QkbklADE9Z2lcxFr0chQd+UWRJ1TOSrXcCyh5XDqutfnjjcTZ0T0WQCb4MrVzt9nsv tb+VSNQaC9RLo2b3dblP/rKxolRe/XLGLIaqep7GZId2hrYnTSJE6pVBoDcfLs1fgz ScW/iJpeT8EPi8+CzzGIJLPNuzFIaOWvSvj4srGCGx1KfqZ0wup77JEkkP4B4qMacW 84brR8XvaVlUA== From: Sasha Levin To: stable@vger.kernel.org Cc: Yao Kai , Sebastian Andrzej Siewior , Thomas Gleixner , Sasha Levin Subject: [PATCH 5.15.y 6/6] futex: Prevent rcuwait use-after-free during requeue PI Date: Thu, 10 Sep 2026 10:06:17 -0400 Message-ID: <20260910140617.564852-6-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260910140617.564852-1-sashal@kernel.org> References: <2026090826-jeep-tassel-64cd@gregkh> <20260910140617.564852-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Yao Kai [ Upstream commit a3b8d46fe401cba3a5c46dea610e6eb3dc15370e ] 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: Sasha Levin --- kernel/futex/requeue.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c index 0378f603fb04f..6dac1abfed902 100644 --- a/kernel/futex/requeue.c +++ b/kernel/futex/requeue.c @@ -148,8 +148,16 @@ 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 } -- 2.53.0