From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout01.his.huawei.com (canpmsgout01.his.huawei.com [113.46.200.216]) (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 E6E4078F2E for ; Mon, 20 Jul 2026 02:51:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.216 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784515870; cv=none; b=jE7pE9zsdmW3KpvuM1H3wcvf/0cesMsNgCwNpXzJif+H3JV6QFyBbaFS1Z84xMTQAgEYlOV8dYrWEhLeNZzSOibZsFNIZrajT1223pRrSA7STK/er2jgHdXyDI4fSXd11LZO5Ff7akZ/XiFKq+kLyzizkmNp9JO3XhY8lbh8aLQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784515870; c=relaxed/simple; bh=U9H7hs9Vd5f7q4kTkcWoCNdGjPfXXqT/kE3J8atSGzs=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=Obbij5k7RiOc+KpzGUyonv/k4LJFNs9s9G4+R/xFdVYc6kkPFQMQb4uhF6VXSepA45MX3V28p+mhm6F8RTQoUeU7sX0b+KBVs6tbYOt5BqaV0VjqUiE48PnWgKH8/lZJ3QhP3R3UFsmJ375gw5fP2odK5gjlyBLkqNtHcXf8pqo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=Es3+m4jc; arc=none smtp.client-ip=113.46.200.216 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="Es3+m4jc" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=28AQbmdzD1QdeBmDn3aQuznfetit+ippHiAocTrvD5k=; b=Es3+m4jcEcINOsFznTVkbWIErK2LKQNF8R8mIP6lE31tS+6Kk9RPaDX+Z2MgAGcu/T1FKHMVH UeQIF+6S0DaDHrvk4CsQ2ngxmQDtWFriXDYJpOVfnJFltyLhRGDAaTSQU5UWYpvcV8Nc/yn/3Mj lURYKQm2qIeY7SUjALRFNdU= Received: from mail.maildlp.com (unknown [172.19.162.144]) by canpmsgout01.his.huawei.com (SkyGuard) with ESMTPS id 4h3Psv2WD2z1T4Kf; Mon, 20 Jul 2026 10:41:39 +0800 (CST) Received: from dggpemr500006.china.huawei.com (unknown [7.185.36.185]) by mail.maildlp.com (Postfix) with ESMTPS id AF84440538; Mon, 20 Jul 2026 10:50:56 +0800 (CST) Received: from [100.103.109.15] (100.103.109.15) by dggpemr500006.china.huawei.com (7.185.36.185) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Mon, 20 Jul 2026 10:50:55 +0800 Message-ID: <235279a2-e6e4-4b6a-b76b-8a972a3e2af8@huawei.com> Date: Mon, 20 Jul 2026 10:50:55 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] futex/requeue: Prevent rcuwait use-after-free during requeue PI To: Sebastian Andrzej Siewior CC: , , , , , , , References: <20260717084922.4153317-1-yaokai34@huawei.com> <20260717084922.4153317-3-yaokai34@huawei.com> <20260717093829.K07Gk1tS@linutronix.de> Content-Language: en-US From: Yao Kai In-Reply-To: <20260717093829.K07Gk1tS@linutronix.de> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: kwepems100002.china.huawei.com (7.221.188.206) To dggpemr500006.china.huawei.com (7.185.36.185) On 7/17/2026 5:38 PM, Sebastian Andrzej Siewior wrote: > On 2026-07-17 16:49:22 [+0800], Yao Kai wrote: >> On PREEMPT_RT, FUTEX_CMP_REQUEUE_PI can trigger a KASAN report: >> >> BUG: KASAN: slab-out-of-bounds in _raw_spin_lock_irqsave+0x76/0xe0 >> Call Trace: >> _raw_spin_lock_irqsave+0x76/0xe0 >> try_to_wake_up+0xab/0x1540 >> rcuwait_wake_up+0x39/0x60 >> futex_requeue+0x18c3/0x1e10 >> >> 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() >> >> * timeout/signal wakes waiter * >> >> futex_requeue_pi_wakeup_sync() >> IN_PROGRESS -> WAIT >> rcuwait_wait_event() >> requeue_pi_wake_futex() >> task = READ_ONCE(q->task) >> futex_requeue_pi_complete() >> WAIT -> LOCKED >> return LOCKED >> return >> // q lifetime ends >> rcuwait_wake_up() >> >> futex_requeue_pi_complete() publishes LOCKED before calling >> rcuwait_wake_up(). Once the waiter observes LOCKED, it can return from >> futex_wait_requeue_pi() and let q go out of scope before rcuwait_wake_up() >> reads q->requeue_wait.task and passes the stale pointer to >> try_to_wake_up(). >> >> Skip rcuwait_wake_up() for Q_REQUEUE_PI_LOCKED. requeue_pi_wake_futex() >> already saves q->task before publishing LOCKED and wakes the saved task >> afterward. >> >> Fixes: 07d91ef510fb1 ("futex: Prevent requeue_pi() lock nesting issue on RT") >> Cc: stable@vger.kernel.org >> Signed-off-by: Yao Kai >> --- >> kernel/futex/requeue.c | 9 +++++++-- >> 1 file changed, 7 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c >> index abc652b5b2dd..59e587775d9b 100644 >> --- a/kernel/futex/requeue.c >> +++ b/kernel/futex/requeue.c >> @@ -155,8 +155,13 @@ 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)) >> + /* >> + * If the waiter interleaved with the requeue, let it know. For LOCKED, >> + * q may already be invalid, so requeue_pi_wake_futex() wakes the saved >> + * task instead. >> + */ >> + if (unlikely(old == Q_REQUEUE_PI_WAIT) && >> + new != Q_REQUEUE_PI_LOCKED) >> rcuwait_wake_up(&q->requeue_wait); > > Your whole assumption is based on the requeue_state in > futex_requeue_pi_wakeup_sync() changes from Q_REQUEUE_PI_IN_PROGRESS to > Q_REQUEUE_PI_WAIT and the rcuwait_wait_event() does not wait because the > condition becomes true before that happens. So the rcuwait_wake_up() > could access q.requeue_wait which is allocated on behalf of the waiter > which is gone. Certainly possible. But if we skip the wait in thise case > we probably miss the 99% cases where the waiter did wait, no? > > This looks similar to commit b549113738e8c ("futex: Prevent > use-after-free during requeue-PI"). > >> #endif >> } > > Sebastian No wakeup is missed. Q_REQUEUE_PI_LOCKED is only published by requeue_pi_wake_futex(), which saves q->task before publishing the state and calls wake_up_state(task, TASK_NORMAL) afterwards. The other futex_requeue_pi_complete() callers produce DONE or an error state, and the rcuwait wakeup is retained for those paths. Thanks, Yao