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 BAD2343849F; Tue, 21 Jul 2026 07:55:14 +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=1784620515; cv=none; b=pG58AVBQyr3fYAN7UHJ5Vq3x0N6oGN9YDn64+Irlrq8egg0j5OVAxato5lq9iqBq4llIFrk8lOjo/fd0bOUcd0vqC/R8EsfPcQEaEaInOJBwyVQpEFHBPZQSJrlxGRdtKs0jDru4igSBVbibSQiwBU/jo2/lZFE1tkkNuSc3R9Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784620515; c=relaxed/simple; bh=aXk5FmK+bvqFiumDZjWwP0Ma45DgCw3NdhhJM01AFAg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Ry2rkOTVG9zmy10rKbJW1IOV6VbslZ2NOgjy8brdGhudl5SXOIphD2Oj3mKaGEvXsTBfsemIKJJTTfPipBW3gXhfZfoYWF3mnwb8vMtMfwzitdrdoScbPyEWks8JV6TOeccIG7a+L0zVRm9AUnadzUCK5JQu+C9SdJk3tCipwyI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IEcMi55r; 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="IEcMi55r" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7CABB1F00A3A; Tue, 21 Jul 2026 07:55:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784620514; bh=BVfgm0xC115uBaQmqio50UdqDv74qrMywulQ/rq6+/8=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=IEcMi55ro+KVNzsTRji/XC3zEGUW7CEqrE5H0FOwIsSFjKovTNxeVS3Cvrk9YRMeR COSfUr/wdrHRfpgmR111FNpm1VX+HH9LpVwPM2ZNuJaXXuzmH52xQxH3WsJoOpRAKn VObEBFO3/cGNIlZviP7Uvt8ItFYnXPN7MtDno9qHtiSWtTv6oR8DjRyFuWtOhejrwx +JCwUuXSGvucUP0SVdNLwUTi81GVF+DQutgwaYQSfAiTy0kwe+0EWqg2e7tZT5QRyj UCjmHSzHvg7ocBhQ4SDQlgepFj4DkGgOrMU6EFmYGfSnw5JRMfcOqeZlAW4enCH3Go QnSWQd9CampAw== From: Lee Jones To: lee@kernel.org, Peter Zijlstra , Ingo Molnar , Will Deacon , Steven Rostedt , Lai Jiangshan , Keenan Dong , Thomas Gleixner , linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org, Davidlohr Bueso , syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com, Sasha Levin , Greg Kroah-Hartman Subject: [STABLE v5.10 2/2] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued Date: Tue, 21 Jul 2026 08:54:54 +0100 Message-ID: <20260721075459.1970132-2-lee@kernel.org> X-Mailer: git-send-email 2.55.0.229.g6434b31f56-goog In-Reply-To: <20260721075459.1970132-1-lee@kernel.org> References: <20260721075459.1970132-1-lee@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Davidlohr Bueso [ Upstream commit 40a25d59e85b3c8709ac2424d44f65610467871e ] syzbot triggered the following splat in remove_waiter() via FUTEX_CMP_REQUEUE_PI: KASAN: null-ptr-deref in range [0x0000000000000a88-0x0000000000000a8f] class_raw_spinlock_constructor remove_waiter+0x159/0x1200 kernel/locking/rtmutex.c:1561 rt_mutex_start_proxy_lock+0x103/0x120 futex_requeue+0x10e4/0x20d0 __x64_sys_futex+0x34f/0x4d0 task_blocks_on_rt_mutex() does not arm the waiter upon deadlock detection, leaving waiter->task nil, where 3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()") made this fatal. Furthermore, rt_mutex_start_proxy_lock() should not be calling into remove_waiter() upon a successfully grabbing the rtmutex. 1a1fb985f2e2 ("futex: Handle early deadlock return correctly"), moved the remove_waiter() out of __rt_mutex_start_proxy_lock() (where 'ret' was only ever 0 or < 0) into the wrapper. Tighten this check to account for try_to_take_rt_mutex(). Fixes: 3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()") Reported-by: syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Closes: https://lore.kernel.org/all/69f114ac.050a0220.ac8b.0003.GAE@google.com/ Link: https://patch.msgid.link/20260507112913.1019537-1-dave@stgolabs.net Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman (cherry picked from commit 4afda3a1da02129568a3a2f1898aa13e6763bcba) [Lee: Backported to linux-5.10.y from linux-6.1.y] Signed-off-by: Lee Jones --- kernel/locking/rtmutex.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 2ab2d6cce004..9cfe7f0aa04e 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1074,6 +1074,9 @@ static void remove_waiter(struct rt_mutex *lock, lockdep_assert_held(&lock->wait_lock); + if (!waiter_task) /* never enqueued */ + return; + raw_spin_lock(&waiter_task->pi_lock); rt_mutex_dequeue(lock, waiter); waiter_task->pi_blocked_on = NULL; @@ -1803,7 +1806,7 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock, raw_spin_lock_irq(&lock->wait_lock); ret = __rt_mutex_start_proxy_lock(lock, waiter, task); - if (unlikely(ret)) + if (unlikely(ret < 0)) remove_waiter(lock, waiter); raw_spin_unlock_irq(&lock->wait_lock); -- 2.55.0.229.g6434b31f56-goog