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 149913B9930; Tue, 21 Jul 2026 21:55:03 +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=1784670904; cv=none; b=UwMCbL/W2J8sxLChggO+lvDvgczr3ozoSYml+HjDslKIck084mJea6z5mP+mzZfQYIpl+6xfiwEYUWFPQY6qDJISKFxpvopY3EUT+Ckxk7CNkYFDVHQXEXYkE1RqEaBgmC1N+VBl+pp8gBGPG5sidPsKow2Nu534a/dycYg+z60= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784670904; c=relaxed/simple; bh=57XWhT1TtH6T0slNQbS6riCW2TUgoBo/0Ay1jp952Is=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=A4EneysOiU6wwZmsfdpv1wGF+4XjQsR8fjI1gztTKXSZD++JAoiy4TL7yF7ZYap7I22ci2M03FFx0Ykcs1uFlM44S7gi2N7ir2oOFBxzxOtkN+GhigZlGM6PE1Xh5ykRb4Mq0LfmFujsUUjO7upJD3EhWY7c6bOOvjFQ5nRQhrY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=AfqIaahk; 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="AfqIaahk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7A05B1F000E9; Tue, 21 Jul 2026 21:55:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784670903; bh=zZoQnkT3coWygGD0KG7j4vuQ20RofLG8ly/cyituF08=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=AfqIaahkRvvN4qV37G/KdGvGUqepAEZLSB7Qa8uDcsdLZ7BIOcdEBoi5eSLV2QM1l U1CPx1strmXy0UsG8vhnXdWVKKlEcxbrWIvk/EPIt+09NaNjp+wC3ZqIVcBiZQ5U3N yZRWYqpHqMcOzIrqOGiKBdK2ePS7VDuojKojwMgw= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com, Davidlohr Bueso , Thomas Gleixner , Sasha Levin Subject: [PATCH 5.15 045/843] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued Date: Tue, 21 Jul 2026 17:14:40 +0200 Message-ID: <20260721152406.993733332@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260721152405.946368001@linuxfoundation.org> References: <20260721152405.946368001@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 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ 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 --- kernel/locking/rtmutex.c | 3 +++ kernel/locking/rtmutex_api.c | 2 +- 2 files changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index f79c9286c7c0c1..2118f8928eb7cb 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1513,6 +1513,9 @@ static void __sched remove_waiter(struct rt_mutex_base *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; diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c index 56d1938cb52a1f..c4e191340c59b3 100644 --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -322,7 +322,7 @@ int __sched rt_mutex_start_proxy_lock(struct rt_mutex_base *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); -- 2.53.0