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 A462835F19A for ; Wed, 22 Jul 2026 16:15:07 +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=1784736908; cv=none; b=GI+tNzEbXwMqbB6rYhfByBjK7b/vjHF7AqGuWG2LdKn7u7dMki+cot06cUrWG6rYch8odT++9Y3z5R/6qeDYddogGzd9b3o4EdoWksaaRLIHAoQER5zdUDGS330Ujw8T0C6MtxpNiGDVyrOPvQQxkoBPVn96YE91Zz93c9Dbmq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784736908; c=relaxed/simple; bh=JbD34QVVjz9RnrLVLEaBjJenwz01tFHsDCyVnBgUV6I=; h=Date:Message-ID:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=i+nAq+82ELDq0XPdMQXCjUF6vASc0Mn17aKRZcljWNebCvIYbwwnq/FGuDqsbjPn3NGd1eWqEDQA0kWm2m2UFwO/6t/m7ijKccnlrh3HGHvFU3FIQudUc+Um64qBUpDo7FldZSL73e2k1znkwO3bABykpsHc9UKH3UNjVzCp9vA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VIR7tXL2; 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="VIR7tXL2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8173A1F000E9; Wed, 22 Jul 2026 16:15:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784736907; bh=dseg9bglq/UiPptAnUgSyAnxnYa/49PE8CXkEWpjiVU=; h=Date:From:To:Cc:Subject:References; b=VIR7tXL2uUCKFP0uO2EinIHIXjMpdGxh74BuCRhZMr21SaJm/Eq/b6XFQ7w2wRfDN 8g6dSYsFpf+Ct2Wnnmmn2gqEkn5DI5m3C/ARaqyBYMULi5pHot5ygkdIdhpXnbC/fx zrz+pWOuj0wgRhLE+OPQlPJqbAnlJXlKxTxNJ7fQ0G7lNi5wL7WvPZ/QQeQlVP0H61 npBUxU/UNSFUNLc6dQNTdiBatIVSRuVYwHKew+I27deHM8SaqnIKEO9jwUUN3ps0Xr TNCRAp7Sg3ztjo/XvJzTm28x19e0qy+EYxlchKkT5BrRI1Xyc2W6mCh2lyasrSbYYd z4jBs1Ezorz+w== Date: Wed, 22 Jul 2026 18:15:04 +0200 Message-ID: <20260722155239.024902788@kernel.org> User-Agent: quilt/0.69 From: Thomas Gleixner To: stable@vger.kernel.org Cc: tglx@kernel.org, syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com, Davidlohr Bueso Subject: [patch 5.15.y 2/2] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued References: <2026072141-ravage-display-0696@gregkh> <20260722155049.973195593@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 From: Davidlohr Bueso commit 40a25d59e85b3c8709ac2424d44f65610467871e upstream. 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 --- kernel/locking/rtmutex.c | 3 +++ kernel/locking/rtmutex_api.c | 2 +- 2 files changed, 4 insertions(+), 1 deletion(-) --- --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1513,6 +1513,9 @@ static void __sched remove_waiter(struct 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; --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -322,7 +322,7 @@ int __sched rt_mutex_start_proxy_lock(st 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);