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 C78473F1AB2 for ; Wed, 17 Jun 2026 12:46:10 +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=1781700371; cv=none; b=blnp2uJ7mwdG7M/oZApE185ovamMIb+o4vdoNl/fgiS88HkSyn86LoZB1xFyF8czlmC07QPrAjjQD8CW/NKn7YXGhSkzIG3d3A8t97BpLjodV+Iomdegwx4q7jScdyrw6VV5tu+8Vhzvr83l8smOnevsDElK+z7EraAEV75Srog= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781700371; c=relaxed/simple; bh=w/sB2l7gXJTakhJ0wWIIgmulvqcW/BvVtvS64eq14H8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=uZiL4fCpSh9UlirDeyV/zZYQuhRLVdbv+4SZekd1DCSqmjo3RPnoBzGd778l7u1Nmiwuv3liPiqPwTNQD8ESf+uL2QPMkiGIn3gRmoKK/FlaGaYYMfXkDm2AeaTpBivBy5NxwB6mu7rTPT3svNdiBBWcYAvMgRVWb+rkNpnQzpo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G4e9fO1c; 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="G4e9fO1c" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 259821F00A3E; Wed, 17 Jun 2026 12:46:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781700370; bh=UyL3MLqsl1QwRb+An2uZnxrWd4KDwtYihrrKlA6XsOI=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=G4e9fO1c4V2I6svSC8YFfZuUqxSNVbsQtm9Ga8Qz6nKmIgUT2qExtw4hd/j/6KZgo ULJQLN2arKBbxkFqD9AOWcWCeODRcvdN3+gWEDStTY0cD2SXY+DZJnIAEoqdzWRI68 4YFD8wdVvpTjx+XzdS7VXUFZyD+HsYtExLjNHp6B+23xBeodCdnQm5AAUKePEnHuwE F+UDgGhwWKJw1qMsJcGuyS3LkuUC3/SSYXglsmL5U/3pCrExCC4IwCev262xCkVYuF uR87lNT0FjW6Hv588rDJXk5EXoxUjtCHXAT2shrgLZ5QiueNsa5u5R3hd1h43sXiXQ 3h6zHO++ZSpRg== From: Sasha Levin To: stable@vger.kernel.org Cc: Davidlohr Bueso , syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com, Thomas Gleixner , Sasha Levin Subject: [PATCH 6.1.y] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued Date: Wed, 17 Jun 2026 08:46:07 -0400 Message-ID: <20260617124607.3857087-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026061518-quilt-plaything-413e@gregkh> References: <2026061518-quilt-plaything-413e@gregkh> Precedence: bulk X-Mailing-List: stable@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 --- 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 def1f8fbc85cda..6df2f674712a47 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1524,6 +1524,9 @@ static void __sched remove_waiter(struct rt_mutex_base *lock, lockdep_assert_held(&lock->wait_lock); + if (!waiter_task) /* never enqueued */ + return; + scoped_guard(raw_spinlock, &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 a6974d04459301..06c3db9f6cf1c5 100644 --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -344,7 +344,7 @@ 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