From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 6323D41B36B for ; Tue, 31 Mar 2026 15:39:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774971584; cv=none; b=rJBPBMc0wkmpEsBIwidQW2iBOfN7CzlPkW1k2pdy1HQpGWGm6zJBylC/MKYVIHFFLxWghzAEOzHVW6pwM7LlTHGBlcVzr/FMtShO1EYtDfOBiKAt6Q9nJJCKVjz4w6xWFNOFSLuf5eQ9fUUIfU5UCPf9/exTZQ9Cr5D6CG+3KSs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774971584; c=relaxed/simple; bh=osElbW/uT3dhIbCgERqN13ChlHueSZNPb9IiJjEccK4=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=o07vpHCQTSSHE1yEi3yUah+BFC4KTKQehhMDa3zl24UCO94290w3Nakntxpw9dOUKo7nW0kenBgTORAsaQk+9GoC6AuJFHm3/UBsyLPQtA4dIjfyIN/hSdQGpzgMdzAoGAvvQw77vkRNs8/O/p3em6wjp03Ev9efohS2+i/hQEY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LMS3f86P; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LMS3f86P" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 658A5C19423; Tue, 31 Mar 2026 15:39:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774971584; bh=osElbW/uT3dhIbCgERqN13ChlHueSZNPb9IiJjEccK4=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=LMS3f86PITCqrxHI9vMNZHUUYoDS3qxXa3yp385Ktbp5mkd6YVHt9FNnG/FAlVOrx Vb8PmkLpzcZLNjYf+xSzYKnXfp43zdAvBVcl455Eu8QqZ7crhD7nt1QCrsFhoC9LXm 9jiJ3C5LTPotXYZKsZMC93G50zVL3LDgBqCnDF1cGhVL/LDFHwpQjMelxLdeVq1BuM XA39I1ckI+EuS6eiWFuu4RPiBa8Mx+H0XIMEKq1sqq+pWHdqPOc2r98s13chna7pw6 qo4Dqej4ID9LSjmIUsmRZKkEPOUEUS6I8j0v9vxx11cWIPw0kxN/pMZLFlodIVqAQY KivvfamZ61jKA== From: Thomas Gleixner To: gregkh@linuxfoundation.org, dave@stgolabs.net Cc: stable@vger.kernel.org Subject: [PATCH 5.10.y] futex: Clear stale exiting pointer in futex_lock_pi() retry In-Reply-To: <2026033035-underrate-yogurt-49bf@gregkh> References: <2026033035-underrate-yogurt-49bf@gregkh> Date: Tue, 31 Mar 2026 17:39:40 +0200 Message-ID: <87wlys80ub.ffs@tglx> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain From: Davidlohr Bueso Commit 210d36d892de5195e6766c45519dfb1e65f3eb83 upstream. Fuzzying/stressing futexes triggered: WARNING: kernel/futex/core.c:825 at wait_for_owner_exiting+0x7a/0x80, CPU#11: futex_lock_pi_s/524 When futex_lock_pi_atomic() sees the owner is exiting, it returns -EBUSY and stores a refcounted task pointer in 'exiting'. After wait_for_owner_exiting() consumes that reference, the local pointer is never reset to nil. Upon a retry, if futex_lock_pi_atomic() returns a different error, the bogus pointer is passed to wait_for_owner_exiting(). CPU0 CPU1 CPU2 futex_lock_pi(uaddr) // acquires the PI futex exit() futex_cleanup_begin() futex_state = EXITING; futex_lock_pi(uaddr) futex_lock_pi_atomic() attach_to_pi_owner() // observes EXITING *exiting = owner; // takes ref return -EBUSY wait_for_owner_exiting(-EBUSY, owner) put_task_struct(); // drops ref // exiting still points to owner goto retry; futex_lock_pi_atomic() lock_pi_update_atomic() cmpxchg(uaddr) *uaddr ^= WAITERS // whatever // value changed return -EAGAIN; wait_for_owner_exiting(-EAGAIN, exiting) // stale WARN_ON_ONCE(exiting) Fix this by resetting upon retry, essentially aligning it with requeue_pi. Fixes: 3ef240eaff36 ("futex: Prevent exit livelock") Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260326001759.4129680-1-dave@stgolabs.net --- kernel/futex/core.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -2785,9 +2785,9 @@ static int futex_lock_pi(u32 __user *uad ktime_t *time, int trylock) { struct hrtimer_sleeper timeout, *to; - struct task_struct *exiting = NULL; struct rt_mutex_waiter rt_waiter; struct futex_hash_bucket *hb; + struct task_struct *exiting; struct futex_q q = futex_q_init; int res, ret; @@ -2800,6 +2800,7 @@ static int futex_lock_pi(u32 __user *uad to = futex_setup_timer(time, &timeout, FLAGS_CLOCKRT, 0); retry: + exiting = NULL; ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE); if (unlikely(ret != 0)) goto out;