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 79BB6423177 for ; Tue, 31 Mar 2026 15:38:23 +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=1774971503; cv=none; b=FQW0Mf6vLIx0zVWe2DVPwq32KqO2fkwIceKKcXdkIbKTWk6cTihuAoVVirW5bxhJ7nTuOqxU/ZU/jC9T94GJ8Yw/EW8FDfSQcLhWZ4zMcakqyV8BYqS0RqyjrD1MyPhK4vfMLDZbz6hXyZN3Yu+D/6RmLZQMbd6WW4aIS2Rzim0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774971503; c=relaxed/simple; bh=b0wv4DNysW6+BvyRts/3Sc80xdRiFiIEm7VfKpDhZSI=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=D/3PPIMpqmiYlXB9aK+kcXoK8NEkHN+wHpltHI4Dc5HuK2oABp/z+TJbJntBYS3QCewx8t0+qAO2MvyMa/hIJBZw8zk6r6s9R1l6m8i+BGzsLl18j9G9A9MwHylGN5hes6WeJH2uS+28zTpLUSQnaoFQj1vM36R2LL8dAU9mBms= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=imzPdkbN; 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="imzPdkbN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 85E8CC19424; Tue, 31 Mar 2026 15:38:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774971503; bh=b0wv4DNysW6+BvyRts/3Sc80xdRiFiIEm7VfKpDhZSI=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=imzPdkbNhZSgrmLcgCrPg7mtsv80PLamIRz06KbOUByKq8fVP7/IVS0fiFiQz2gh7 vk8w4zH99MGLd4yI6aZbR2dkubBh4Jrs8QK7/LVMPK8Fl/spvtiK2kTvyBVyxYgr17 /Giju7p9HYhhhlTJ6da4qqWU3HNVHrlEJSOBghSjmYIfp0QP2fV2K50OGuRpHxE0+Q RcA+fIKSrzn8m6dOxsQMKwZtOF20a/mH4I7zEXndiun3+CSzsDxNc4xwmYnhKiwLj5 +9CRNGvgkho6iEgamB2dQfK3ruP9/69vUldqWduIp2Ik+G4JE4a1aJxujnujtnywaF 0N3QgKH6E+jDA== From: Thomas Gleixner To: gregkh@linuxfoundation.org, dave@stgolabs.net Cc: stable@vger.kernel.org Subject: [PATCH 5.15.y] futex: Clear stale exiting pointer in futex_lock_pi() retry In-Reply-To: <2026033034-city-docile-1900@gregkh> References: <2026033034-city-docile-1900@gregkh> Date: Tue, 31 Mar 2026 17:38:20 +0200 Message-ID: <871ph09fgz.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 @@ -3029,9 +3029,9 @@ 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; @@ -3044,6 +3044,7 @@ static int futex_lock_pi(u32 __user *uad to = futex_setup_timer(time, &timeout, flags, 0); retry: + exiting = NULL; ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE); if (unlikely(ret != 0)) goto out;