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 C5FE32FDC5E; Tue, 31 Mar 2026 17:06:26 +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=1774976786; cv=none; b=tsubA6yvBduDHkbCauohe/bDyoF/2yMnA0mN3bFWjMNeqTt7QzXLhxo4HfCmGtuWljAkkB0eo9DqInM8uQWp4BSGeFzeTF/vJhp8I09i9v7EO6BA0VSq3zYPCsAm0l3v4r9V62FOyfLqwS5SHbWnwsdaLmTlmZR3L2SRvitx5o0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774976786; c=relaxed/simple; bh=hDQXUPIrOlxifmm+H1G4SIJqOPFlUE1V9zezLy71GTg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=kPoER0/1r4v+cUe1X5tw/ytZw+FsT6J6QRIcefw1tSlH+2ryeAMyDf9FvhnE5fbLmSGfRzmB5voyMf2BZUttow/1YldYyGO7OSkv7obNYD5d3oh4MNyCtHbBuCzecvICWL+svsVm+YavoLoV5hSpJ+xhs4kPnkpnOqy1qLuC2cs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=zncMszMM; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="zncMszMM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5A8D0C19423; Tue, 31 Mar 2026 17:06:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774976786; bh=hDQXUPIrOlxifmm+H1G4SIJqOPFlUE1V9zezLy71GTg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=zncMszMMDsbSh36T7I27PK9EpVnjvg+lrp6+F/KhQOie7VTHgHfnR4d2GkM04v1qy F+acjqqDLIkldBlU0oC/fPVA+PVPAh37bSvtfy8eP1mwTjo1k3tnEVx8X40Mfo+6XP YCqucXAmDwDdCefySmB94I2wrvM+hTmN1+arO1i8= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Davidlohr Bueso , Thomas Gleixner Subject: [PATCH 6.18 227/309] futex: Clear stale exiting pointer in futex_lock_pi() retry path Date: Tue, 31 Mar 2026 18:22:10 +0200 Message-ID: <20260331161801.802407247@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260331161753.468533260@linuxfoundation.org> References: <20260331161753.468533260@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ 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 Signed-off-by: Greg Kroah-Hartman --- kernel/futex/pi.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/kernel/futex/pi.c +++ b/kernel/futex/pi.c @@ -918,7 +918,7 @@ int fixup_pi_owner(u32 __user *uaddr, st int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock) { struct hrtimer_sleeper timeout, *to; - struct task_struct *exiting = NULL; + struct task_struct *exiting; struct rt_mutex_waiter rt_waiter; struct futex_q q = futex_q_init; DEFINE_WAKE_Q(wake_q); @@ -933,6 +933,7 @@ int futex_lock_pi(u32 __user *uaddr, uns to = futex_setup_timer(time, &timeout, flags, 0); retry: + exiting = NULL; ret = get_futex_key(uaddr, flags, &q.key, FUTEX_WRITE); if (unlikely(ret != 0)) goto out;