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 C94483B8412; Tue, 21 Jul 2026 21:50:37 +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=1784670639; cv=none; b=CWWHrYxXRH/UEtU/Cjfes4rh6bGg0fcxRy9gtxnmJWaYrY/flv8968cRNsCilqtrTZLBTBiaSdSjpjXtlpJYJSX59QriKgo9ld3V62ZgNx5xLNck9kkgm7ciY8jsw1kF1CE4anyQ3cvmhICYH2HYZDORjlDlKiIlcIuNe5+f360= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784670639; c=relaxed/simple; bh=oUJCJKNpxKxOb6EPfTzSL+KSsNtpjQNKDXV6ZUv3jFs=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MMnVjvlv4K51is5qKK7sbQaLCvHfN6RCbXFEWpFSgpZkiIbLNlqu8t4NVVhdUZsgRsGXVFrJM3ILPgivsgY31o1HE0FFOaQxLM1dibQEKfTQo0ZX/7MLFYkM/1JEhIMo2O+QlmLhRliJUU7V5ujUqBq/WNdELti94Q+U8LUtTRM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=gHU/M180; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="gHU/M180" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D54341F00A3D; Tue, 21 Jul 2026 21:50:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784670637; bh=hiGNV5pNFgc3QlcD8/BHGysN6RDvFgxYb4+AavvlgGQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=gHU/M180WSau6rutWdmAgIRuNJV2m6iV2XwAxe7iZoCvu/GGdKwJZSocdHpHGMy87 t3xywtu1U/O8eE9iPh7jpM2830xIaU0B3b/FlDK4MZ5eRaNZ6h+Wl7VQSk2RzzKA+J kH8x4t32SJ0tajdq0qwOY8TlzqS5WDVTaQaXld1g= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jan Kara , Baokun Li , Tejun Heo , "Christian Brauner (Amutable)" , Sasha Levin Subject: [PATCH 6.1 1007/1067] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() Date: Tue, 21 Jul 2026 17:26:48 +0200 Message-ID: <20260721152447.097641972@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260721152424.521567757@linuxfoundation.org> References: <20260721152424.521567757@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Baokun Li [ Upstream commit cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d ] When a container exits, the following BUG_ON() is occasionally triggered: ================================================================== VFS: Busy inodes after unmount of sdb (ext4) ------------[ cut here ]------------ kernel BUG at fs/super.c:695! CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1 pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--) pc : generic_shutdown_super+0xf0/0x100 lr : generic_shutdown_super+0xf0/0x100 Call trace: generic_shutdown_super+0xf0/0x100 kill_block_super+0x20/0x48 ext4_kill_sb+0x28/0x60 deactivate_locked_super+0x54/0x130 deactivate_super+0x84/0xa0 cleanup_mnt+0xa4/0x140 __cleanup_mnt+0x18/0x28 task_work_run+0x78/0xe0 do_notify_resume+0x204/0x240 ================================================================== The root cause is a race between cgroup_writeback_umount() and inode_switch_wbs()/cleanup_offline_cgwb(). There is a window between inode_prepare_wbs_switch() returning true and the subsequent wb_queue_isw() call. Following is the process that triggers the issue: CPU A (umount) | CPU B (writeback) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ inode_switch_wbs/cleanup_offline_cgwb atomic_inc(&isw_nr_in_flight) inode_prepare_wbs_switch -> passes SB_ACTIVE check __iget(inode) generic_shutdown_super sb->s_flags &= ~SB_ACTIVE cgroup_writeback_umount(sb) smp_mb() atomic_read(&isw_nr_in_flight) rcu_barrier() -> no pending RCU callbacks flush_workqueue(isw_wq) -> nothing queued, returns evict_inodes(sb) -> Inode skipped as isw still holds a ref. sop->put_super(sb) /* destroys percpu counters */ -> VFS: Busy inodes after unmount! wb_queue_isw() queue_work(isw_wq, ...) /* later in work function */ inode_switch_wbs_work_fn process_inode_switch_wbs iput() -> evict percpu_counter_dec() // UAF! Fix this by extending the RCU read-side critical section in inode_switch_wbs() and cleanup_offline_cgwb() to cover from inode_prepare_wbs_switch() through wb_queue_isw(). Since there is no sleep in this window, rcu_read_lock() can be used. Then add a synchronize_rcu() in cgroup_writeback_umount() before the existing rcu_barrier(), so that all in-flight switchers that have passed the SB_ACTIVE check have completed queue_work() before flush_workqueue() is called. The existing rcu_barrier() is intentionally retained so this fix can be backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that still queue switches via queue_rcu_work(). It is a no-op on current mainline (since commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")) and is removed in a follow-up patch. Fixes: a1a0e23e4903 ("writeback: flush inode cgroup wb switches instead of pinning super_block") Cc: stable@vger.kernel.org Suggested-by: Jan Kara Link: https://lore.kernel.org/all/mxnjq2l6guusfchvauxr3v7c4bwjasybxlleqbbh4efloeqspz@iqylk76ohufz Reviewed-by: Jan Kara Signed-off-by: Baokun Li Link: https://patch.msgid.link/20260521095016.2791354-2-libaokun@linux.alibaba.com Acked-by: Tejun Heo Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- fs/fs-writeback.c | 31 +++++++++++++++++++++++++++++-- 1 file changed, 29 insertions(+), 2 deletions(-) --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -623,12 +623,19 @@ static void inode_switch_wbs(struct inod atomic_inc(&isw_nr_in_flight); - /* find and pin the new wb */ + /* + * Paired with synchronize_rcu() in cgroup_writeback_umount(): + * holding rcu_read_lock across inode_prepare_wbs_switch() + * (covering the SB_ACTIVE check and the inode grab) and + * wb_queue_isw() ensures synchronize_rcu() cannot return until + * the work is queued, so the subsequent flush_workqueue() will + * wait for the switch. + */ rcu_read_lock(); + /* find and pin the new wb */ memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys); if (memcg_css && !css_tryget(memcg_css)) memcg_css = NULL; - rcu_read_unlock(); if (!memcg_css) goto out_free; @@ -643,9 +650,11 @@ static void inode_switch_wbs(struct inod isw->inodes[0] = inode; wb_queue_isw(new_wb, isw); + rcu_read_unlock(); return; out_free: + rcu_read_unlock(); atomic_dec(&isw_nr_in_flight); if (new_wb) wb_put(new_wb); @@ -704,6 +713,14 @@ bool cleanup_offline_cgwb(struct bdi_wri new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */ nr = 0; + /* + * Paired with synchronize_rcu() in cgroup_writeback_umount(). + * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab + * and wb_queue_isw() ensures synchronize_rcu() cannot return until + * the work is queued, so the subsequent flush_workqueue() will wait + * for the switch. + */ + rcu_read_lock(); spin_lock(&wb->list_lock); /* * In addition to the inodes that have completed writeback, also switch @@ -721,6 +738,7 @@ bool cleanup_offline_cgwb(struct bdi_wri /* no attached inodes? bail out */ if (nr == 0) { + rcu_read_unlock(); atomic_dec(&isw_nr_in_flight); wb_put(new_wb); kfree(isw); @@ -728,6 +746,7 @@ bool cleanup_offline_cgwb(struct bdi_wri } wb_queue_isw(new_wb, isw); + rcu_read_unlock(); return restart; } @@ -1159,6 +1178,14 @@ void cgroup_writeback_umount(void) if (atomic_read(&isw_nr_in_flight)) { /* + * Paired with rcu_read_lock() in inode_switch_wbs() and + * cleanup_offline_cgwb(). synchronize_rcu() waits for any + * in-flight switcher that already passed the SB_ACTIVE check + * to finish queueing its work, so flush_workqueue() below + * will then drain it. + */ + synchronize_rcu(); + /* * Use rcu_barrier() to wait for all pending callbacks to * ensure that all in-flight wb switches are in the workqueue. */