writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

Summary

In the Linux kernel, the following vulnerability has been resolved: writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() 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.

Timeline

1. Jul 25, 2026, 8:50 AM UTCCVE published
2. Oct 16, 2026, 3:39 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Oct 19, 2026, 8:38 PM UTCRecord last updated

Affected versions

• Linux Linux

  From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in 087d5b8b501c570f84bf655164e6698c3ce146e0; From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in 3c9c9648f77e4d14e50676bc51c2174ba9c8d361; From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in 5c3265f3252b2ee50707adaaa3f9bd0df3df72de; From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in c923cc3cb5cd8945ceaf08252754110643446593; From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in 685fc15a410885b6d4dee64de0dce721b9428b12; From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in 53eeaf4d63068dbc7708b0c7adb20151c812feca; From a1a0e23e49037c23ea84bc8cc146a03584d13577 · fixed in cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d; From 4.4.5 · fixed in 4.5

• Linux Linux

  4.5

• KernelLinux

  From 4.5.0 · fixed in 5.10.261; From 5.11.0 · fixed in 6.1.178; From 6.2.0 · fixed in 6.6.145; From 6.7.0 · fixed in 6.12.96; From 6.13.0 · fixed in 6.18.39; From 6.19.0 · fixed in 7.1.4

References 7