{"id":"CVE-2026-64378","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-64378","generatedAt":"2026-10-28T02:11:56.815Z","title":"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:…","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nwriteback: fix race between cgroup_writeback_umount() and inode_switch_wbs()\n\nWhen a container exits, the following BUG_ON() is occasionally triggered:\n\n==================================================================\n VFS: Busy inodes after unmount of sdb (ext4)\n ------------[ cut here ]------------\n kernel BUG at fs/super.c:695!\n CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1\n pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)\n pc : generic_shutdown_super+0xf0/0x100\n lr : generic_shutdown_super+0xf0/0x100\n Call trace:\n  generic_shutdown_super+0xf0/0x100\n  kill_block_super+0x20/0x48\n  ext4_kill_sb+0x28/0x60\n  deactivate_locked_super+0x54/0x130\n  deactivate_super+0x84/0xa0\n  cleanup_mnt+0xa4/0x140\n  __cleanup_mnt+0x18/0x28\n  task_work_run+0x78/0xe0\n  do_notify_resume+0x204/0x240\n==================================================================\n\nThe root cause is a race between cgroup_writeback_umount() and\ninode_switch_wbs()/cleanup_offline_cgwb(). There is a window between\ninode_prepare_wbs_switch() returning true and the subsequent\nwb_queue_isw() call. Following is the process that triggers the issue:\n\n      CPU A (umount)           |          CPU B (writeback)\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n                                 inode_switch_wbs/cleanup_offline_cgwb\n                                  atomic_inc(&isw_nr_in_flight)\n                                  inode_prepare_wbs_switch\n                                   -> passes SB_ACTIVE check\n                                   __iget(inode)\n generic_shutdown_super\n  sb->s_flags &= ~SB_ACTIVE\n  cgroup_writeback_umount(sb)\n   smp_mb()\n   atomic_read(&isw_nr_in_flight)\n   rcu_barrier()\n    -> no pending RCU callbacks\n   flush_workqueue(isw_wq)\n    -> nothing queued, returns\n  evict_inodes(sb)\n   -> Inode skipped as isw still holds a ref.\n  sop->put_super(sb)\n   /* destroys percpu counters */\n  -> VFS: Busy inodes after unmount!\n                                  wb_queue_isw()\n                                   queue_work(isw_wq, ...)\n                                  /* later in work function */\n                                  inode_switch_wbs_work_fn\n                                   process_inode_switch_wbs\n                                    iput() -> evict\n                                     percpu_counter_dec() // UAF!\n\nFix this by extending the RCU read-side critical section in\ninode_switch_wbs() and cleanup_offline_cgwb() to cover from\ninode_prepare_wbs_switch() through wb_queue_isw().  Since there is\nno sleep in this window, rcu_read_lock() can be used.  Then add a\nsynchronize_rcu() in cgroup_writeback_umount() before the existing\nrcu_barrier(), so that all in-flight switchers that have passed the\nSB_ACTIVE check have completed queue_work() before flush_workqueue()\nis called.\n\nThe existing rcu_barrier() is intentionally retained so this fix can\nbe backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that\nstill queue switches via queue_rcu_work(). It is a no-op on current\nmainline (since commit e1b849cfa6b6 (\"writeback: Avoid contention on\nwb->list_lock when switching inodes\")) and is removed in a follow-up\npatch.","published":"2026-07-25T10:17:21.000Z","lastModified":"2026-10-19T21:17:34.000Z","status":"Modified","sourceIdentifier":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","cvss":{"version":"3.1","score":7.8,"severity":"HIGH","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"},"cwe":["CWE-362"],"affected":["linux/linux kernel"],"epss":{"score":0.001,"percentile":0.00846,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"high","text":"High impact if exploited, but EPSS currently rates exploitation as unlikely (0.10%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit."},"changes":[],"references":[{"url":"https://git.kernel.org/stable/c/087d5b8b501c570f84bf655164e6698c3ce146e0","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/3c9c9648f77e4d14e50676bc51c2174ba9c8d361","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/53eeaf4d63068dbc7708b0c7adb20151c812feca","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/5c3265f3252b2ee50707adaaa3f9bd0df3df72de","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/685fc15a410885b6d4dee64de0dce721b9428b12","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/c923cc3cb5cd8945ceaf08252754110643446593","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-64378","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}