GHSA-325C-M2Q7-75HG
Vulnerability from github – Published: 2026-07-25 12:31 – Updated: 2026-07-27 06:30In 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.
{
"affected": [],
"aliases": [
"CVE-2026-64378"
],
"database_specific": {
"cwe_ids": [
"CWE-362"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-07-25T10:17:20Z",
"severity": "HIGH"
},
"details": "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(\u0026isw_nr_in_flight)\n inode_prepare_wbs_switch\n -\u003e passes SB_ACTIVE check\n __iget(inode)\n generic_shutdown_super\n sb-\u003es_flags \u0026= ~SB_ACTIVE\n cgroup_writeback_umount(sb)\n smp_mb()\n atomic_read(\u0026isw_nr_in_flight)\n rcu_barrier()\n -\u003e no pending RCU callbacks\n flush_workqueue(isw_wq)\n -\u003e nothing queued, returns\n evict_inodes(sb)\n -\u003e Inode skipped as isw still holds a ref.\n sop-\u003eput_super(sb)\n /* destroys percpu counters */\n -\u003e 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() -\u003e 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-\u003elist_lock when switching inodes\")) and is removed in a follow-up\npatch.",
"id": "GHSA-325c-m2q7-75hg",
"modified": "2026-07-27T06:30:34Z",
"published": "2026-07-25T12:31:33Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-64378"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/087d5b8b501c570f84bf655164e6698c3ce146e0"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3c9c9648f77e4d14e50676bc51c2174ba9c8d361"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/53eeaf4d63068dbc7708b0c7adb20151c812feca"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/5c3265f3252b2ee50707adaaa3f9bd0df3df72de"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/685fc15a410885b6d4dee64de0dce721b9428b12"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/c923cc3cb5cd8945ceaf08252754110643446593"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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