Linux Kernel up to 7.1.3 Writeback fs/super.c inode_switch_wbs/cleanup_offline_cgwb s_flags use after free

CVSS Meta Temp Score
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CTI Interest Score
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7.8$0-$5k2.39

Summaryinfo

A vulnerability was found in Linux Kernel up to 7.1.3. It has been classified as very critical. This affects the function inode_switch_wbs/cleanup_offline_cgwb of the file fs/super.c of the component Writeback. The manipulation of the argument s_flags leads to use after free. This vulnerability is documented as CVE-2026-64378. The attack needs to be performed locally. There is not any exploit available.

Detailsinfo

A vulnerability was found in Linux Kernel up to 7.1.3 and classified as very critical. This issue affects the function inode_switch_wbs/cleanup_offline_cgwb of the file fs/super.c of the component Writeback. The manipulation of the argument s_flags with an unknown input leads to a use after free vulnerability. Using CWE to declare the problem leads to CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at git.kernel.org. The identification of this vulnerability is CVE-2026-64378 since 07/19/2026. The exploitation is known to be easy. Attacking locally is a requirement. The exploitation needs additional levels of successful authentication. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 07/25/2026).

Upgrading to version 5.10.261, 6.1.178, 6.6.145, 6.12.96, 6.18.39, 7.1.4 or 7.2-rc1 eliminates this vulnerability. Applying the patch 087d5b8b501c570f84bf655164e6698c3ce146e0/3c9c9648f77e4d14e50676bc51c2174ba9c8d361/5c3265f3252b2ee50707adaaa3f9bd0df3df72de/c923cc3cb5cd8945ceaf08252754110643446593/685fc15a410885b6d4dee64de0dce721b9428b12/53eeaf4d63068dbc7708b0c7adb20151c812feca/cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d is able to eliminate this problem.

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Productinfo

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CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 8.2
VulDB Meta Temp Score: 7.8

VulDB Base Score: 8.2
VulDB Temp Score: 7.8
VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv2info

AVACAuCIA
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💳💳💳💳💳💳
💳💳💳💳💳💳
VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Use after free
CWE: CWE-416 / CWE-119
CAPEC: 🔒
ATT&CK: 🔒

Physical: Partially
Local: Yes
Remote: No

Availability: 🔒
Status: Not defined

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Kernel 5.10.261/6.1.178/6.6.145/6.12.96/6.18.39/7.1.4/7.2-rc1
Patch: 087d5b8b501c570f84bf655164e6698c3ce146e0/3c9c9648f77e4d14e50676bc51c2174ba9c8d361/5c3265f3252b2ee50707adaaa3f9bd0df3df72de/c923cc3cb5cd8945ceaf08252754110643446593/685fc15a410885b6d4dee64de0dce721b9428b12/53eeaf4d63068dbc7708b0c7adb20151c812feca/cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d

Timelineinfo

07/19/2026 CVE reserved
07/25/2026 +6 days Advisory disclosed
07/25/2026 +0 days VulDB entry created
07/25/2026 +0 days VulDB entry last update

Sourcesinfo

Vendor: kernel.org

Advisory: git.kernel.org
Status: Confirmed

CVE: CVE-2026-64378 (🔒)
GCVE (CVE): GCVE-0-2026-64378
GCVE (VulDB): GCVE-100-383171

Entryinfo

Created: 07/25/2026 17:56
Changes: 07/25/2026 17:56 (61)
Complete: 🔍
Cache ID: 216::103

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