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Fencer Vulns
high 7.8

CVE-2026-64378 json

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

published 2026-07-25 3 mo ago modified 2026-10-19 20 d ago also EUVD-2026-48922

cvss
7.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.1%
EPSS 0.1% chance of exploitation in 30 days, percentile 0%
public exploits
0
none known

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.

Patch / fix

  • Linux Linux 4.5, 5.10.261, 6.1.178, 6.6.145, 6.12.96, 6.18.39, 7.1.4, 7.2
  • linux Debian 12 6.1.180-1
  • linux Debian 13 6.12.96-1
  • linux Debian 14 7.1.4-1
  • linux-6.12 Debian 12 6.12.100-1~deb12u1

debian advisories DEBIAN-CVE-2026-64378

  • linux Ubuntu 24.04 LTS 6.8.0-147.147
  • linux Ubuntu 26.04 LTS 7.0.0-31.31
  • linux-aws Ubuntu 24.04 LTS 6.8.0-1067.70
  • linux-aws Ubuntu 26.04 LTS 7.0.0-1012.12
  • linux-gcp Ubuntu 24.04 LTS 6.8.0-1071.79
  • linux-gcp Ubuntu 26.04 LTS 7.0.0-1011.11
  • linux-gke Ubuntu 24.04 LTS 6.8.0-1066.74
  • linux-gke Ubuntu 26.04 LTS 7.0.0-1006.7

ubuntu advisories UBUNTU-CVE-2026-64378

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel-64kb SLE Module for Basesystem 15 SP7 6.4.0-150700.53.81.1
  • kernel-syms SLE Module for Development Tools 15 SP7 6.4.0-150700.53.81.1
  • kernel-azure SLE Module for Public Cloud 15 SP7 6.4.0-150700.53.81.1
  • kernel-source SLE Module for Development Tools 15 SP7, SLE Module for Basesystem 15 SP7 6.4.0-150700.53.81.1
  • kernel-default SLE Workstation Extension 15 SP7, SLE Module for Legacy 15 SP7, SLE Module for Basesystem 15 SP7 +2 more 6.4.0-150700.53.81.1
  • kernel-zfcpdump SLE Module for Basesystem 15 SP7 6.4.0-150700.53.81.1
  • kernel-obs-build SLE Module for Development Tools 15 SP7 6.4.0-150700.53.81.1
  • kernel-default-base SLE Module for Basesystem 15 SP7 6.4.0-150700.53.81.1.150700.17.45.1

suse advisories SUSE-SU-2026:4687-1SUSE-SU-2026:4254-1SUSE-SU-2026:23887-1SUSE-SU-2026:23902-1SUSE-SU-2026:23897-1SUSE-SU-2026:23881-1

  • kernel-rt openSUSE Leap 16.0 6.12.0-160000.38.1
  • dtb-aarch64 openSUSE Leap 16.0 6.12.0-160000.38.1
  • kernel-64kb openSUSE Leap 16.0 6.12.0-160000.38.1
  • kernel-syms openSUSE Leap 16.0 6.12.0-160000.38.1
  • kernel-azure openSUSE Leap 16.0 6.12.0-160000.38.1
  • kernel-obs-qa openSUSE Leap 16.0 6.12.0-160000.38.1
  • kernel-source openSUSE Leap 16.0 6.12.0-160000.38.1
  • kernel-default openSUSE Leap 16.0 6.12.0-160000.38.1

opensuse advisories openSUSE-SU-2026:21910-1openSUSE-SU-2026:11476-1

  • kernel AlmaLinux 10 6.12.0-211.74.1.el10_2
  • rv AlmaLinux 10 6.12.0-211.74.1.el10_2
  • perf AlmaLinux 10 6.12.0-211.74.1.el10_2
  • rtla AlmaLinux 10 6.12.0-211.74.1.el10_2
  • libperf AlmaLinux 10 6.12.0-211.74.1.el10_2
  • kernel-rt AlmaLinux 10 6.12.0-211.74.1.el10_2
  • kernel-64k AlmaLinux 10 6.12.0-211.74.1.el10_2
  • kernel-core AlmaLinux 10 6.12.0-211.74.1.el10_2

almalinux advisories ALSA-2026:95367ALSA-2026:89082ALSA-2026:89083

  • kernel Rocky Linux 10 6.12.0-211.74.1.el10_2
  • kernel-rt Rocky Linux 8 4.18.0-553.178.1.rt7.519.el8_10
  • kernel Rocky Linux 8 4.18.0-553.178.1.el8_10

rocky linux advisories RLSA-2026:95367RLSA-2026:89082RLSA-2026:89083

  • kernel Azure Linux 3.0 6.6.145.2-1

azure linux advisories AZL-93945

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-64378 updated cvss 7.0

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

  • Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more
    affected: workaround, no fix available
  • Red Hat Enterprise Linux kernel-modules-internal 6.12.0-211.74.1.el10_2 Red Hat Enterprise Linux AppStream (v. 10) rv 6.12.0-211.74.1.el10_2, perf 6.12.0-211.74.1.el10_2, rtla 6.12.0-211.74.1.el10_2, python3-perf 6.12.0-211.74.1.el10_2, kernel-devel-matched 6.12.0-211.74.1.el10_2, kernel-64k-devel-matched 6.12.0-211.74.1.el10_2, +7 more
    fixed: vendor fix RHSA-2026:95367
  • Red Hat Enterprise Linux BaseOS (v. 8) perf 4.18.0-553.178.1.el8_10, kernel 4.18.0-553.178.1.el8_10, bpftool 4.18.0-553.178.1.el8_10, kernel-core 4.18.0-553.178.1.el8_10, kernel-tools 4.18.0-553.178.1.el8_10, python3-perf 4.18.0-553.178.1.el8_10, +15 more
    fixed: vendor fix RHSA-2026:89083
  • Red Hat Enterprise Linux BaseOS (v. 10) kernel 6.12.0-211.74.1.el10_2, kernel-64k 6.12.0-211.74.1.el10_2, kernel-core 6.12.0-211.74.1.el10_2, kernel-tools 6.12.0-211.74.1.el10_2, kernel-modules 6.12.0-211.74.1.el10_2, kernel-64k-core 6.12.0-211.74.1.el10_2, +28 more Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) libperf 6.12.0-211.74.1.el10_2, kernel-debuginfo-common-s390x 6.12.0-211.74.1.el10_2, kernel-debuginfo-common-x86_64 6.12.0-211.74.1.el10_2, kernel-debuginfo-common-aarch64 6.12.0-211.74.1.el10_2, kernel-debuginfo-common-ppc64le 6.12.0-211.74.1.el10_2
    fixed: vendor fix RHSA-2026:95367
  • Red Hat Enterprise Linux CRB (v. 8) kernel-debuginfo-common-x86_64 4.18.0-553.178.1.el8_10, kernel-debuginfo-common-aarch64 4.18.0-553.178.1.el8_10, kernel-debuginfo-common-ppc64le 4.18.0-553.178.1.el8_10
    fixed: vendor fix RHSA-2026:89083
  • Red Hat Enterprise Linux Hidden (v. 10) kernel-modules-internal 6.12.0-211.74.1.el10_2, kernel-debuginfo-common-x86_64 6.12.0-211.74.1.el10_2
    fixed: vendor fix RHSA-2026:95367
  • Red Hat Enterprise Linux NFV (v. 8) kernel-rt 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-kvm 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-core 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-modules 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-kvm 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-core 4.18.0-553.178.1.rt7.519.el8_10, +4 more
    fixed: vendor fix RHSA-2026:89082
  • Red Hat Enterprise Linux Real Time (v. 10) kernel-rt 6.12.0-211.74.1.el10_2, kernel-rt-64k 6.12.0-211.74.1.el10_2, kernel-rt-core 6.12.0-211.74.1.el10_2, kernel-rt-modules 6.12.0-211.74.1.el10_2, kernel-rt-64k-core 6.12.0-211.74.1.el10_2, kernel-rt-debug-core 6.12.0-211.74.1.el10_2, +14 more Red Hat Enterprise Linux Real Time for NFV (v. 10) kernel-rt 6.12.0-211.74.1.el10_2, kernel-rt-core 6.12.0-211.74.1.el10_2, kernel-rt-modules 6.12.0-211.74.1.el10_2, kernel-rt-debug-core 6.12.0-211.74.1.el10_2, kernel-rt-modules-core 6.12.0-211.74.1.el10_2, kernel-rt-debug-modules 6.12.0-211.74.1.el10_2, +4 more
    fixed: vendor fix RHSA-2026:95367
show 2 more
  • Red Hat Enterprise Linux RT (v. 8) kernel-rt 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-core 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-modules 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-core 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-modules 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-modules-extra 4.18.0-553.178.1.rt7.519.el8_10, +2 more
    fixed: vendor fix RHSA-2026:89082
  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
total
Assessed
23 d ago

Exploit prediction

0.10% chance of exploitation in the next 30 days, higher than 0% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Affected products

  • Linux Linux

Classification

Weakness
  • CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.