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Fencer Vulns
high 7.3

CVE-2025-39738 json

btrfs: do not allow relocation of partially dropped subvolumes

published 2025-09-11 13 mo ago modified 2026-10-19 20 d ago also EUVD-2025-28973

cvss
7.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 7%
public exploits
0
none known

Summary

In the Linux kernel, the following vulnerability has been resolved:

btrfs: do not allow relocation of partially dropped subvolumes

[BUG] There is an internal report that balance triggered transaction abort, with the following call trace:

item 85 key (594509824 169 0) itemoff 12599 itemsize 33 extent refs 1 gen 197740 flags 2 ref#0: tree block backref root 7 item 86 key (594558976 169 0) itemoff 12566 itemsize 33 extent refs 1 gen 197522 flags 2 ref#0: tree block backref root 7 ... BTRFS error (device loop0): extent item not found for insert, bytenr 594526208 num_bytes 16384 parent 449921024 root_objectid 934 owner 1 offset 0 BTRFS error (device loop0): failed to run delayed ref for logical 594526208 num_bytes 16384 type 182 action 1 ref_mod 1: -117 ------------[ cut here ]------------ BTRFS: Transaction aborted (error -117) WARNING: CPU: 1 PID: 6963 at ../fs/btrfs/extent-tree.c:2168 btrfs_run_delayed_refs+0xfa/0x110 [btrfs]

And btrfs check doesn't report anything wrong related to the extent tree.

[CAUSE] The cause is a little complex, firstly the extent tree indeed doesn't have the backref for 594526208.

The extent tree only have the following two backrefs around that bytenr on-disk:

item 65 key (594509824 METADATA_ITEM 0) itemoff 13880 itemsize 33 refs 1 gen 197740 flags TREE_BLOCK tree block skinny level 0 (176 0x7) tree block backref root CSUM_TREE item 66 key (594558976 METADATA_ITEM 0) itemoff 13847 itemsize 33 refs 1 gen 197522 flags TREE_BLOCK tree block skinny level 0 (176 0x7) tree block backref root CSUM_TREE

But the such missing backref item is not an corruption on disk, as the offending delayed ref belongs to subvolume 934, and that subvolume is being dropped:

item 0 key (934 ROOT_ITEM 198229) itemoff 15844 itemsize 439 generation 198229 root_dirid 256 bytenr 10741039104 byte_limit 0 bytes_used 345571328 last_snapshot 198229 flags 0x1000000000001(RDONLY) refs 0 drop_progress key (206324 EXTENT_DATA 2711650304) drop_level 2 level 2 generation_v2 198229

And that offending tree block 594526208 is inside the dropped range of that subvolume. That explains why there is no backref item for that bytenr and why btrfs check is not reporting anything wrong.

But this also shows another problem, as btrfs will do all the orphan subvolume cleanup at a read-write mount.

So half-dropped subvolume should not exist after an RW mount, and balance itself is also exclusive to subvolume cleanup, meaning we shouldn't hit a subvolume half-dropped during relocation.

The root cause is, there is no orphan item for this subvolume. In fact there are 5 subvolumes from around 2021 that have the same problem.

It looks like the original report has some older kernels running, and caused those zombie subvolumes.

Thankfully upstream commit 8d488a8c7ba2 ("btrfs: fix subvolume/snapshot deletion not triggered on mount") has long fixed the bug.

[ENHANCEMENT] For repairing such old fs, btrfs-progs will be enhanced.

Considering how delayed the problem will show up (at run delayed ref time) and at that time we have to abort transaction already, it is too late.

Instead here we reject any half-dropped subvolume for reloc tree at the earliest time, preventing confusion and extra time wasted on debugging similar bugs.

Patch / fix

  • Linux Linux 5.15.190, 6.1.149, 6.6.103, 6.12.43, 6.15.11, 6.16.2, 6.17
  • Siemens SIMATIC CN 4100 V5.0
  • linux Debian 12 6.1.153-1
  • linux Debian 13 6.12.43-1
  • linux Debian 14 6.16.3-1

debian advisories DEBIAN-CVE-2025-39738

  • linux Ubuntu 22.04 LTS 5.15.0-163.173
  • linux Ubuntu 24.04 LTS 6.8.0-100.100
  • linux-aws Ubuntu 22.04 LTS 5.15.0-1097.104
  • linux-aws Ubuntu 24.04 LTS 6.8.0-1046.49
  • linux-gcp Ubuntu 22.04 LTS 5.15.0-1097.106
  • linux-gcp Ubuntu 24.04 LTS 6.8.0-1047.50
  • linux-gke Ubuntu 22.04 LTS 5.15.0-1093.99
  • linux-gke Ubuntu 24.04 LTS 6.8.0-1043.48

ubuntu advisories UBUNTU-CVE-2025-39738

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-rt Linux Micro 6.2 6.12.0-160000.6.1
  • kernel-64kb SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1
  • kernel-source SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1
  • kernel-default SLES 16.0, SLE High Availability Extension 16.0, Linux Micro 6.2 +1 more 6.12.0-160000.6.1
  • kernel-default-base SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1.160000.2.4
  • kernel-syms SLES 16.0, Linux Micro Extras 6.2, SLES for SAP applications 16.0 6.12.0-160000.6.1
  • kernel-obs-qa SLES for SAP applications 16.0, SLES 16.0 6.12.0-160000.6.1
  • kernel-kvmsmall SLES for SAP applications 16.0, SLES 16.0 6.12.0-160000.6.1

suse advisories SUSE-SU-2025:21074-1SUSE-SU-2025:21139-1SUSE-SU-2026:20560-1SUSE-SU-2025:21179-1SUSE-SU-2025:20870-1SUSE-SU-2025:20898-1

  • kernel-rt openSUSE Leap 16.0 6.12.0-160000.6.1
  • dtb-aarch64 openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-64kb openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-syms openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-obs-qa openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-source openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-default openSUSE Leap 16.0 6.12.0-160000.6.1
  • kernel-kvmsmall openSUSE Leap 16.0 6.12.0-160000.6.1

opensuse advisories openSUSE-SU-2025:20081-1

  • kernel Azure Linux 3.0 6.6.104.2-1
  • kernel CBL-Mariner 2.0 5.15.200.1-1

azure linux advisories AZL-67244AZL-73938

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-2025-39738 updated cvss 5.5

kernel: Linux kernel: Denial of Service in Btrfs due to relocation of partially dropped subvolumes

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more Red Hat Enterprise Linux 10 kernel
    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
partial
Assessed
24 d ago

Exploit prediction

0.18% chance of exploitation in the next 30 days, higher than 7% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.3 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
NVD 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
  • Siemens SIMATIC CN 4100

Classification

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.