CVE-2025-39738 — linux / linux_kernel
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.
| Software | From | Fixed in |
|---|---|---|
| linux / linux_kernel | 5.11.1 | 5.15.190 |
| linux / linux_kernel | 5.16 | 6.1.149 |
| linux / linux_kernel | 6.2 | 6.6.103 |
| linux / linux_kernel | 6.7 | 6.12.43 |
| linux / linux_kernel | 6.13 | 6.15.11 |
| linux / linux_kernel | 6.16 | 6.16.2 |
| linux / linux_kernel | 5.11 | 5.11.x |
| linux / linux_kernel | 5.11-rc3 | 5.11-rc3.x |
| linux / linux_kernel | 5.11-rc4 | 5.11-rc4.x |
| linux / linux_kernel | 5.11-rc5 | 5.11-rc5.x |
| linux / linux_kernel | 5.11-rc6 | 5.11-rc6.x |
| linux / linux_kernel | 5.11-rc7 | 5.11-rc7.x |
| linux / linux_kernel | 6.17-rc1 | 6.17-rc1.x |
| debian / debian_linux | 11.0 | 11.0.x |
- https://cert-portal.siemens.com/productcert/html/ssa-032379.html
- https://git.kernel.org/stable/c/125e94a4b76b7b75d194f85bedd628097d2121f0
- https://git.kernel.org/stable/c/39a93e1c9dbf7e11632efeb20fcf0fc1dcf64d51
- https://git.kernel.org/stable/c/4289b494ac553e74e86fed1c66b2bf9530bc1082
- https://git.kernel.org/stable/c/4e403bd8e127d40dc7c05f06ee969c1ba1537ec5
- https://git.kernel.org/stable/c/f83d4c81bda3b7d1813268ab77408f7a0ce691ff
- https://git.kernel.org/stable/c/fa086b1398cf7e5f7dee7241bd5f2855cb5df8dc
- https://git.kernel.org/stable/c/fcb1f77b8ed8795608ca7a1f6505e2b07236c1f3
- https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
Deep Security Visibility Without the Complexity
SynScan provides clear, real-time security insights so you can monitor your attack surface, spot risks early, and act fast—without extra complexity.
- No setup fees
- 5-min deployment
- Cancel anytime
Frequently Asked Questions
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.