# CVE-2025-21942: btrfs: zoned: fix extent range end unlock in cow_file_range()
> A missing extent unlock in the btrfs zoned filesystem code causes kernel hangs, leading to denial of service.
- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 Apr 2025
- **Updated:** 21 Oct 2026
- **Also known as:** GHSA-f349-gqv8-wh65, UBUNTU-CVE-2025-21942
## What is CVE-2025-21942?
CVE-2025-21942 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.
### Official description
In the Linux kernel, the following vulnerability has been resolved:
btrfs: zoned: fix extent range end unlock in cow_file_range()
Running generic/751 on the for-next branch often results in a hang like
below. They are both stack by locking an extent. This suggests someone
forget to unlock an extent.
INFO: task kworker/u128:1:12 blocked for more than 323 seconds.
Not tainted 6.13.0-BTRFS-ZNS+ #503
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u128:1 state:D stack:0 pid:12 tgid:12 ppid:2 flags:0x00004000
Workqueue: btrfs-fixup btrfs_work_helper [btrfs]
Call Trace:
__schedule+0x534/0xdd0
schedule+0x39/0x140
__lock_extent+0x31b/0x380 [btrfs]
? __pfx_autoremove_wake_function+0x10/0x10
btrfs_writepage_fixup_worker+0xf1/0x3a0 [btrfs]
btrfs_work_helper+0xff/0x480 [btrfs]
? lock_release+0x178/0x2c0
process_one_work+0x1ee/0x570
? srso_return_thunk+0x5/0x5f
worker_thread+0x1d1/0x3b0
? __pfx_worker_thread+0x10/0x10
kthread+0x10b/0x230
? __pfx_kthread+0x10/0x10
ret_from_fork+0x30/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
INFO: task kworker/u134:0:184 blocked for more than 323 seconds.
Not tainted 6.13.0-BTRFS-ZNS+ #503
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u134:0 state:D stack:0 pid:184 tgid:184 ppid:2 flags:0x00004000
Workqueue: writeback wb_workfn (flush-btrfs-4)
Call Trace:
__schedule+0x534/0xdd0
schedule+0x39/0x140
__lock_extent+0x31b/0x380 [btrfs]
? __pfx_autoremove_wake_function+0x10/0x10
find_lock_delalloc_range+0xdb/0x260 [btrfs]
writepage_delalloc+0x12f/0x500 [btrfs]
? srso_return_thunk+0x5/0x5f
extent_write_cache_pages+0x232/0x840 [btrfs]
btrfs_writepages+0x72/0x130 [btrfs]
do_writepages+0xe7/0x260
? srso_return_thunk+0x5/0x5f
? lock_acquire+0xd2/0x300
? srso_return_thunk+0x5/0x5f
? find_held_lock+0x2b/0x80
? wbc_attach_and_unlock_inode.part.0+0x102/0x250
? wbc_attach_and_unlock_inode.part.0+0x102/0x250
__writeback_single_inode+0x5c/0x4b0
writeback_sb_inodes+0x22d/0x550
__writeback_inodes_wb+0x4c/0xe0
wb_writeback+0x2f6/0x3f0
wb_workfn+0x32a/0x510
process_one_work+0x1ee/0x570
? srso_return_thunk+0x5/0x5f
worker_thread+0x1d1/0x3b0
? __pfx_worker_thread+0x10/0x10
kthread+0x10b/0x230
? __pfx_kthread+0x10/0x10
ret_from_fork+0x30/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
This happens because we have another success path for the zoned mode. When
there is no active zone available, btrfs_reserve_extent() returns
-EAGAIN. In this case, we have two reactions.
(1) If the given range is never allocated, we can only wait for someone
to finish a zone, so wait on BTRFS_FS_NEED_ZONE_FINISH bit and retry
afterward.
(2) Or, if some allocations are already done, we must bail out and let
the caller to send IOs for the allocation. This is because these IOs
may be necessary to finish a zone.
The commit 06f364284794 ("btrfs: do proper folio cleanup when
cow_file_range() failed") moved the unlock code from the inside of the
loop to the outside. So, previously, the allocated extents are unlocked
just after the allocation and so before returning from the function.
However, they are no longer unlocked on the case (2) above. That caused
the hang issue.
Fix the issue by modifying the 'end' to the end of the allocated
range. Then, we can exit the loop and the same unlock code can properly
handle the case.
Weakness: CWE-835 (Loop with Unreachable Exit Condition)
## Which versions are affected by CVE-2025-21942?
It affects 1 product.
Products: Linux kernel.
## How do I fix CVE-2025-21942?
Apply the vendor's patch.
## Is CVE-2025-21942 being exploited?
There is no known exploitation of CVE-2025-21942 in the wild and no public exploit code.
## Other vulnerabilities in Linux kernel
- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)
## Fix prompt for AI coding agents
```text
Check this codebase for CVE-2025-21942 and fix it if it is affected.
What it is: CVE-2025-21942 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-21942 in the wild and no public exploit code.
Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.
Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.
Reference: https://insights.sbomify.com/CVE-2025-21942/ (Markdown: https://insights.sbomify.com/CVE-2025-21942/index.md)
```
## Sources
- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21942
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21942
- OSV: https://osv.dev/vulnerability/CVE-2025-21942
---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21942/
Summaries are generated and may contain mistakes; the official description above is the record.