# CVE-2026-23157: btrfs: do not strictly require dirty metadata threshold for metadata writepages

> A flaw in the Linux kernel's btrfs filesystem can cause system hangs and kernel coredumps when dirty page throttling interacts with internal writeback thresholds.

- **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:** 14 Feb 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-77820, BELL-CVE-2026-23157, DEBIAN-CVE-2026-23157, GHSA-pw2v-cmfh-x2p3, ROOT-OS-DEBIAN-11-CVE-2026-23157, ROOT-OS-DEBIAN-12-CVE-2026-23157, ROOT-OS-DEBIAN-13-CVE-2026-23157, ROOT-OS-UBUNTU-2204-CVE-2026-23157, ROOT-OS-UBUNTU-2404-CVE-2026-23157, UBUNTU-CVE-2026-23157

## What is CVE-2026-23157?

CVE-2026-23157 is a medium-severity denial of service vulnerability in Linux kernel, published on 14 Feb 2026. 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: do not strictly require dirty metadata threshold for metadata writepages

[BUG]
There is an internal report that over 1000 processes are
waiting at the io_schedule_timeout() of balance_dirty_pages(), causing
a system hang and trigger a kernel coredump.

The kernel is v6.4 kernel based, but the root problem still applies to
any upstream kernel before v6.18.

[CAUSE]
From Jan Kara for his wisdom on the dirty page balance behavior first.

  This cgroup dirty limit was what was actually playing the role here
  because the cgroup had only a small amount of memory and so the dirty
  limit for it was something like 16MB.

  Dirty throttling is responsible for enforcing that nobody can dirty
  (significantly) more dirty memory than there's dirty limit. Thus when
  a task is dirtying pages it periodically enters into balance_dirty_pages()
  and we let it sleep there to slow down the dirtying.

  When the system is over dirty limit already (either globally or within
  a cgroup of the running task), we will not let the task exit from
  balance_dirty_pages() until the number of dirty pages drops below the
  limit.

  So in this particular case, as I already mentioned, there was a cgroup
  with relatively small amount of memory and as a result with dirty limit
  set at 16MB. A task from that cgroup has dirtied about 28MB worth of
  pages in btrfs btree inode and these were practically the only dirty
  pages in that cgroup.

So that means the only way to reduce the dirty pages of that cgroup is
to writeback the dirty pages of btrfs btree inode, and only after that
those processes can exit balance_dirty_pages().

Now back to the btrfs part, btree_writepages() is responsible for
writing back dirty btree inode pages.

The problem here is, there is a btrfs internal threshold that if the
btree inode's dirty bytes are below the 32M threshold, it will not
do any writeback.

This behavior is to batch as much metadata as possible so we won't write
back those tree blocks and then later re-COW them again for another
modification.

This internal 32MiB is higher than the existing dirty page size (28MiB),
meaning no writeback will happen, causing a deadlock between btrfs and
cgroup:

- Btrfs doesn't want to write back btree inode until more dirty pages

- Cgroup/MM doesn't want more dirty pages for btrfs btree inode
  Thus any process touching that btree inode is put into sleep until
  the number of dirty pages is reduced.

Thanks Jan Kara a lot for the analysis of the root cause.

[ENHANCEMENT]
Since kernel commit b55102826d7d ("btrfs: set AS_KERNEL_FILE on the
btree_inode"), btrfs btree inode pages will only be charged to the root
cgroup which should have a much larger limit than btrfs' 32MiB
threshold.
So it should not affect newer kernels.

But for all current LTS kernels, they are all affected by this problem,
and backporting the whole AS_KERNEL_FILE may not be a good idea.

Even for newer kernels I still think it's a good idea to get
rid of the internal threshold at btree_writepages(), since for most cases
cgroup/MM has a better view of full system memory usage than btrfs' fixed
threshold.

For internal callers using btrfs_btree_balance_dirty() since that
function is already doing internal threshold check, we don't need to
bother them.

But for external callers of btree_writepages(), just respect their
requests and write back whatever they want, ignoring the internal
btrfs threshold to avoid such deadlock on btree inode dirty page
balancing.

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2026-23157?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23157?

Apply the vendor's patch.

## Is CVE-2026-23157 being exploited?

There is no known exploitation of CVE-2026-23157 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-2026-23157 and fix it if it is affected.

What it is: CVE-2026-23157 is a medium-severity denial of service vulnerability in Linux kernel, published on 14 Feb 2026. 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-2026-23157 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-2026-23157/ (Markdown: https://insights.sbomify.com/CVE-2026-23157/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23157
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23157
- OSV: https://osv.dev/vulnerability/CVE-2026-23157

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23157/
Summaries are generated and may contain mistakes; the official description above is the record.