# CVE-2026-23181: btrfs: sync read disk super and set block size

> A race condition in the Linux kernel btrfs filesystem can cause a null pointer dereference and system crash during mount operations.

- **Severity:** Not scored
- **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:** 11 May 2026
- **Also known as:** BELL-CVE-2026-23181, DEBIAN-CVE-2026-23181, GHSA-4fxm-jm89-7f7q, ROOT-OS-UBUNTU-2204-CVE-2026-23181, ROOT-OS-UBUNTU-2404-CVE-2026-23181, UBUNTU-CVE-2026-23181

## What is CVE-2026-23181?

CVE-2026-23181 is a vulnerability in Linux, 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: sync read disk super and set block size

When the user performs a btrfs mount, the block device is not set
correctly. The user sets the block size of the block device to 0x4000
by executing the BLKBSZSET command.
Since the block size change also changes the mapping->flags value, this
further affects the result of the mapping_min_folio_order() calculation.

Let's analyze the following two scenarios:

Scenario 1: Without executing the BLKBSZSET command, the block size is
0x1000, and mapping_min_folio_order() returns 0;

Scenario 2: After executing the BLKBSZSET command, the block size is
0x4000, and mapping_min_folio_order() returns 2.

do_read_cache_folio() allocates a folio before the BLKBSZSET command
is executed. This results in the allocated folio having an order value
of 0. Later, after BLKBSZSET is executed, the block size increases to
0x4000, and the mapping_min_folio_order() calculation result becomes 2.

This leads to two undesirable consequences:

1. filemap_add_folio() triggers a VM_BUG_ON_FOLIO(folio_order(folio) <
mapping_min_folio_order(mapping)) assertion.

2. The syzbot report [1] shows a null pointer dereference in
create_empty_buffers() due to a buffer head allocation failure.

Synchronization should be established based on the inode between the
BLKBSZSET command and read cache page to prevent inconsistencies in
block size or mapping flags before and after folio allocation.

[1]
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:create_empty_buffers+0x4d/0x480 fs/buffer.c:1694
Call Trace:
 folio_create_buffers+0x109/0x150 fs/buffer.c:1802
 block_read_full_folio+0x14c/0x850 fs/buffer.c:2403
 filemap_read_folio+0xc8/0x2a0 mm/filemap.c:2496
 do_read_cache_folio+0x266/0x5c0 mm/filemap.c:4096
 do_read_cache_page mm/filemap.c:4162 [inline]
 read_cache_page_gfp+0x29/0x120 mm/filemap.c:4195
 btrfs_read_disk_super+0x192/0x500 fs/btrfs/volumes.c:1367

## Which versions are affected by CVE-2026-23181?


## How do I fix CVE-2026-23181?

Apply the vendor's patch.

## Is CVE-2026-23181 being exploited?

There is no known exploitation of CVE-2026-23181 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23181 and fix it if it is affected.

What it is: CVE-2026-23181 is a vulnerability in Linux, 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-2026-23181 in the wild and no public exploit code.

Affected product: Linux. 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-2026-23181/ (Markdown: https://insights.sbomify.com/CVE-2026-23181/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23181
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23181
- OSV: https://osv.dev/vulnerability/CVE-2026-23181

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23181/
Summaries are generated and may contain mistakes; the official description above is the record.