# CVE-2026-74571: btrfs: skip global block reserve accounting for rescue mounts

> A local attacker with physical access or mount privileges can cause a kernel crash by mounting a corrupted Btrfs filesystem with rescue options.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-96371, BELL-CVE-2026-74571, DEBIAN-CVE-2026-74571, GHSA-2vh7-793m-f7hf, UBUNTU-CVE-2026-74571

## What is CVE-2026-74571?

CVE-2026-74571 is a vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

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

btrfs: skip global block reserve accounting for rescue mounts

[BUG]
Mounting with rescue=ibadroots after corrupting the block group tree
root triggers a NULL pointer dereference:

  BUG: kernel NULL pointer dereference, address: 0000000000000100
  RIP: 0010:btrfs_update_global_block_rsv+0x9d/0x1c0 [btrfs]
  Call Trace:
   fill_dummy_bgs+0xd4/0x120 [btrfs]
   open_ctree+0xc6e/0x1ca0 [btrfs]
   btrfs_get_tree+0x50d/0xa40 [btrfs]

The same crash occurs with a corrupted raid stripe tree root, via
btrfs_read_block_groups() instead of fill_dummy_bgs().

[CAUSE]
With rescue=ibadroots, btrfs_read_roots() allows the mount to continue
when either root cannot be read, leaving the corresponding root pointer
NULL while its on-disk feature bit remains set.

btrfs_update_global_block_rsv() then dereferences the missing root based
on the feature bit alone.

[FIX]
Rescue mounts are fully read-only and cannot start transactions, so the
global reserve is never consumed. Under btrfs_is_full_ro(), mark the
reserve as full and return before performing the accounting.

And since we need to check if the fs is mount fully RO, export
fs_is_full_ro() as btrfs_is_full_ro(), and move it to fs.h.

[ Squash the fs_is_full_ro() export commit into this one. ]

## Which versions are affected by CVE-2026-74571?


## How do I fix CVE-2026-74571?

Apply the vendor's patch.

## Is CVE-2026-74571 being exploited?

There is no known exploitation of CVE-2026-74571 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74571 and fix it if it is affected.

What it is: CVE-2026-74571 is a vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-74571 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-2026-74571/ (Markdown: https://insights.sbomify.com/CVE-2026-74571/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74571
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74571
- OSV: https://osv.dev/vulnerability/CVE-2026-74571

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74571/
Summaries are generated and may contain mistakes; the official description above is the record.