# CVE-2022-49850: nilfs2: fix deadlock in nilfs_count_free_blocks()

> A deadlock in the nilfs2 filesystem can cause a denial of service when metadata corruption coincides with superblock writeback.

- **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 May 2025
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2022-49850, GHSA-m8x7-f64c-28w3, UBUNTU-CVE-2022-49850

## What is CVE-2022-49850?

CVE-2022-49850 is a medium-severity vulnerability in Linux kernel, published on 1 May 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:

nilfs2: fix deadlock in nilfs_count_free_blocks()

A semaphore deadlock can occur if nilfs_get_block() detects metadata
corruption while locating data blocks and a superblock writeback occurs at
the same time:

task 1                               task 2
------                               ------
* A file operation *
nilfs_truncate()
  nilfs_get_block()
    down_read(rwsem A) <--
    nilfs_bmap_lookup_contig()
      ...                            generic_shutdown_super()
                                       nilfs_put_super()
                                         * Prepare to write superblock *
                                         down_write(rwsem B) <--
                                         nilfs_cleanup_super()
      * Detect b-tree corruption *         nilfs_set_log_cursor()
      nilfs_bmap_convert_error()             nilfs_count_free_blocks()
        __nilfs_error()                        down_read(rwsem A) <--
          nilfs_set_error()
            down_write(rwsem B) <--

                           *** DEADLOCK ***

Here, nilfs_get_block() readlocks rwsem A (= NILFS_MDT(dat_inode)->mi_sem)
and then calls nilfs_bmap_lookup_contig(), but if it fails due to metadata
corruption, __nilfs_error() is called from nilfs_bmap_convert_error()
inside the lock section.

Since __nilfs_error() calls nilfs_set_error() unless the filesystem is
read-only and nilfs_set_error() attempts to writelock rwsem B (=
nilfs->ns_sem) to write back superblock exclusively, hierarchical lock
acquisition occurs in the order rwsem A -> rwsem B.

Now, if another task starts updating the superblock, it may writelock
rwsem B during the lock sequence above, and can deadlock trying to
readlock rwsem A in nilfs_count_free_blocks().

However, there is actually no need to take rwsem A in
nilfs_count_free_blocks() because it, within the lock section, only reads
a single integer data on a shared struct with
nilfs_sufile_get_ncleansegs().  This has been the case after commit
aa474a220180 ("nilfs2: add local variable to cache the number of clean
segments"), that is, even before this bug was introduced.

So, this resolves the deadlock problem by just not taking the semaphore in
nilfs_count_free_blocks().

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2022-49850?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2022-49850?

Apply the vendor's patch.

## Is CVE-2022-49850 being exploited?

There is no known exploitation of CVE-2022-49850 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-2022-49850 and fix it if it is affected.

What it is: CVE-2022-49850 is a medium-severity vulnerability in Linux kernel, published on 1 May 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-2022-49850 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-2022-49850/ (Markdown: https://insights.sbomify.com/CVE-2022-49850/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2022-49850
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2022-49850
- OSV: https://osv.dev/vulnerability/CVE-2022-49850

---
From sbomify Insights: https://insights.sbomify.com/CVE-2022-49850/
Summaries are generated and may contain mistakes; the official description above is the record.