# CVE-2026-31702: f2fs: fix use-after-free of sbi in f2fs_compress_write_end_io()

> A use-after-free in the Linux kernel's f2fs compressed writeback path allows attackers to potentially execute code or escalate privileges via a race condition during unmount.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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 2026
- **Updated:** 14 Jun 2026
- **Also known as:** BELL-CVE-2026-31702, DEBIAN-CVE-2026-31702, GHSA-5c3m-p5m6-p659, ROOT-OS-DEBIAN-11-CVE-2026-31702, ROOT-OS-DEBIAN-12-CVE-2026-31702, ROOT-OS-UBUNTU-2204-CVE-2026-31702, ROOT-OS-UBUNTU-2404-CVE-2026-31702, UBUNTU-CVE-2026-31702

## What is CVE-2026-31702?

CVE-2026-31702 is a high-severity use after free vulnerability in Linux kernel, published on 1 May 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:

f2fs: fix use-after-free of sbi in f2fs_compress_write_end_io()

In f2fs_compress_write_end_io(), dec_page_count(sbi, type) can bring
the F2FS_WB_CP_DATA counter to zero, unblocking
f2fs_wait_on_all_pages() in f2fs_put_super() on a concurrent unmount
CPU. The unmount path then proceeds to call
f2fs_destroy_page_array_cache(sbi), which destroys
sbi->page_array_slab via kmem_cache_destroy(), and eventually
kfree(sbi). Meanwhile, the bio completion callback is still executing:
when it reaches page_array_free(sbi, ...), it dereferences
sbi->page_array_slab — a destroyed slab cache — to call
kmem_cache_free(), causing a use-after-free.

This is the same class of bug as CVE-2026-23234 (which fixed the
equivalent race in f2fs_write_end_io() in data.c), but in the
compressed writeback completion path that was not covered by that fix.

Fix this by moving dec_page_count() to after page_array_free(), so
that all sbi accesses complete before the counter decrement that can
unblock unmount. For non-last folios (where atomic_dec_return on
cic->pending_pages is nonzero), dec_page_count is called immediately
before returning — page_array_free is not reached on this path, so
there is no post-decrement sbi access. For the last folio,
page_array_free runs while the F2FS_WB_CP_DATA counter is still
nonzero (this folio has not yet decremented it), keeping sbi alive,
and dec_page_count runs as the final operation.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-31702?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31702?

Apply the vendor's patch.

## Is CVE-2026-31702 being exploited?

There is no known exploitation of CVE-2026-31702 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-2026-31702 and fix it if it is affected.

What it is: CVE-2026-31702 is a high-severity use after free vulnerability in Linux kernel, published on 1 May 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-2026-31702 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-2026-31702/ (Markdown: https://insights.sbomify.com/CVE-2026-31702/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31702
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31702
- OSV: https://osv.dev/vulnerability/CVE-2026-31702

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31702/
Summaries are generated and may contain mistakes; the official description above is the record.