# CVE-2026-31715: f2fs: fix UAF caused by decrementing sbi->nr_pages[] in f2fs_write_end_io()

> A use-after-free vulnerability in the Linux kernel's F2FS filesystem allows local attackers to cause a system crash 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:** 19 Jun 2026
- **Also known as:** BELL-CVE-2026-31715, DEBIAN-CVE-2026-31715, GHSA-5pg7-fr46-jx8j, ROOT-OS-DEBIAN-11-CVE-2026-31715, ROOT-OS-DEBIAN-12-CVE-2026-31715, ROOT-OS-DEBIAN-13-CVE-2026-31715, ROOT-OS-UBUNTU-2204-CVE-2026-31715, ROOT-OS-UBUNTU-2404-CVE-2026-31715, UBUNTU-CVE-2026-31715

## What is CVE-2026-31715?

CVE-2026-31715 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 UAF caused by decrementing sbi->nr_pages[] in f2fs_write_end_io()

The xfstests case "generic/107" and syzbot have both reported a NULL
pointer dereference.

The concurrent scenario that triggers the panic is as follows:

F2FS_WB_CP_DATA write callback          umount
                                        - f2fs_write_checkpoint
                                         - f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA)
- blk_mq_end_request
 - bio_endio
  - f2fs_write_end_io
   : dec_page_count(sbi, F2FS_WB_CP_DATA)
   : wake_up(&sbi->cp_wait)
                                        - kill_f2fs_super
                                         - kill_block_super
                                          - f2fs_put_super
                                           : iput(sbi->node_inode)
                                           : sbi->node_inode = NULL
   : f2fs_in_warm_node_list
    - is_node_folio // sbi->node_inode is NULL and panic

The root cause is that f2fs_put_super() calls iput(sbi->node_inode) and
sets sbi->node_inode to NULL after sbi->nr_pages[F2FS_WB_CP_DATA] is
decremented to zero. As a result, f2fs_in_warm_node_list() may
dereference a NULL node_inode when checking whether a folio belongs to
the node inode, leading to a panic.

This patch fixes the issue by calling f2fs_in_warm_node_list() before
decrementing sbi->nr_pages[F2FS_WB_CP_DATA], thus preventing the
use-after-free condition.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-31715?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31715?

Apply the vendor's patch.

## Is CVE-2026-31715 being exploited?

There is no known exploitation of CVE-2026-31715 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-2026-31715 and fix it if it is affected.

What it is: CVE-2026-31715 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-2026-31715 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-2026-31715/ (Markdown: https://insights.sbomify.com/CVE-2026-31715/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31715
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31715
- OSV: https://osv.dev/vulnerability/CVE-2026-31715

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31715/
Summaries are generated and may contain mistakes; the official description above is the record.