# CVE-2026-23233: f2fs: fix to avoid mapping wrong physical block for swapfile

> A flaw in the Linux kernel F2FS filesystem causes incorrect physical block mapping for small swapfiles, leading to data corruption and system crashes.

- **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:** 4 Mar 2026
- **Updated:** 23 May 2026
- **Also known as:** BELL-CVE-2026-23233, DEBIAN-CVE-2026-23233, GHSA-p99w-mp33-jg8x, ROOT-OS-DEBIAN-13-CVE-2026-23233, ROOT-OS-UBUNTU-2404-CVE-2026-23233, UBUNTU-CVE-2026-23233

## What is CVE-2026-23233?

CVE-2026-23233 is a high-severity memory corruption vulnerability in Linux kernel, published on 4 Mar 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 to avoid mapping wrong physical block for swapfile

Xiaolong Guo reported a f2fs bug in bugzilla [1]

[1] https://bugzilla.kernel.org/show_bug.cgi?id=220951

Quoted:

"When using stress-ng's swap stress test on F2FS filesystem with kernel 6.6+,
the system experiences data corruption leading to either:
1 dm-verity corruption errors and device reboot
2 F2FS node corruption errors and boot hangs

The issue occurs specifically when:
1 Using F2FS filesystem (ext4 is unaffected)
2 Swapfile size is less than F2FS section size (2MB)
3 Swapfile has fragmented physical layout (multiple non-contiguous extents)
4 Kernel version is 6.6+ (6.1 is unaffected)

The root cause is in check_swap_activate() function in fs/f2fs/data.c. When the
first extent of a small swapfile (< 2MB) is not aligned to section boundaries,
the function incorrectly treats it as the last extent, failing to map
subsequent extents. This results in incorrect swap_extent creation where only
the first extent is mapped, causing subsequent swap writes to overwrite wrong
physical locations (other files' data).

Steps to Reproduce
1 Setup a device with F2FS-formatted userdata partition
2 Compile stress-ng from https://github.com/ColinIanKing/stress-ng
3 Run swap stress test: (Android devices)
adb shell "cd /data/stressng; ./stress-ng-64 --metrics-brief --timeout 60
--swap 0"

Log:
1 Ftrace shows in kernel 6.6, only first extent is mapped during second
f2fs_map_blocks call in check_swap_activate():
stress-ng-swap-8990: f2fs_map_blocks: ino=11002, file offset=0, start
blkaddr=0x43143, len=0x1
(Only 4KB mapped, not the full swapfile)
2 in kernel 6.1, both extents are correctly mapped:
stress-ng-swap-5966: f2fs_map_blocks: ino=28011, file offset=0, start
blkaddr=0x13cd4, len=0x1
stress-ng-swap-5966: f2fs_map_blocks: ino=28011, file offset=1, start
blkaddr=0x60c84b, len=0xff

The problematic code is in check_swap_activate():
if ((pblock - SM_I(sbi)->main_blkaddr) % blks_per_sec ||
    nr_pblocks % blks_per_sec ||
    !f2fs_valid_pinned_area(sbi, pblock)) {
    bool last_extent = false;

    not_aligned++;

    nr_pblocks = roundup(nr_pblocks, blks_per_sec);
    if (cur_lblock + nr_pblocks > sis->max)
        nr_pblocks -= blks_per_sec;

    /* this extent is last one */
    if (!nr_pblocks) {
        nr_pblocks = last_lblock - cur_lblock;
        last_extent = true;
    }

    ret = f2fs_migrate_blocks(inode, cur_lblock, nr_pblocks);
    if (ret) {
        if (ret == -ENOENT)
            ret = -EINVAL;
        goto out;
    }

    if (!last_extent)
        goto retry;
}

When the first extent is unaligned and roundup(nr_pblocks, blks_per_sec)
exceeds sis->max, we subtract blks_per_sec resulting in nr_pblocks = 0. The
code then incorrectly assumes this is the last extent, sets nr_pblocks =
last_lblock - cur_lblock (entire swapfile), and performs migration. After
migration, it doesn't retry mapping, so subsequent extents are never processed.
"

In order to fix this issue, we need to lookup block mapping info after
we migrate all blocks in the tail of swapfile.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-23233?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23233?

Apply the vendor's patch.

## Is CVE-2026-23233 being exploited?

There is no known exploitation of CVE-2026-23233 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-2026-23233 and fix it if it is affected.

What it is: CVE-2026-23233 is a high-severity memory corruption vulnerability in Linux kernel, published on 4 Mar 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-2026-23233 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-2026-23233/ (Markdown: https://insights.sbomify.com/CVE-2026-23233/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23233
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23233
- OSV: https://osv.dev/vulnerability/CVE-2026-23233

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23233/
Summaries are generated and may contain mistakes; the official description above is the record.