# CVE-2026-89640: cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0

> An integer underflow in the Linux kernel CIFS module allows corrupting file range requests, potentially leading to data corruption or denial of service.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/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:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100292, BELL-CVE-2026-89640, DEBIAN-CVE-2026-89640, GHSA-hwfc-7pp7-m5j3, UBUNTU-CVE-2026-89640

## What is CVE-2026-89640?

CVE-2026-89640 is a high-severity vulnerability in Linux, published on 11 Sep 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:

cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0

With len == 0 (clone to EOF), the effective length is computed as:

    len = src_inode->i_size - off;

If off > i_size, this is a negative loff_t, corrupting the ByteCount
in the FSCTL_DUPLICATE_EXTENTS_TO_FILE request and inverting the range
in filemap_write_and_wait_range().  The existing off >= i_size check
fires only after the ioctl has already been sent.

Snapshot i_size_read() once for both the bounds check and the length
calculation, eliminating the TOCTOU and 32-bit torn-read risk.  Reject
off > src_size with -EINVAL.  Treat off == src_size as a no-op,
consistent with __generic_remap_file_range_prep().

## Which versions are affected by CVE-2026-89640?


## How do I fix CVE-2026-89640?

Apply the vendor's patch.

## Is CVE-2026-89640 being exploited?

There is no known exploitation of CVE-2026-89640 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89640 and fix it if it is affected.

What it is: CVE-2026-89640 is a high-severity vulnerability in Linux, published on 11 Sep 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-2026-89640 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-2026-89640/ (Markdown: https://insights.sbomify.com/CVE-2026-89640/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89640
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89640
- OSV: https://osv.dev/vulnerability/CVE-2026-89640

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89640/
Summaries are generated and may contain mistakes; the official description above is the record.