# CVE-2026-90391: lib/test_hmm: fail dmirror_fault() when the mirrored mm is gone

> A local attacker can cause an infinite loop in the Linux kernel's HMM test driver by triggering a fault on an exited memory map, leading to denial of service.

- **Severity:** Not scored
- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102867, BELL-CVE-2026-90391, DEBIAN-CVE-2026-90391, GHSA-q8fg-m6xh-5pw6, UBUNTU-CVE-2026-90391

## What is CVE-2026-90391?

CVE-2026-90391 is a denial of service vulnerability in Linux, published on 17 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:

lib/test_hmm: fail dmirror_fault() when the mirrored mm is gone

dmirror_fault() is called from the dmirror_read() and dmirror_write()
retry loops after dmirror_do_read() or dmirror_do_write() finds a missing
device page table entry.

If the mirrored mm has already exited, mmget_not_zero() fails.  The
current code returns 0 in that case, which tells the caller that faulting
succeeded even though no page was faulted and no device page table entry
was installed.  The caller then retries the same address, hits -ENOENT
again, and can loop forever without making progress.

Return -EFAULT instead, so the ioctl fails when the mirrored mm is no
longer faultable.

## Which versions are affected by CVE-2026-90391?


## How do I fix CVE-2026-90391?

Apply the vendor's patch.

## Is CVE-2026-90391 being exploited?

There is no known exploitation of CVE-2026-90391 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90391 and fix it if it is affected.

What it is: CVE-2026-90391 is a denial of service vulnerability in Linux, published on 17 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-2026-90391 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-2026-90391/ (Markdown: https://insights.sbomify.com/CVE-2026-90391/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90391
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90391
- OSV: https://osv.dev/vulnerability/CVE-2026-90391

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90391/
Summaries are generated and may contain mistakes; the official description above is the record.