# CVE-2025-21939: drm/xe/hmm: Don't dereference struct page pointers without notifier lock

> A null pointer dereference in the Linux kernel's Xe GPU driver can cause a system crash when accessing page structures without proper locking.

- **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 Apr 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-21939, DEBIAN-CVE-2025-21939, GHSA-5jw6-g45h-9h97, UBUNTU-CVE-2025-21939

## What is CVE-2025-21939?

CVE-2025-21939 is a high-severity vulnerability in Linux kernel, published on 1 Apr 2025. 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:

drm/xe/hmm: Don't dereference struct page pointers without notifier lock

The pnfs that we obtain from hmm_range_fault() point to pages that
we don't have a reference on, and the guarantee that they are still
in the cpu page-tables is that the notifier lock must be held and the
notifier seqno is still valid.

So while building the sg table and marking the pages accesses / dirty
we need to hold this lock with a validated seqno.

However, the lock is reclaim tainted which makes
sg_alloc_table_from_pages_segment() unusable, since it internally
allocates memory.

Instead build the sg-table manually. For the non-iommu case
this might lead to fewer coalesces, but if that's a problem it can
be fixed up later in the resource cursor code. For the iommu case,
the whole sg-table may still be coalesced to a single contigous
device va region.

This avoids marking pages that we don't own dirty and accessed, and
it also avoid dereferencing struct pages that we don't own.

v2:
- Use assert to check whether hmm pfns are valid (Matthew Auld)
- Take into account that large pages may cross range boundaries
  (Matthew Auld)

v3:
- Don't unnecessarily check for a non-freed sg-table. (Matthew Auld)
- Add a missing up_read() in an error path. (Matthew Auld)

(cherry picked from commit ea3e66d280ce2576664a862693d1da8fd324c317)

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2025-21939?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21939?

Apply the vendor's patch.

## Is CVE-2025-21939 being exploited?

There is no known exploitation of CVE-2025-21939 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-2025-21939 and fix it if it is affected.

What it is: CVE-2025-21939 is a high-severity vulnerability in Linux kernel, published on 1 Apr 2025. 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-2025-21939 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-2025-21939/ (Markdown: https://insights.sbomify.com/CVE-2025-21939/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21939
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21939
- OSV: https://osv.dev/vulnerability/CVE-2025-21939

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21939/
Summaries are generated and may contain mistakes; the official description above is the record.