# CVE-2025-39992: mm: swap: check for stable address space before operating on the VMA

> A race condition in the Linux kernel swap subsystem causes a null pointer dereference, leading to a system crash.

- **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:** 15 Oct 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-39992, DEBIAN-CVE-2025-39992, GHSA-fcm8-3qw6-4427, ROOT-OS-DEBIAN-13-CVE-2025-39992, ROOT-OS-UBUNTU-2204-CVE-2025-39992, ROOT-OS-UBUNTU-2404-CVE-2025-39992, UBUNTU-CVE-2025-39992

## What is CVE-2025-39992?

CVE-2025-39992 is a vulnerability in Linux, published on 15 Oct 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:

mm: swap: check for stable address space before operating on the VMA

It is possible to hit a zero entry while traversing the vmas in unuse_mm()
called from swapoff path and accessing it causes the OOPS:

Unable to handle kernel NULL pointer dereference at virtual address
0000000000000446--> Loading the memory from offset 0x40 on the
XA_ZERO_ENTRY as address.
Mem abort info:
  ESR = 0x0000000096000005
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x05: level 1 translation fault

The issue is manifested from the below race between the fork() on a
process and swapoff:
fork(dup_mmap())			swapoff(unuse_mm)
---------------                         -----------------
1) Identical mtree is built using
   __mt_dup().

2) copy_pte_range()-->
	copy_nonpresent_pte():
       The dst mm is added into the
    mmlist to be visible to the
    swapoff operation.

3) Fatal signal is sent to the parent
process(which is the current during the
fork) thus skip the duplication of the
vmas and mark the vma range with
XA_ZERO_ENTRY as a marker for this process
that helps during exit_mmap().

				     4) swapoff is tried on the
					'mm' added to the 'mmlist' as
					part of the 2.

				     5) unuse_mm(), that iterates
					through the vma's of this 'mm'
					will hit the non-NULL zero entry
					and operating on this zero entry
					as a vma is resulting into the
					oops.

The proper fix would be around not exposing this partially-valid tree to
others when droping the mmap lock, which is being solved with [1].  A
simpler solution would be checking for MMF_UNSTABLE, as it is set if
mm_struct is not fully initialized in dup_mmap().

Thanks to Liam/Lorenzo/David for all the suggestions in fixing this
issue.

## Which versions are affected by CVE-2025-39992?


## How do I fix CVE-2025-39992?

Apply the vendor's patch.

## Is CVE-2025-39992 being exploited?

There is no known exploitation of CVE-2025-39992 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39992 and fix it if it is affected.

What it is: CVE-2025-39992 is a vulnerability in Linux, published on 15 Oct 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-2025-39992 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-2025-39992/ (Markdown: https://insights.sbomify.com/CVE-2025-39992/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39992
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39992
- OSV: https://osv.dev/vulnerability/CVE-2025-39992

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39992/
Summaries are generated and may contain mistakes; the official description above is the record.