# CVE-2026-63950: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

> A logic error in the Linux kernel's memory management can cause refcount corruption and kernel crashes when unmapping specific anonymous folios.

- **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:** 19 Jul 2026
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2026-63950, DEBIAN-CVE-2026-63950, GHSA-9fqf-4mvh-6fx9, UBUNTU-CVE-2026-63950

## What is CVE-2026-63950?

CVE-2026-63950 is a high-severity memory corruption vulnerability in Linux, published on 19 Jul 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:

mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

Initialize nr_pages to 1 at the start of each loop iteration, like
folio_referenced_one() does.

Without this, nr_pages computed by a previous folio_unmap_pte_batch() call
can be reused on a later iteration that does not run
folio_unmap_pte_batch() again.

mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call
madvise(MADV_FREE), then make the last page device-exclusive via
HMM_DMIRROR_EXCLUSIVE.

Trigger node reclaim through sysfs.  Now, in try_to_unmap_one(), we will
first clear the first 15 out of 16 entries mapping the lazyfree folio. 
This will set nr_pages to 15.  In the next pvmw walk, this nr_pages gets
reused on a device-exclusive pte, thus potentially corrupting folio
refcount/mapcount.

At the moment, I have a userspace program which can make the kernel spit
out a trace, but the blow up is in folio_referenced_one(), because there
are existing bugs in the interaction between device-private and rmap
(which too I am investigating).  I did a one liner kernel change to avoid
going into folio_referenced_one(), and the kernel blows up at
folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted.

Note that the bug is there not since file folio batching but lazyfree
folio batching, since device-exclusive only works for anonymous folios.

Userspace visible effect is simply kernel crashing somewhere due to
refcount/mapcount corruption.

## Which versions are affected by CVE-2026-63950?


## How do I fix CVE-2026-63950?

Apply the vendor's patch.

## Is CVE-2026-63950 being exploited?

There is no known exploitation of CVE-2026-63950 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63950 and fix it if it is affected.

What it is: CVE-2026-63950 is a high-severity memory corruption vulnerability in Linux, published on 19 Jul 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-2026-63950 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-2026-63950/ (Markdown: https://insights.sbomify.com/CVE-2026-63950/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63950
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63950
- OSV: https://osv.dev/vulnerability/CVE-2026-63950

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63950/
Summaries are generated and may contain mistakes; the official description above is the record.