# CVE-2026-98939: x86/mm: Drop unnecessary PMD page copy when freeing


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 23 Oct 2026
- **Also known as:** BELL-CVE-2026-98939, DEBIAN-CVE-2026-98939, GHSA-6vp7-hj7x-73jj, UBUNTU-CVE-2026-98939

## What is CVE-2026-98939?

CVE-2026-98939 is a vulnerability in Linux, published on 23 Oct 2026. 

### Official description

In the Linux kernel, the following vulnerability has been resolved:

x86/mm: Drop unnecessary PMD page copy when freeing

On a box with a discrete GPU, lockdep reports a possible deadlock as
soon as kswapd shrinks the TTM page pool. The immediate cause is an
x86 commit that added an mmap_read_lock() to kernel page protection
munging code.

The huge vmap code holds the same lock over a GFP_KERNEL allocation,
which is a no-no now that reclaim can take it. That allocation is in a
page table *free* path and ends up being for dubious purposes[1].
Basically, it tries to avoid hardware setting Accessed=1 in page table
entries that are unreachable by the hardware, a non-issue.

Remove the PMD copy. Detach the original PMD page at the PUD, flush
the mid-level caches, and free the PTE tables straight from the
detached PMD page. With no allocation left, the locking issue is gone.

Lockdep splat/analysis:

  WARNING: possible circular locking dependency detected
  7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G     U
  ------------------------------------------------------
  kswapd0/269 is trying to acquire lock:
  ((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0
  but task is already holding lock:
  (pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm]
  Chain exists of:
    (init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem

The cycle is built from three edges:

  1) pool_shrink_rwsem -> (init_mm).mmap_lock

     The TTM shrinker restores the caching attribute of every page it
     frees, while holding pool_shrink_rwsem:

       ttm_pool_shrink()
       -> ttm_pool_dispose_list()
       -> ttm_pool_free_page()
       -> set_pages_wb()
       -> change_page_attr_set_clr()   [ init_mm mmap read lock ]

  2) fs_reclaim -> pool_shrink_rwsem

     The same shrinker, called from reclaim.

  3) (init_mm).mmap_lock -> fs_reclaim

     ioremap() installing a huge PUD mapping over an existing PMD table:

       ioremap_page_range()
       -> vmap_range_noflush()
       -> vmap_try_huge_pud()          [ init_mm mmap read lock ]
       -> pud_free_pmd_page()
       -> __get_free_page(GFP_KERNEL)  [ enters reclaim ]

[ dhansen: Lots of changelog munging/trimming and merged comments from my
	   version of the fix. ]

## Which versions are affected by CVE-2026-98939?


## How do I fix CVE-2026-98939?

Upgrade to a fixed version.

## Is CVE-2026-98939 being exploited?

There is no known exploitation of CVE-2026-98939 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98939 and fix it if it is affected.

What it is: CVE-2026-98939 is a vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-98939 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-2026-98939/ (Markdown: https://insights.sbomify.com/CVE-2026-98939/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98939
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98939
- OSV: https://osv.dev/vulnerability/CVE-2026-98939

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98939/
Summaries are generated and may contain mistakes; the official description above is the record.