# CVE-2026-23100: mm/hugetlb: fix hugetlb_pmd_shared()

> Incorrect shared PMD detection in Linux kernel hugetlb allows non-exclusive folio migration and incorrect memory accounting, potentially leading to data integrity issues.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 4 Feb 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-80285, BELL-CVE-2026-23100, DEBIAN-CVE-2026-23100, GHSA-4rhr-9xj2-x9gx, ROOT-OS-DEBIAN-11-CVE-2026-23100, ROOT-OS-DEBIAN-12-CVE-2026-23100, ROOT-OS-DEBIAN-13-CVE-2026-23100, ROOT-OS-UBUNTU-2204-CVE-2026-23100, ROOT-OS-UBUNTU-2404-CVE-2026-23100, UBUNTU-CVE-2026-23100

## What is CVE-2026-23100?

CVE-2026-23100 is a medium-severity vulnerability in Linux kernel, published on 4 Feb 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/hugetlb: fix hugetlb_pmd_shared()

Patch series "mm/hugetlb: fixes for PMD table sharing (incl.  using
mmu_gather)", v3.

One functional fix, one performance regression fix, and two related
comment fixes.

I cleaned up my prototype I recently shared [1] for the performance fix,
deferring most of the cleanups I had in the prototype to a later point. 
While doing that I identified the other things.

The goal of this patch set is to be backported to stable trees "fairly"
easily. At least patch #1 and #4.

Patch #1 fixes hugetlb_pmd_shared() not detecting any sharing
Patch #2 + #3 are simple comment fixes that patch #4 interacts with.
Patch #4 is a fix for the reported performance regression due to excessive
IPI broadcasts during fork()+exit().

The last patch is all about TLB flushes, IPIs and mmu_gather.
Read: complicated

There are plenty of cleanups in the future to be had + one reasonable
optimization on x86. But that's all out of scope for this series.

Runtime tested, with a focus on fixing the performance regression using
the original reproducer [2] on x86.

This patch (of 4):

We switched from (wrongly) using the page count to an independent shared
count.  Now, shared page tables have a refcount of 1 (excluding
speculative references) and instead use ptdesc->pt_share_count to identify
sharing.

We didn't convert hugetlb_pmd_shared(), so right now, we would never
detect a shared PMD table as such, because sharing/unsharing no longer
touches the refcount of a PMD table.

Page migration, like mbind() or migrate_pages() would allow for migrating
folios mapped into such shared PMD tables, even though the folios are not
exclusive.  In smaps we would account them as "private" although they are
"shared", and we would be wrongly setting the PM_MMAP_EXCLUSIVE in the
pagemap interface.

Fix it by properly using ptdesc_pmd_is_shared() in hugetlb_pmd_shared().

## Which versions are affected by CVE-2026-23100?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23100?

Apply the vendor's patch.

## Is CVE-2026-23100 being exploited?

There is no known exploitation of CVE-2026-23100 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-2026-23100 and fix it if it is affected.

What it is: CVE-2026-23100 is a medium-severity vulnerability in Linux kernel, published on 4 Feb 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-2026-23100 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-2026-23100/ (Markdown: https://insights.sbomify.com/CVE-2026-23100/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23100
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23100
- OSV: https://osv.dev/vulnerability/CVE-2026-23100

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23100/
Summaries are generated and may contain mistakes; the official description above is the record.