# CVE-2026-63875: arm64: tlb: Flush walk cache when unsharing PMD tables

> A missing TLB walk cache flush in the Linux kernel arm64 subsystem could lead to incorrect page table walks and potential memory corruption.

- **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:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-63875, DEBIAN-CVE-2026-63875, GHSA-h9hg-3wv2-x6j7, ROOT-OS-UBUNTU-2204-CVE-2026-63875, UBUNTU-CVE-2026-63875

## What is CVE-2026-63875?

CVE-2026-63875 is a high-severity 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:

arm64: tlb: Flush walk cache when unsharing PMD tables

When huge_pmd_unshare() is called to unshare a PMD table, the
tlb_unshare_pmd_ptdesc() function sets tlb->unshared_tables=true
but the aarch64 tlb_flush() only checked tlb->freed_tables to
determine whether to use TLBF_NONE (vae1is, invalidates walk
cache) or TLBF_NOWALKCACHE (vale1is, leaf-only).

This caused the stale PMD page table entry to remain in the walk cache
after unshare, potentially leading to incorrect page table walks.

Fix by including unshared_tables in the check, so that when
unsharing tables, TLBF_NONE is used and the walk cache is properly
invalidated.

Here is the detailed distinction between vae1is and vale1is:

| Instruction Combination  | Actual Invalidation Scope                         |
| ------------------------ | --------------------------------------------------|
| `VAE1IS`  + TTL=`0`      | All entries at all levels (full invalidation)     |
| `VAE1IS`  + TTL=`2` (L2) | Non-leaf at Level 0/1 + leaf at Level 2           |
| `VALE1IS` + TTL=`0`      | Leaf entries at all levels (non-leaf not cleared) |
| `VALE1IS` + TTL=`2` (L2) | Leaf entry at Level 2 only                        |

Weakness: CWE-459 (Incomplete Cleanup)

## Which versions are affected by CVE-2026-63875?


## How do I fix CVE-2026-63875?

Apply the vendor's patch.

## Is CVE-2026-63875 being exploited?

There is no known exploitation of CVE-2026-63875 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63875 and fix it if it is affected.

What it is: CVE-2026-63875 is a high-severity 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-2026-63875 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-2026-63875/ (Markdown: https://insights.sbomify.com/CVE-2026-63875/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63875
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63875
- OSV: https://osv.dev/vulnerability/CVE-2026-63875

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63875/
Summaries are generated and may contain mistakes; the official description above is the record.