# CVE-2026-99082: KVM: x86: Use active memslots for the per-vCPU MMIO cache


- **Severity:** High 8.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 24 Oct 2026
- **Also known as:** AZL-110987, BELL-CVE-2026-99082, DEBIAN-CVE-2026-99082, GHSA-58r9-ch9w-px7w, UBUNTU-CVE-2026-99082

## What is CVE-2026-99082?

CVE-2026-99082 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 

### Official description

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

KVM: x86: Use active memslots for the per-vCPU MMIO cache

KVM tags the per-vCPU MMIO cache with a memslot generation so that a
memslot update invalidates cached MMIO information.  Both the fill and
validation paths use kvm_memslots(), which unconditionally selects address
space 0, even when the vCPU is running in SMM and using address space 1.

Commit 56f17dd3fbc4 ("kvm: x86: fix stale mmio cache bug") added the
memslot generation to the cache key so that a memslot update could not
leave a stale entry valid.  When commit 699023e23965 ("KVM: x86: add SMM
to the MMU role, support SMRAM address space") added the SMM address space,
these helpers were not converted to use the active memslots.

Consequently, an update to the SMM memslots can leave an entry from the old
SMM address space apparently valid after the vCPU returns to the normal
address space.  A guest can then cause an access to valid RAM at the same
GFN to be returned to userspace as KVM_EXIT_MMIO.

Completing that exit through the VMM's RAM address space writes the backing
page without going through KVM's write-tracking path.  If the page backs a
nested EPT table, KVM can continue using shadow translations derived from
the old contents.  The WARN_ON_ONCE() in kvm_mmu_write_protect_fault()
catches this invalid cache and memslot combination.

Use the vCPU's active memslots when caching and validating the entry.
Memslot generations are unique across address spaces, so the generation
also distinguishes the normal and SMM views.  This matches the MMIO SPTE
cache, which already uses kvm_vcpu_memslots().

On an unpatched current-mainline kernel, a regression test that fills the
cache in SMM and updates only the SMM memslots fails the intended RAM write
and triggers the kvm_mmu_write_protect_fault() warning.  With this change,
the same test completes the RAM write without a warning.  The x86/smm_test,
set_memory_region_test, and memslot_modification_stress_test selftests also
pass.

## Which versions are affected by CVE-2026-99082?


## How do I fix CVE-2026-99082?

Upgrade to a fixed version.

## Is CVE-2026-99082 being exploited?

There is no known exploitation of CVE-2026-99082 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99082 and fix it if it is affected.

What it is: CVE-2026-99082 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99082 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-2026-99082/ (Markdown: https://insights.sbomify.com/CVE-2026-99082/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99082
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99082
- OSV: https://osv.dev/vulnerability/CVE-2026-99082

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99082/
Summaries are generated and may contain mistakes; the official description above is the record.