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Linux kernel KVM x86: Stale MMIO cache entry after memslot update

CVE-2026-99082High 8.8

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Summary

The KVM x86 code of the Linux kernel uses the wrong memory slots for the per-vCPU MMIO cache when a vCPU runs in System Management Mode (SMM). After the vCPU returns to the normal address space, an outdated cache entry can still be treated as valid. A local guest with low privileges on the host can use this to make RAM accesses be handled as MMIO exits, which can corrupt shadow page translations and may cause memory corruption or a denial of service on the host. The CVE record scores it 8.8, while Red Hat scores it 5.6 because it rates the impact and attack complexity differently.

Affected

  • Linux kernel from commit 699023e23965 (added SMM address space support) onward, in the stable branches listed below
  • Unaffected per CVE record: 5.10.273 and later in 5.10.x, 5.15.224 and later in 5.15.x, 6.1.191 and later in 6.1.x, 6.6.160 and later in 6.6.x, 6.12.114 and later in 6.12.x, 6.18.58 and later in 6.18.x, 7.2.12 and later in 7.2.x
  • Distribution kernels: Unknown. Check the advisories of your distribution.

Fixed in

  • Linux kernel 5.10.273, 5.15.224, 6.1.191, 6.6.160, 6.12.114, 6.18.58 or 7.2.12 and later (per CVE record and OSV)
  • Distribution kernel packages: Unknown.

Workaround

No vendor workaround documented.

Exploitation

  • Exploited in the wild: Not known.
  • Public exploit code: None indexed in Metasploit or Exploit-DB.
  • The upstream commit includes a kernel selftest that reproduces the condition; no exploit code is published.

Our recommendation

  • Update hypervisor host kernels to a fixed version as soon as your distribution provides it.
  • Prioritise hosts that run untrusted or multi-tenant guests; single-tenant hosts with trusted guests carry lower risk.
  • Check the kernel version of each KVM host and confirm that the distribution backport is installed.
  • Monitor the vendor advisories until the packages for your distribution are released.

Public exploits

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 31 Oct 2026, 10:42.

Original description (NVD)

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.

NVD references (8)

Sources

The tracker combines official, freely usable data sources – updated continuously:

This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.

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