Cloudanix VulnDB

CVE-2026-99082

High · 8.8

CVSS

8.8

EPSS

0.2%

Published

Oct 23, 2026

CISA due date

—

Cloudanix analysis

Next patch cycle SSVC: Track

A bug in the Linux kernel's handling of virtual-machine memory can let one guest interfere with the hypervisor's memory protections, potentially allowing it to affect other guests or the host itself.

Exploitation
Local Privilege
Attacker needs
Low Privilege
Complexity
Low
Impact
Full Compromise
Component
OS kernel
Class
Privilege Escalation
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

The flaw allows a guest to cause KVM to use stale MMIO cache entries across SMM address spaces, bypassing write-tracking so that a VMM write to a page backing a nested EPT table can leave KVM using invalid shadow translations.

Why it matters to the business

In virtualized environments this can let a compromised guest corrupt hypervisor memory structures or escape isolation, threatening the confidentiality and integrity of other guests and the host.

How an attack unfolds

An attacker running as a guest enters SMM to populate the MMIO cache, exits SMM, modifies only the SMM memslots, then triggers an access that KVM treats as an MMIO exit; completing the exit from userspace writes the backing page without KVM's write-protection path, leaving stale nested EPT translations in use.

How to fix it

Upgrade the Linux kernel to a version containing the fix that calls kvm_vcpu_memslots() instead of kvm_memslots() for the MMIO cache; apply the change from the referenced commit that ensures active memslots are used for both fill and validation paths.

How to confirm the fix worked

Run the x86/smm_test, set_memory_region_test, and memslot_modification_stress_test selftests; confirm absence of the kvm_mmu_write_protect_fault() warning after memslot updates in SMM; verify the memslot generation check now uses per-vCPU active memslots.

Typical attack path

  1. Gain code execution inside a KVM guest
  2. Enter SMM and populate the per-vCPU MMIO cache
  3. Exit SMM and update only SMM memslots
  4. Trigger a memory access that produces KVM_EXIT_MMIO
  5. Complete the exit from the VMM address space to bypass write tracking
  6. Continue using stale shadow translations for nested EPT

How to detect it

Search hypervisor kernel logs for the kvm_mmu_write_protect_fault() warning; inspect guest-triggered KVM_EXIT_MMIO exits that occur immediately after SMM memslot changes; correlate with guest SMM entry/exit events in tracepoints.

  • kvm_mmu_write_protect_fault
  • KVM_EXIT_MMIO.*SMM
  • memslot generation mismatch

What your security team should do

  • Prioritize kernel updates on all KVM hosts within the next maintenance window
  • Monitor for the kvm_mmu_write_protect_fault() warning in dmesg on production hosts
  • Restrict guest access to SMM unless required by workload
  • Review nested virtualization configurations for additional hardening

The record

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.

Affected packages

Ecosystem Package Introduced Fixed