CVE-2026-99082: Linux Vulnerability (CVSS 8.8)

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.

Metrics

CVSS 3.1

8.8/10

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS Probability

0.16%

4.7th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Source: CNA advisory (CVE.org). NVD analysis pending.

Vendor	Product	Versions
Linux	Linux	>= 699023e239658e62da6f42f47d31b54788521ec1, < 485a5286675b15b00ee850d90eaa178cc588445f; >= 699023e239658e62da6f42f47d31b54788521ec1, < 445e3369ac8cc1b18aedc8e8f977c68a0a326fa9; >= 699023e239658e62da6f42f47d31b54788521ec1, < 1c77d33d85786d36e56ca1b580fb55347fd65b47; >= 699023e239658e62da6f42f47d31b54788521ec1, < a63f79d9da9468952396af73e1e53330845cf876; >= 699023e239658e62da6f42f47d31b54788521ec1, < 7fd4e64f4bbc8c35ae8f0d2a1fb5b77a76d5b4fd; >= 699023e239658e62da6f42f47d31b54788521ec1, < 2566780c684c9c21241d409d790087c73070f312; >= 699023e239658e62da6f42f47d31b54788521ec1, < e792a9ace39d2812a5b342a1987186c7bc653204; >= 699023e239658e62da6f42f47d31b54788521ec1, < b3e6212dea2e4328a841ceb37f377583176dc735
Linux	Linux	4.2

References

Timeline

Published: Oct 23, 2026
Last Modified: Oct 30, 2026
Status: Received

Frequently Asked Questions

What is CVE-2026-99082?

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.

How severe is CVE-2026-99082?

CVE-2026-99082 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 0.16% probability of exploitation in the next 30 days.

How do I fix CVE-2026-99082?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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