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

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 ]------------ isshadowpresentpte(sptep) WARNING: arch/x86/kvm/mmu/mmu.c:484 at markmmiospte+0xb2/0xc0 [kvm], CPU#0: vmxeptstaler/4292 Modules linked in: kvmintel kvm irqbypass CPU: 0 UID: 1000 PID: 4292 Comm: vmxeptstaler 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:markmmiospte+0xb2/0xc0 [kvm] Call Trace: TASK mmusetspte+0x237/0x440 [kvm] eptpagefault+0x535/0x7f0 [kvm] kvmmmudopagefault+0xee/0x1f0 [kvm] kvmmmupagefault+0x8d/0x620 [kvm] vmxhandleexit+0x18c/0x5a0 [kvmintel] kvmarchvcpuioctlrun+0xc55/0x1c20 [kvm] kvmvcpuioctl+0x2d5/0x980 [kvm] x64sysioctl+0x8a/0xd0 dosyscall64+0xb5/0x730 entrySYSCALL64afterhwframe+0x4b/0x53 RIP: 0033:0x47fa3f /TASK ---[ end trace 0000000000000000 ]---

— IBM