CVE-2026-89930: KVM: nVMX: Service local TLB flushes on failed nested VM-Enter

In the Linux kernel, the following vulnerability has been resolved:

KVM: nVMX: Service local TLB flushes on failed nested VM-Enter

KVM services local TLB flushes on "full" nested VM-Exits (through nestedvmxvmexit()), but not if a nested VM-Enter fails (e.g. due to failed VMCS checks in nestedvmxenternonrootmode()).

However, it is possible that KVM had queued TLB flushes that need to be performed, even if the nested VM-Enter was not successful. For example, if VPID is disabled for L2 (via nestedvmxtransitiontlbflush(), or if via the MSR load lists, as the SDM says:

If any MSR is being loaded in such a way that would architecturally require a TLB flush, the TLBs are updated so that, after VM entry, the logical processor will not use any translations that were cached before the transition.

The SDM is unclear about when the TLB flush should occur, and whether or not a failed VM entry would flush the TLB, so it is safer to always do the TLB flush in this case.

More concretely, KVM also updates the last VPID L1 used for L2 in nestedvmxtransitiontlbflush() (i.e. lastvpid), even if the VM entry ultimately fails. With the current code, KVM could miss a TLB flush if L1 changes L2's VPID, then does a failed VM entry followed by a successful one, as the failed VM entry would update lastvpid but not actually flush the TLB. Servicing local TLB flushes on failed VM entries makes sure that the TLB is always flushed when lastvpid is updated.