Re: [Bug 221841] KVM: nested VMX eVMCS VMPTRLD/VMPTRST causes infinite VM-Exit loop due to missing RIP advance

On Fri, Aug 07, 2026, [email protected] wrote:
> > > The eVMCS path should behave similarly to other unsupported nested VMX
> > > instructions.
> > 
> > Only if the TLFS allows it.  I assume it just says "unsupported" or
> > "undefined behavior", i.e. KVM can probably do whatever it wants.  Vitaly?
> 
> VMREAD/VMWRITE with eVMCS are explicitly called 'unsupported' in the TLFS:
> 
> "Any VMREAD or VMWRITE instructions while an enlightened VMCS is active is
> unsupported and can result in unexpected behavior.". 
> 
> As for VMPTRLD, we only have
> "No VMPTRLD instruction must be executed to make an enlightened VMCS active or
> current." but there's nothing about the expected result there. VMPTRST is not
> mentioned. 
> 
> So I guess we can do whatever we want in KVM and skipping the instruction is
> not a bad thing. Ideally, we can try and see what genuine Hyper-V gives us in
> this situation (by e.g. running an instrumented version of KVM there).

Heh, I agree, so long as Hyper-V's behavior is to either signal VM-Fail or inject
an exception of some kind.  If Hyper-V loops (like KVM does) or skips the
instruction without synthesizing an error of some kind, then I think we should
diverge from Hyper-V, as not signalling failure of some kind is pretty gross.

If we diverge from Hyper-V, think my vote would be to inject #UD or #GP, because
for all intents and purposes VMPTR{LD,ST} become unsupported instructions.  E.g.

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 7ed79894d11d..e743c892eeb8 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5888,6 +5888,12 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
        if (!nested_vmx_check_permission(vcpu))
                return 1;
 
+       /* Forbid normal VMPTRLD if Enlightened version was used */
+       if (nested_vmx_is_evmptr12_valid(vmx)) {
+               kvm_queue_exception(vcpu, UD_VECTOR);
+               return 1;
+       }
+
        if (nested_vmx_get_vmptr(vcpu, &vmptr, &r))
                return r;
 
@@ -5897,10 +5903,6 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
        if (vmptr == vmx->nested.vmxon_ptr)
                return nested_vmx_fail(vcpu, VMXERR_VMPTRLD_VMXON_POINTER);
 
-       /* Forbid normal VMPTRLD if Enlightened version was used */
-       if (nested_vmx_is_evmptr12_valid(vmx))
-               return 1;
-
        if (vmx->nested.current_vmptr != vmptr) {
                struct gfn_to_hva_cache *ghc = &vmx->nested.vmcs12_cache;
                struct vmcs_hdr hdr;
@@ -5961,8 +5963,10 @@ static int handle_vmptrst(struct kvm_vcpu *vcpu)
        if (!nested_vmx_check_permission(vcpu))
                return 1;
 
-       if (unlikely(nested_vmx_is_evmptr12_valid(to_vmx(vcpu))))
+       if (unlikely(nested_vmx_is_evmptr12_valid(to_vmx(vcpu)))) {
+               kvm_queue_exception(vcpu, UD_VECTOR);
                return 1;
+       }
 
        if (get_vmx_mem_address(vcpu, exit_qual, instr_info,
                                true, sizeof(gpa_t), &gva))