Re: [PATCH v2] KVM: x86: Mark nested RSM-created VM-Enter as untrusted

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 21, 2026, sashiko-bot@kernel.org wrote:
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 4d2bacd00ec4f..f54757bd83821 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> 
> [ ... ]
> 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index cc75feec05daa..333072cc9bce4 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6720,7 +6720,8 @@ static int __vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
> >  	 * invalid guest state should never happen as that means KVM knowingly
> >  	 * allowed a nested VM-Enter with an invalid vmcs12.  More below.
> >  	 */
> > -	if (KVM_BUG_ON(vcpu->arch.nested_run_pending, vcpu->kvm))
> > +	if (KVM_BUG_ON(vcpu->arch.nested_run_pending == KVM_NESTED_RUN_PENDING,
> > +		       vcpu->kvm))
> >  		return -EIO;
> 
> [Severity: High]
> Does this allow the KVM_NESTED_RUN_PENDING_UNTRUSTED flag to leak and lock up
> the vCPU?
> 
> If a malicious L2 guest modifies SMRAM to create an invalid nested state, then
> executes RSM, vmx_leave_smm() restores this state and sets nested_run_pending
> to KVM_NESTED_RUN_PENDING_UNTRUSTED.

LOL, that is an impressively convoluted hallucination from Sashiko.  SMI is an
unconditional VM-Exit on Intel, and KVM doesn't support emulating parallel SMM,
i.e. it's impossible for L2 to have access to "live" SMRAM.

That said, leaving nested_run_pending set isn't good, and I missed that my
suggested "fix" would require tweaking the above KVM_BUG_ON().  I'll respond to
v3 with an alternative solution.