KVM: nSVM: Always intercept VMMCALL when L2 is active

Always intercept VMMCALL now that KVM properly synthesizes a #UD as
appropriate, i.e. when L1 doesn't want to intercept VMMCALL, to avoid
putting L2 into an infinite #UD loop if KVM_X86_QUIRK_FIX_HYPERCALL_INSN
is enabled.

By letting L2 execute VMMCALL natively and thus #UD, for all intents and
purposes KVM morphs the VMMCALL intercept into a #UD intercept (KVM always
intercepts #UD).  When the hypercall quirk is enabled, KVM "emulates"
VMMCALL in response to the #UD by trying to fixup the opcode to the "right"
vendor, then restarts the guest, without skipping the VMMCALL.  As a
result, the guest sees an endless stream of #UDs since it's already
executing the correct vendor hypercall instruction, i.e. the emulator
doesn't anticipate that the #UD could be due to lack of interception, as
opposed to a truly undefined opcode.

Fixes: 0d945bd93511 ("KVM: SVM: Don't allow nested guest to VMMCALL into host")
Cc: stable@vger.kernel.org
Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://patch.msgid.link/20260304002223.1105129-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

2 files changed, 0 insertions, 11 deletions

diff --git a/arch/x86/kvm/svm/hyperv.h b/arch/x86/kvm/svm/hyperv.h
index 9af03970d40c..f70d076911a6 100644
--- a/arch/x86/kvm/svm/hyperv.h
+++ b/arch/x86/kvm/svm/hyperv.h
@@ -51,10 +51,6 @@ static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
 void svm_hv_inject_synthetic_vmexit_post_tlb_flush(struct kvm_vcpu *vcpu);
 #else /* CONFIG_KVM_HYPERV */
 static inline void nested_svm_hv_update_vm_vp_ids(struct kvm_vcpu *vcpu) {}
-static inline bool nested_svm_l2_tlb_flush_enabled(struct kvm_vcpu *vcpu)
-{
-	return false;
-}
 static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
 {
 	return false;
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 0f7893a7cb04..fb86f09985e7 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -158,13 +158,6 @@ void recalc_intercepts(struct vcpu_svm *svm)
 			vmcb_clr_intercept(c, INTERCEPT_VINTR);
 	}
 
-	/*
-	 * We want to see VMMCALLs from a nested guest only when Hyper-V L2 TLB
-	 * flush feature is enabled.
-	 */
-	if (!nested_svm_l2_tlb_flush_enabled(&svm->vcpu))
-		vmcb_clr_intercept(c, INTERCEPT_VMMCALL);
-
 	for (i = 0; i < MAX_INTERCEPT; i++)
 		c->intercepts[i] |= g->intercepts[i];