From 33d3617a52f9930d22b2af59f813c2fbdefa6dd5 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Tue, 3 Mar 2026 16:22:23 -0800 Subject: [PATCH] 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 Reviewed-by: Vitaly Kuznetsov Link: https://patch.msgid.link/20260304002223.1105129-3-seanjc@google.com Signed-off-by: Sean Christopherson --- arch/x86/kvm/svm/hyperv.h | 4 ---- arch/x86/kvm/svm/nested.c | 7 ------- 2 files changed, 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];