x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested (d391f120) · Commits · 戴 / test

I was investigating an issue with seabios >= 1.10 which stopped working for nested KVM on Hyper-V. The problem appears to be in handle_ept_violation() function: when we do fast mmio we need to skip the instruction so we do kvm_skip_emulated_instruction(). This, however, depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS. However, this is not the case. Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when EPT MISCONFIG occurs. While on real hardware it was observed to be set, some hypervisors follow the spec and don't set it; we end up advancing IP with some random value. I checked with Microsoft and they confirmed they don't fill VM_EXIT_INSTRUCTION_LEN on EPT MISCONFIG. Fix the issue by doing instruction skip through emulator when running nested. Fixes: Suggested-by:

Radim Krčmář <rkrcmar@redhat.com> Suggested-by:

Paolo Bonzini <pbonzini@redhat.com> Signed-off-by:

Vitaly Kuznetsov <vkuznets@redhat.com> Acked-by:

Michael S. Tsirkin <mst@redhat.com> Signed-off-by:

Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -6577,7 +6577,21 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
		if (!is_guest_mode(vcpu) &&
		!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
		trace_kvm_fast_mmio(gpa);
		/*
		* Doing kvm_skip_emulated_instruction() depends on undefined
		* behavior: Intel's manual doesn't mandate
		* VM_EXIT_INSTRUCTION_LEN to be set in VMCS when EPT MISCONFIG
		* occurs and while on real hardware it was observed to be set,
		* other hypervisors (namely Hyper-V) don't set it, we end up
		* advancing IP with some random value. Disable fast mmio when
		* running nested and keep it for real hardware in hope that
		* VM_EXIT_INSTRUCTION_LEN will always be set correctly.
		*/
		if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
		return kvm_skip_emulated_instruction(vcpu);
		else
		return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
		NULL, 0) == EMULATE_DONE;
		}

		ret = kvm_mmu_page_fault(vcpu, gpa, PFERR_RSVD_MASK, NULL, 0);

arch/x86/kvm/x86.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -5773,7 +5773,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
		* handle watchpoints yet, those would be handled in
		* the emulate_ops.
		*/
		if (kvm_vcpu_check_breakpoint(vcpu, &r))
		if (!(emulation_type & EMULTYPE_SKIP) &&
		kvm_vcpu_check_breakpoint(vcpu, &r))
		return r;

		ctxt->interruptibility = 0;

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