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Commit d391f120 authored by Vitaly Kuznetsov's avatar Vitaly Kuznetsov Committed by Radim Krčmář
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x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested 



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: 68c3b4d1
Suggested-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
Suggested-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: default avatarMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
parent e46b4692

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;