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x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

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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: 68c3b4d167
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>
This commit is contained in:
Vitaly Kuznetsov 2018-01-25 16:37:07 +01:00 committed by Radim Krčmář
parent e46b469278
commit d391f12070
2 changed files with 17 additions and 2 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

16
arch/x86/kvm/vmx.c
View file

@ -6577,7 +6577,21 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
if (!is_guest_mode(vcpu) && if (!is_guest_mode(vcpu) &&
!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) { !kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
trace_kvm_fast_mmio(gpa); trace_kvm_fast_mmio(gpa);
return kvm_skip_emulated_instruction(vcpu); /*
* 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); ret = kvm_mmu_page_fault(vcpu, gpa, PFERR_RSVD_MASK, NULL, 0);

3
arch/x86/kvm/x86.c
View file

@ -5773,7 +5773,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
* handle watchpoints yet, those would be handled in * handle watchpoints yet, those would be handled in
* the emulate_ops. * the emulate_ops.
*/ */
if (kvm_vcpu_check_breakpoint(vcpu, &r)) if (!(emulation_type & EMULTYPE_SKIP) &&
kvm_vcpu_check_breakpoint(vcpu, &r))
return r; return r;
ctxt->interruptibility = 0; ctxt->interruptibility = 0;