From: Vitaly Kuznetsov vkuznets@redhat.com
[ Upstream commit d391f1207067268261add0485f0f34503539c5b0 ]
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: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae 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 Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- arch/x86/kvm/vmx.c | 16 +++++++++++++++- arch/x86/kvm/x86.c | 3 ++- 2 files changed, 17 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index ae4803b213d0..bdd84ce4491e 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -6765,7 +6765,21 @@ 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); - 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); diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index d7728bcd9a3c..3b2c3aa2cd07 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5699,7 +5699,8 @@ 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;