[ Backport of 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 [mhaboustak: backport to 4.9.y] Signed-off-by: Mike Haboustak haboustak@gmail.com --- arch/x86/kvm/vmx.c | 19 +++++++++++++++++-- arch/x86/kvm/x86.c | 3 ++- 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 011050820608..9446a3a2fc69 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -6548,9 +6548,24 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)

gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS); if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) { - skip_emulated_instruction(vcpu); trace_kvm_fast_mmio(gpa); - return 1; + /* + * 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)) { + skip_emulated_instruction(vcpu); + return 1; + } + else + return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP, + NULL, 0) == EMULATE_DONE; }

ret = handle_mmio_page_fault(vcpu, gpa, true); diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 27d13b870e07..46e0ad71b4da 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5707,7 +5707,8 @@ 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;