[PATCH] x86/kvm: disable fast MMIO when running nested

Vitaly Kuznetsov
2018-01-24T15:12:50Z

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 disabling fast mmio when running nested.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
arch/x86/kvm/vmx.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c829d89e2e63..54afb446f38e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6558,9 +6558,16 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
/*
* A nested guest cannot optimize MMIO vmexits, because we have an
* nGPA here instead of the required GPA.
+ * Skipping instruction below 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.
*/
gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
- if (!is_guest_mode(vcpu) &&
+ if (!static_cpu_has(X86_FEATURE_HYPERVISOR) && !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);
--
2.14.3