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

[ 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index ae4803b213d070a1d5000d69706ba270d4dd6988..bdd84ce4491e5375ffab668b2d9fe01207173fc7 100644 (file)
--- 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 d7728bcd9a3c7d890faf974eea6b6021e445e099..3b2c3aa2cd076bdc7c4e23bc6befe4a6e8ff40b3 100644 (file)
--- 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;