Re: [PATCH] KVM: nVMX: fix instruction skipping during emulated vm-entry

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-12-19 19:15-0800, Kyle Huey:
> On Mon, Dec 19, 2016 at 12:48 PM, David Matlack <dmatlack@google.com> wrote:
>> kvm_skip_emulated_instruction() should not be called after emulating
>> a VM-entry failure during or after loading guest state
>> (nested_vmx_entry_failure()). Otherwise the L1 hypervisor is resumed
>> some number of bytes past vmcs->host_rip.
> 
> Ah, I see.  Sorry for that regression.
> 
> These paths are supposed to trigger TF-induced singlestep exceptions
> though.  Quoting from the Intel SDM (Vol 3, Chapter 26)
> 
> "EFLAGS.TF = 1 causes a VM-entry instruction to generate a single-step
> debug exception only if failure of one of the checks in Section 26.1
> and Section 26.2 causes control to pass to the following instruction.
> A VM-entry does not generate a single-step debug exception in any of
> the following cases: (1) the instruction generates a fault; (2)
> failure of one of the checks in Section 26.3 or in loading MSRs causes
> processor state to be loaded from the hoststate area of the VMCS; or
> (3) the instruction passes all checks in Section 26.1, Section 26.2,
> and Section 26.3 and there is no failure in loading MSRs"

Changed cases are in section 26.3 => not generating #DB is correct,

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>

Sorry for missing that while applying, I wonder if there is a reason why
we didn't check them after enter_guest_mode() ...

>> Fixes: eb2775621701e6ee3ea2a474437d04e93ccdcb2f
>> Signed-off-by: David Matlack <dmatlack@google.com>
>> ---
>>  arch/x86/kvm/vmx.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index c41d7ff..0e7ad72 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -10474,12 +10474,12 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>>             !nested_guest_cr4_valid(vcpu, vmcs12->guest_cr4)) {
>>                 nested_vmx_entry_failure(vcpu, vmcs12,
>>                         EXIT_REASON_INVALID_STATE, ENTRY_FAIL_DEFAULT);
>> -               goto out;
>> +               return 1;
>>         }
>>         if (vmcs12->vmcs_link_pointer != -1ull) {
>>                 nested_vmx_entry_failure(vcpu, vmcs12,
>>                         EXIT_REASON_INVALID_STATE, ENTRY_FAIL_VMCS_LINK_PTR);
>> -               goto out;
>> +               return 1;
>>         }
>>
>>         /*
>> @@ -10499,7 +10499,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>>                      ia32e != !!(vmcs12->guest_ia32_efer & EFER_LME))) {
>>                         nested_vmx_entry_failure(vcpu, vmcs12,
>>                                 EXIT_REASON_INVALID_STATE, ENTRY_FAIL_DEFAULT);
>> -                       goto out;
>> +                       return 1;
>>                 }
>>         }
>>
>> @@ -10517,7 +10517,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>>                     ia32e != !!(vmcs12->host_ia32_efer & EFER_LME)) {
>>                         nested_vmx_entry_failure(vcpu, vmcs12,
>>                                 EXIT_REASON_INVALID_STATE, ENTRY_FAIL_DEFAULT);
>> -                       goto out;
>> +                       return 1;
>>                 }
>>         }
>>
>> --
>> 2.8.0.rc3.226.g39d4020
>>