Re: [PATCH v4 2/7] x86: kvm: svm: propagate errors from skip_emulated_instruction()

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> On Tue, Aug 13, 2019 at 03:53:30PM +0200, Vitaly Kuznetsov wrote:
>> @@ -3899,20 +3898,25 @@ static int task_switch_interception(struct vcpu_svm *svm)
>>  	if (reason != TASK_SWITCH_GATE ||
>>  	    int_type == SVM_EXITINTINFO_TYPE_SOFT ||
>>  	    (int_type == SVM_EXITINTINFO_TYPE_EXEPT &&
>> -	     (int_vec == OF_VECTOR || int_vec == BP_VECTOR)))
>> -		skip_emulated_instruction(&svm->vcpu);
>> +	     (int_vec == OF_VECTOR || int_vec == BP_VECTOR))) {
>> +		if (skip_emulated_instruction(&svm->vcpu) != EMULATE_DONE)
>
> This isn't broken in the current code, but it's flawed in the sense that
> it assumes skip_emulated_instruction() never returns EMULATE_USER_EXIT.
>
> Note, both SVM and VMX make the opposite assumption when handling
> kvm_task_switch() and kvm_inject_realmode_interrupt().
>
> More below...
>
>> +			goto fail;
>> +	}
>>  
>>  	if (int_type != SVM_EXITINTINFO_TYPE_SOFT)
>>  		int_vec = -1;
>>  
>>  	if (kvm_task_switch(&svm->vcpu, tss_selector, int_vec, reason,
>> -				has_error_code, error_code) == EMULATE_FAIL) {
>> -		svm->vcpu.run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
>> -		svm->vcpu.run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
>> -		svm->vcpu.run->internal.ndata = 0;
>> -		return 0;
>> -	}
>> +				has_error_code, error_code) == EMULATE_FAIL)
>> +		goto fail;
>> +
>>  	return 1;
>> +
>> +fail:
>> +	svm->vcpu.run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
>> +	svm->vcpu.run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
>> +	svm->vcpu.run->internal.ndata = 0;
>> +	return 0;
>>  }
>>  
>>  static int cpuid_interception(struct vcpu_svm *svm)
>
> ...
>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c6d951cbd76c..e8f797fe9d9e 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -6383,9 +6383,11 @@ static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
>>  int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
>>  {
>>  	unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
>> -	int r = EMULATE_DONE;
>> +	int r;
>>  
>> -	kvm_x86_ops->skip_emulated_instruction(vcpu);
>> +	r = kvm_x86_ops->skip_emulated_instruction(vcpu);
>> +	if (unlikely(r != EMULATE_DONE))
>> +		return 0;
>
> x86_emulate_instruction() doesn't set vcpu->run->exit_reason when emulation
> fails with EMULTYPE_SKIP, i.e. this will exit to userspace with garbage in
> the exit_reason.

Oh, nice catch, will take a look!

>
> handle_ept_misconfig() also has the same (pre-existing) flaw.
>
> Given the handle_ept_misconfig() bug and that kvm_emulate_instruction()
> sets vcpu->run->exit_reason when it returns EMULATE_FAIL in the normal
> case, I think it makes sense to fix the issue in x86_emulate_instruction().
> That would also eliminate the need to worry about EMULATE_USER_EXIT in
> task_switch_interception(), e.g. the SVM code can just return 0 when it
> gets a non-EMULATE_DONE return type.
>
> E.g.:
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 07ab14d73094..73b86f81ed9c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6201,7 +6201,8 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
>         if (emulation_type & EMULTYPE_NO_UD_ON_FAIL)
>                 return EMULATE_FAIL;
>
> -       if (!is_guest_mode(vcpu) && kvm_x86_ops->get_cpl(vcpu) == 0) {
> +       if ((!is_guest_mode(vcpu) && kvm_x86_ops->get_cpl(vcpu) == 0) ||
> +           (emulation_type & EMULTYPE_SKIP)) {
>                 vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
>                 vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
>                 vcpu->run->internal.ndata = 0;
> @@ -6525,8 +6526,6 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>                                 return EMULATE_DONE;
>                         if (ctxt->have_exception && inject_emulated_exception(vcpu))
>                                 return EMULATE_DONE;
> -                       if (emulation_type & EMULTYPE_SKIP)
> -                               return EMULATE_FAIL;
>                         return handle_emulation_failure(vcpu, emulation_type);
>                 }
>         }
>
>
> As for the kvm_task_switch() handling and other cases, I think it's
> possible to rework all of the functions and callers that return/handle
> EMULATE_DONE to instead return 0/1, i.e. contain EMULATE_* to x86.c.
> I'll put together a series, I think you've suffered more than enough scope
> creep as it is :-)

No problem at all, you seem to have a lot of great ideas on how to
improve things :-) Thanks for your review!

-- 
Vitaly