Re: [RESEND PATCH 07/13] KVM: x86: Add explicit flag for forced emulation on #UD

[Liran Alon]

> On 23 Aug 2019, at 17:44, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> 
> On Fri, Aug 23, 2019 at 04:47:14PM +0300, Liran Alon wrote:
>> 
>> 
>>> On 23 Aug 2019, at 4:07, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>>> 
>>> Add an explicit emulation type for forced #UD emulation and use it to
>>> detect that KVM should unconditionally inject a #UD instead of falling
>>> into its standard emulation failure handling.
>>> 
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> 
>> The name "forced emulation on #UD" is not clear to me.
>> 
>> If I understand correctly, EMULTYPE_TRAP_UD is currently used to indicate
>> that in case the x86 emulator fails to decode instruction, the caller would
>> like the x86 emulator to fail early such that it can handle this condition
>> properly.  Thus, I would rename it EMULTYPE_TRAP_DECODE_FAILURE.
> 
> EMULTYPE_TRAP_UD is used when KVM intercepts a #UD from hardware.  KVM
> only emulates select instructions in this case in order to minmize the
> emulator attack surface, e.g.:
> 
> 	if (unlikely(ctxt->ud) && likely(!(ctxt->d & EmulateOnUD)))
> 		return EMULATION_FAILED;
> 
> To enable testing of the emulator, KVM recognizes a special "opcode" that
> triggers full emulation on #UD, e.g. ctxt->ud is false when the #UD was
> triggered with the magic prefix.  The prefix is only recognized when the
> module param force_emulation_prefix is toggled on, hence the name
> EMULTYPE_TRAP_UD_FORCED.

Ah-ha. This makes sense. Thanks for the explanation.
I would say itâs worth putting a comment in it in codeâ

> 
>> But this new flag seems to do the same. So Iâm left confused.  Iâm probably
>> missing something trivial here.