Re: [Xen-devel] [PATCH 2/2] x86/HVM: re-order operations in hvm_ud_intercept()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.06.16 at 16:27, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 09/06/16 15:13, Jan Beulich wrote:
>>>>> On 09.06.16 at 16:06, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 09/06/16 13:31, Jan Beulich wrote:
>>>>>>> On 09.06.16 at 13:34, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>> On 08/06/16 14:43, Jan Beulich wrote:
>>>>>> Don't fetch CS explicitly, leverage the fact that hvm_emulate_prepare()
>>>>>> already does (and that hvm_virtual_to_linear_addr() doesn't alter it).
>>>>>>
>>>>>> At once increase the length passed to hvm_virtual_to_linear_addr() by
>>>>>> one: There definitely needs to be at least one more opcode byte, and we
>>>>>> can avoid missing a wraparound case this way.
>>>>>>
>>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>> I looked into this when you suggested it, but it latches the wrong eip
>>>>> in the emulation state, and you will end up re-emulating the ud2a
>>>>> instruction, rather than the following instruction.
>>>> Where is there any latching of eip? All hvm_emulate_prepare() does
>>>> is storing the regs pointer.
>>> Oh - so it does.  I clearly looked over it too quickly.
>>>
>>> What wraparound issue are you referring to?  Adding 1 will cause
>>> incorrect behaviour when the emulation prefix ends at the segment limit.
>> I don't think so: The prefix together with the actual instruction
>> encoding should be viewed as a single instruction, and it crossing
>> the segment limit should #GP. It wrapping at the prefix/encoding
>> boundary is the case that I'm specifically referring to (this case
>> should also #GP, but wouldn't without this adjustment).
> 
> But the force emulation prefix specifically doesn't behave like other
> prefixes.
> 
> It doesn't count towards the 15 byte instruction limit, and if the
> emulated instruction does fault, we want the fault pointing at the
> emulated instruction, not the force prefix.  We should avoid making any
> link.

Well, are you saying placing such a prefix right below the boundary
of a flat segment is _expected_ to get the instruction at address 0
emulated? I don't think I could buy that. The patch makes no other
connection between prefix and actual insn. And #GP because of
such a boundary condition should imo point at the prefix; only all
faults associated with the actual insn should point there.

Jan


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