Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

Andy Lutomirski
2017-03-10T02:39:38Z

On Thu, Mar 9, 2017 at 2:10 PM, Stas Sergeev <stsp@list.ru> wrote:
> 09.03.2017 04:15, Ricardo Neri пишет:
>
>> On Wed, 2017-03-08 at 08:46 -0800, Andy Lutomirski wrote:
>>>
>>> On Wed, Mar 8, 2017 at 8:29 AM, Stas Sergeev <stsp@list.ru> wrote:
>>>>
>>>> 08.03.2017 19:06, Andy Lutomirski пишет:
>>>>>
>>>>> On Wed, Mar 8, 2017 at 6:08 AM, Stas Sergeev <stsp@list.ru> wrote:
>>>>>>
>>>>>> 08.03.2017 03:32, Ricardo Neri пишет:
>>>>>>>
>>>>>>> These are the instructions covered by UMIP:
>>>>>>> * SGDT - Store Global Descriptor Table
>>>>>>> * SIDT - Store Interrupt Descriptor Table
>>>>>>> * SLDT - Store Local Descriptor Table
>>>>>>> * SMSW - Store Machine Status Word
>>>>>>> * STR - Store Task Register
>>>>>>>
>>>>>>> This patchset initially treated tasks running in virtual-8086
>>>
>>> mode as a
>>>>>>>
>>>>>>> special case. However, I received clarification that DOSEMU[8]
>>>
>>> does not
>>>>>>>
>>>>>>> support applications that use these instructions.
>>>>>
>>>>> Can you remind me what was special about it? It looks like you
>>>
>>> still
>>>>>
>>>>> emulate them in v8086 mode.
>>>>
>>>> Indeed, sorry, I meant prot mode here. :)
>>>> So I wonder what was cited to be special about v86.
>>
>> Initially my patches disabled UMIP on virtual-8086 instructions, without
>> regards of protected mode (i.e., UMIP was always enabled). I didn't have
>> emulation at the time. Then, I added emulation code that now covers
>> protected and virtual-8086 modes. I guess it is not special anymore.
>
> But isn't SLDT&friends just throw UD in v86?
> How does UMIP affect this? How does your patch affect
> this?

Er, right. Ricardo, your code may need fixing. But don't you have a
test case for this? The behavior should be the same with and without
your patches applied. The exception is #UD, not #GP, so maybe your
code just never executes in the vm86 case.

--Andy