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

Andy Lutomirski
2017-03-10T14:17:45Z

On Fri, Mar 10, 2017 at 3:33 AM, Stas Sergeev <stsp@list.ru> wrote:
> 10.03.2017 05:39, Andy Lutomirski пишет:
>
>> 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?
>
> Why would you need one?
> Or do you really want to allow these instructions
> in v86 by the means of emulation? If so - this wasn't
> clearly stated in the patch description, neither it was
> properly discussed, it seems.

What I meant was: if the patches incorrectly started making these
instructions work in vm86 mode where they used to cause a vm86 exit,
then that's a bug that the selftest should have caught.