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

Ricardo Neri
2017-03-10T23:58:27Z

On Thu, 2017-03-09 at 18:39 -0800, Andy Lutomirski wrote:
> 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.

Ouch! Yes, I am afraid my code will attempt to emulate sldt in vm86
mode. The test cases that I have for vm86 are only for the instructions
that are valid in vm86: smsw, sidt and sgdt.

I will add test cases for str and sldt and make sure that a #UD is
issued.

Would this trigger a v7 series?

Thanks and BR,
Ricardo
>
> --Andy