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

Ricardo Neri
2017-03-11T01:22:28Z

On Fri, 2017-03-10 at 06:17 -0800, Andy Lutomirski wrote:
> 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.

Yes, this is the case. I will fix this behavior... and update the test
cases.