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

[Ricardo Neri]

On Fri, 2017-03-10 at 14:33 +0300, Stas Sergeev wrote:
> 10.03.2017 05:39, Andy Lutomirski ÐÐÑÐÑ:
> > On Thu, Mar 9, 2017 at 2:10 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> >> 09.03.2017 04:15, Ricardo Neri ÐÐÑÐÑ:
> >>
> >>> On Wed, 2017-03-08 at 08:46 -0800, Andy Lutomirski wrote:
> >>>> On Wed, Mar 8, 2017 at 8:29 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> >>>>> 08.03.2017 19:06, Andy Lutomirski ÐÐÑÐÑ:
> >>>>>> On Wed, Mar 8, 2017 at 6:08 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> >>>>>>> 08.03.2017 03:32, Ricardo Neri ÐÐÑÐÑ:
> >>>>>>>> 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.

It str and sldt can be emulated in vm86 but as Andy mention, the
behavior sould be the same with and without emulation.

Thanks and BR,
Ricardo