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

[Ricardo Neri]

On Tue, 2017-03-28 at 12:38 +0300, Stas Sergeev wrote:
> 28.03.2017 02:46, Ricardo Neri ÐÐÑÐÑ:
> > On Tue, 2017-03-14 at 00:25 +0300, Stas Sergeev wrote:
> >> 11.03.2017 02:59, Ricardo Neri ÐÐÑÐÑ:
> >>> On Fri, 2017-03-10 at 14:33 +0300, Stas Sergeev wrote:
> >>>
> >>>> 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.
> >> Why would you do that?
> >> I looked up the dosemu2 CPU simulator code that
> >> is used under x86-64. It says this:
> > Stas, I apologize for the delayed reply; I missed your e-mail.
> >> It only implements smsw.
> >> So maybe you can make your code much
> >> simpler and remove the unneeded emulation?
> >> Same is for prot mode.
> > Do you mean the unneeded emulation for SLDT and STR?
> Not quite, I meant also sgdt and sidt in vm86.	
> Yes that it will be a somewhat "incompatible" change,
> but if there is nothing to stay compatible with,
> then why to worry?

My idea of compatibility was to have the emulation code behave exactly
as a processor without UMIP :)

> Probably you could also remove
> the sldt and str emulation for protected mode, because,
> as I understand from this thread, wine does not
> need those.

I see. I would lean on keeping the emulation because I already
implemented it :), for completeness, and because it is performed in a
single switch. The bulk of the emulation code deals with operands.
> 
> Note that these days dosemu2 uses v86 mode set
> up under kvm rather than vm86(). Your patches
> affect that the same way as they do for vm86()
> syscall, or can there be some differences?
My code does not touch kvm at all. I would need to assess how kvm will
behave.
> Or should
> the UMIP be enabled under kvm by hands?
There was an attempt to emulate UMIP that was submitted a while ago:
https://lkml.org/lkml/2016/7/12/644

> 
> >> You know the wine's
> >> requirements now - they are very small. And
> >> dosemu doesn't need anything at all but smsw.
> >> And even smsw is very rare.
> > But emulation is still needed for SMSW, right?
> Likely so.
> If you want, I can enable the logging of this command
> and see if it is used by some of the DOS programs I have.

It would be great if you could do that, if you don't mind.
> But at least dosemu implements it, so probably it is needed.

Right.

> Of course if it is used by one of 100 DOS progs, then there
> is an option to just add its support to dosemu2 and pretend
> the compatibility problems did not exist. :)
Do you mean relaying the GP fault to dosemu instead of trapping it and
emulating it in the kernel?

Thanks and BR,
Ricardo