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

[Ricardo Neri]

On Fri, 2017-03-31 at 16:11 +0200, Alexandre Julliard wrote:
> Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> writes:
> 
> > On Thu, 2017-03-30 at 13:10 +0300, Stas Sergeev wrote:
> >> 30.03.2017 08:14, Ricardo Neri ÐÐÑÐÑ:
> >> >>>> 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?
> >> >> Yes, that would be optimal if this does not severely break
> >> >> the current setups. If we can find out that smsw is not in
> >> >> the real use, we can probably do exactly that.
> >> >> But other
> >> >> instructions are not in real use in v86 for sure, so I
> >> >> wouldn't be adding the explicit test-cases to the kernel
> >> >> that will make you depend on some particular behaviour
> >> >> that no one may need.
> >> >> My objection was that we shouldn't
> >> >> write tests before we know exactly how we want this to work.
> >> > OK, if only SMSW is used then I'll keep the emulation for SMSW only.
> >> In fact, smsw has an interesting property, which is that
> >> no one will ever want to disable its in-kernel emulation
> >> to provide its own.
> >> So while I'll try to estimate its usage, emulating it in kernel
> >> will not be that problematic in either case.
> >
> > Ah good to know!
> >
> >> As for protected mode, if wine only needs sgdt/sidt, then
> >> again, no one will want to disable its emulation. Not the
> >> case with sldt, but AFAICS wine doesn't need sldt, and so
> >> we can leave sldt without a fixups. Is my understanding
> >> correct?
> >
> > This is my understanding as well. I could not find any use of sldt in
> > wine. Alexandre, would you mind confirming?
> 
> Some versions of the Themida software protection are known to use sldt
> as part of the virtual machine detection code [1]. The check currently
> fails because it expects the LDT to be zero, so the app is already
> broken, but sldt segfaulting would still cause a crash where there
> wasn't one before.
> 
> However, I'm only aware of one application using this, and being able to
> catch and emulate sldt ourselves would actually give us a chance to fix
> this app in newer Wine versions, so I'm not opposed to having it
> segfault.

Great! Then this is in line with what we are aiming to do with dosemu2:
not emulate str and sldt.
> 
> In fact it would be nice to be able to make sidt/sgdt/etc. segfault
> too. I know a new syscall is a pain, but as far as Wine is concerned,
> being able to opt out from any emulation would be potentially useful.

I see. I guess for now there should not be a problem with emulating
sidt/sgdt/smsw, right? In this way we don't break current versions of
winehq and programs using it. In a phase two we can introduce the
syscall so that kernel fixups can be disabled. Does this make sense?

Thanks and BR,
Ricardo