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

[Alexandre Julliard]

Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> writes:

> 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:
>> >> 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?

Yes, that makes sense.

-- 
Alexandre Julliard
julliard@xxxxxxxxxx