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

Ricardo Neri <[email protected]> writes:

> On Thu, 2017-03-30 at 13:10 +0300, Stas Sergeev wrote:
>> 30.03.2017 08:14, Ricardo Neri пишет:
>> >>>> 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.

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.

[1] https://www.winehq.org/pipermail/wine-bugs/2008-February/094470.html

-- 
Alexandre Julliard
[email protected]