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

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Date:   Fri, 31 Mar 2017 16:11:58 +0200
From:   Alexandre Julliard <julliard@...ehq.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Stas Sergeev <stsp@...t.ru>, Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, linux-msdos@...r.kernel.org,
        wine-devel@...ehq.org
Subject: Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> 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
julliard@...ehq.org