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

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Message-ID: <1490762284.2647.24.camel@ranerica-desktop>
Date:   Tue, 28 Mar 2017 21:38:04 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Stas Sergeev <stsp@...t.ru>
Cc:     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>,
        Liang Z Li <liang.z.li@...el.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>,
        Alexandre Julliard <julliard@...ehq.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        "linux-kernel@...r.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

On Tue, 2017-03-28 at 12:38 +0300, Stas Sergeev wrote:
> 28.03.2017 02:46, Ricardo Neri пишет:
> > On Tue, 2017-03-14 at 00:25 +0300, Stas Sergeev wrote:
> >> 11.03.2017 02:59, Ricardo Neri пишет:
> >>> 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