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

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Message-ID: <1489191190.131264.55.camel@ranerica-desktop>
Date:   Fri, 10 Mar 2017 16:13:10 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Stas Sergeev <stsp@...t.ru>
Cc:     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,
        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 Sat, 2017-03-11 at 02:58 +0300, Stas Sergeev wrote:
> 11.03.2017 02:47, Ricardo Neri пишет:
> >>
> >>>> It doesn't need to be a matter of this particular
> >>>> patch set, i.e. this proposal should not trigger a
> >>>> v7 resend of all 21 patches. :) But it would be useful
> >>>> for the future development of dosemu2.
> >>> Would dosemu2 use 32-bit processes in order to keep segmentation? If it
> >>> could use 64-bit processes, emulation is not used in this case and the
> >>> SIGSEGV is delivered to user space.
> >> It does use the mix: 64bit process but some segments
> >> are 32bit for DOS code.
> > Do you mean that dosemu2 will start as a 64-bit process and will jump to
> > 32-bit code segments?
> Yes, so the offending insns are executed only in 32bit
> and 16bit segments, even if the process itself is 64bit.
> I guess you handle 16bit segments same as 32bit ones.

I have code to handle 16-bit and 32-bit address encodings differently.
Segmentation is used if !user_64bit_mode(regs). In such a case, the
emulation code will check the segment descriptor D flag and the
address-size overrides prefix to determine the address size and use
16-bit or 32-bit address encodings as applicable.

> 
> >   My emulation code should work in this case as it
> > will use segmentation in 32-bit code descriptors. Is there anything else
> > needed?
> If I understand you correctly, you are saying that SLDT
> executed in 64bit code segment, will inevitably segfault
> to userspace. 
Correct.

> If this is the case and it makes your code
> simpler, then its perfectly fine with me as dosemu does
> not do this and the 64bit DOS progs are not anticipated.

But if 32-bit or 16-bit code segments are used emulation will be used.

Thanks and BR,
Ricardo