Message-ID: <1489191190.131264.55.camel@ranerica-desktop>
Subject: Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention
From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To: Stas Sergeev <stsp@list.ru>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>, Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Liang Z Li <liang.z.li@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>, Huang Rui <ray.huang@amd.com>,
        Jiri Slaby <jslaby@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Vlastimil Babka <vbabka@suse.cz>, Chen Yucong <slaoub@gmail.com>,
        Alexandre Julliard <julliard@winehq.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-msdos@vger.kernel.org, wine-devel@winehq.org
Date: Fri, 10 Mar 2017 16:13:10 -0800
In-Reply-To: <bb8cf9d3-f746-836c-67ff-76720741b2f9@list.ru>
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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