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

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Date:   Fri, 10 Mar 2017 15:47:08 -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 Fri, 2017-03-10 at 01:01 +0300, Stas Sergeev wrote:
> 09.03.2017 03:46, Ricardo Neri пишет:
> > On Wed, 2017-03-08 at 17:08 +0300, Stas Sergeev wrote:
> >> 08.03.2017 03:32, Ricardo Neri пишет:
> >>> These are the instructions covered by UMIP:
> >>> * SGDT - Store Global Descriptor Table
> >>> * SIDT - Store Interrupt Descriptor Table
> >>> * SLDT - Store Local Descriptor Table
> >>> * SMSW - Store Machine Status Word
> >>> * STR - Store Task Register
> >>>
> >>> This patchset initially treated tasks running in virtual-8086 mode as a
> >>> special case. However, I received clarification that DOSEMU[8] does not
> >>> support applications that use these instructions.
> >> Yes, this is the case.
> >> But at least in the past there was an attempt to
> >> support SLDT as it is used by an ancient pharlap
> >> DOS extender (currently unsupported by dosemu1/2).
> >> So how difficult would it be to add an optional
> >> possibility of delivering such SIGSEGV to userspace
> >> so that the kernel's dummy emulation can be overridden?
> > I suppose a umip=noemulation kernel parameter could be added in this
> > case.
> Why?
> It doesn't need to be global: the app should be
> able to change that on its own. Note that no app currently
> requires this, so its just for the future, and in the
> future the app can start using the new API for this,
> if you provide one.

Right, I missed this detail. Then, yes the API should allow only one app
to relay the SIGSEGV.
> 
> 
> >> 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? My emulation code should work in this case as it
will use segmentation in 32-bit code descriptors. Is there anything else
needed?

Thanks and BR,
Ricardo