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From: Andy Lutomirski <luto@amacapital.net>
Date: Tue, 8 Nov 2016 07:34:00 -0800
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Subject: Re: [PATCH 0/4] x86: enable User-Mode Instruction Prevention
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@suse.de>, Brian Gerst <brgerst@gmail.com>,
        Chen Yucong <slaoub@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>, 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>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Vlastimil Babka <vbabka@suse.cz>, Shuah Khan <shuah@kernel.org>
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On Tue, Nov 8, 2016 at 5:16 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Nov 07, 2016 at 10:12:09PM -0800, Ricardo Neri wrote:
>> There is a caveat, however. Certain applications running in virtual-8086
>> mode, such as DOSEMU[1] and Wine[2], want to utilize the SGDT, SIDT and
>> SLDT instructions for legitimate reasons. In order to keep such
>> applications working, UMIP must be disabled/enabled when entering/exiting
>> virtual-8086 mode.
>
> Would it not be better to emulate these instructions for them? What way
> we can verify they're not malicious.

Forget malice -- if they are really needed for some silly vm86-using
program, let's trap them and emulate them so they return dummy values.

Also, keep in mind that vm86 is already effectively gated behind a
sysctl for non-root.  I think the default should be that, if root has
enabled vm86, it should work.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC