linux-kernel - Re: [PATCH 0/4] x86: enable User-Mode Instruction Prevention

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Date:   Tue, 8 Nov 2016 17:52:25 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...e.de>, Brian Gerst <brgerst@...il.com>,
        Chen Yucong <slaoub@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        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>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 0/4] x86: enable User-Mode Instruction Prevention

On Tue, 8 Nov 2016, Andy Lutomirski wrote:
> On Tue, Nov 8, 2016 at 5:16 AM, Peter Zijlstra <peterz@...radead.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.

handle_vm86_fault() already does instruction emulation, so adding the few
bits there is the right thing to do. Then we just can enable UMIP
unconditionally and be done with it.

Thanks,

	tglx