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

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Date:   Tue, 08 Nov 2016 20:31:29 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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, 2016-11-08 at 07:34 -0800, Andy Lutomirski wrote:
> > 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.

Then should I keep UMIP enabled by default and still provide an option
to disable it via a kernel parameter?

Also, a third option, umip=novm86 would "disable" UMIP in vm86 tasks.
Under the new approach (of emulating the impacted instructions), this
option, a #GP fault would still be generated but the actual values of
GDT/LDT/IDT/MSW would be passed to user space. Does this make sense?

Thanks and BR,
Ricardo