linux-kernel - Re: [tip:x86/asm] x86/umip: Add emulation code for UMIP instructions

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Date:   Wed, 8 Nov 2017 22:17:39 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Anvin <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Jonathan Corbet <corbet@....net>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>, ray.huang@....com,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chen Yucong <slaoub@...il.com>, Ingo Molnar <mingo@...nel.org>,
        Jiri Slaby <jslaby@...e.cz>, Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86/umip: Add emulation code for UMIP instructions

On Wed, Nov 08, 2017 at 06:14:12PM +0100, Paolo Bonzini wrote:
> On 08/11/2017 18:09, Denys Vlasenko wrote:
> > On 11/08/2017 05:57 PM, Linus Torvalds wrote:
> >> On Wed, Nov 8, 2017 at 8:53 AM, Denys Vlasenko <dvlasenk@...hat.com>
> >> wrote:
> >>> We can postpone enabling UMIP by default by a year or so.
> >>> By this time, new Wine will be on majority of users' machines.
> >>
> >> So you are suggesting we run unnecessarily insecure, only in order to
> >> not do the emulation that we already have the code for and that the
> >> patch implements?
> > 
> > We ran insecure in this way for ~25 years.
> > 
> >> Why?
> > 
> > To avoid having to maintain more obscure, rarely executed code.
> 
> As a start, you could propose a patch to disable the emulation code
> through a sysctl or Kconfig symbol.  I would be surprised if it takes
> more time than what you've spent writing emails in this thread.

Indeed, Andy Lutomirski suggested something similar in the past, but as a
separate series [1].

Plus, the Wine team was also interested in receiving the SIGSEGV that UMIP
would generate, but also acknowledged that it would break existing
versions[2].

Thanks and BR,
Ricardo

[1]. https://lkml.org/lkml/2017/3/8/735
[2]. https://www.winehq.org/pipermail/wine-devel/2016-November/115321.html