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

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Message-ID: <dd612f93-1e21-ed53-4955-f753fbf0aa55@redhat.com>
Date:   Wed, 8 Nov 2017 18:14:12 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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>,
        ricardo.neri-calderon@...ux.intel.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 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.

Paolo