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Subject: Re: [tip:x86/asm] x86/umip: Add emulation code for UMIP instructions
To: Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <dd612f93-1e21-ed53-4955-f753fbf0aa55@redhat.com>
Date: Wed, 8 Nov 2017 18:14:12 +0100
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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@redhat.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