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Subject: Re: [PATCH 7/9] x86/mm: Shadow stack page fault error checking
To:     Andy Lutomirski <luto@kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <20180607143705.3531-1-yu-cheng.yu@intel.com>
 <20180607143705.3531-8-yu-cheng.yu@intel.com>
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        Linux-MM <linux-mm@kvack.org>,
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Message-ID: <5764865a-1dd2-ec5b-c67c-1ea322aea203@linux.intel.com>
Date:   Thu, 7 Jun 2018 09:56:36 -0700
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On 06/07/2018 09:26 AM, Andy Lutomirski wrote:
>>
>> +       /*
>> +        * Verify X86_PF_SHSTK is within a shadow stack VMA.
>> +        * It is always an error if there is a shadow stack
>> +        * fault outside a shadow stack VMA.
>> +        */
>> +       if (error_code & X86_PF_SHSTK) {
>> +               if (!(vma->vm_flags & VM_SHSTK))
>> +                       return 1;
>> +               return 0;
>> +       }
>> +
> What, if anything, would go wrong without this change?  It seems like
> it might be purely an optimization.  If so, can you mention that in
> the comment?

This is a fine exercise.  I'm curious what it does, too.

But, I really like it being explicit in the end.  If we depend on
implicit behavior, I really worry that someone breaks it accidentally.