linux-kernel - Re: [PATCH 7/9] x86/mm: Shadow stack page fault error checking

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Date:   Thu, 7 Jun 2018 09:56:36 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com
Subject: Re: [PATCH 7/9] x86/mm: Shadow stack page fault error checking

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.