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From: Kees Cook <keescook@google.com>
Date: Mon, 27 Nov 2017 10:37:19 -0800
Message-ID: <CAGXu5jKyx1T+NZGgPUUPt_-MqZe3-zrpFbAVFzsWbW=aD-D6_Q@mail.gmail.com>
Subject: Re: [patch V2 1/5] x86/kaiser: Respect disabled CPU features
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Rik van Riel <riel@redhat.com>,
        Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
        Hugh Dickins <hughd@google.com>, Linux-MM <linux-mm@kvack.org>,
        michael.schwarz@iaik.tugraz.at, moritz.lipp@iaik.tugraz.at,
        richard.fellner@student.tugraz.at
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On Mon, Nov 27, 2017 at 10:11 AM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
>> --- a/arch/x86/include/asm/pgtable_64.h
>> +++ b/arch/x86/include/asm/pgtable_64.h
>> @@ -222,7 +222,8 @@ static inline pgd_t kaiser_set_shadow_pg
>>                        * wrong CR3 value, userspace will crash
>>                        * instead of running.
>>                        */
>> -                     pgd.pgd |= _PAGE_NX;
>> +                     if (__supported_pte_mask & _PAGE_NX)
>> +                             pgd.pgd |= _PAGE_NX;
>>               }
>
> Thanks for catching that.  It's definitely a bug.  Although,
> practically, it's hard to hit, right?  I think everything 64-bit
> supports NX unless the hypervisor disabled it or something.

There was a very narrow window where x86_64 machines were made without
NX. :( This is reflected in x86_report_nx(), though maybe we should
add a "OMG, why?" when 64-bit but no NX. ;)

-Kees

-- 
Kees Cook
Pixel Security