linux-kernel - Re: [patch V2 1/5] x86/kaiser: Respect disabled CPU features

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Date:   Mon, 27 Nov 2017 10:37:19 -0800
From:   Kees Cook <keescook@...gle.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>, Linux-MM <linux-mm@...ck.org>,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [patch V2 1/5] x86/kaiser: Respect disabled CPU features

On Mon, Nov 27, 2017 at 10:11 AM, Dave Hansen
<dave.hansen@...ux.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