linux-kernel - Re: [tip:x86/mm] x86/mm: Consider effective protection attributes in W+X check

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Message-Id: <5A93EABD02000078001AB9EC@prv-mh.provo.novell.com>
Date:   Mon, 26 Feb 2018 03:08:45 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Andrey Ryabinin" <aryabinin@...tuozzo.com>
Cc:     <bp@...en8.de>, <brgerst@...il.com>, <dvyukov@...gle.com>,
        <glider@...gle.com>, <peterz@...radead.org>, <luto@...nel.org>,
        <mingo@...nel.org>, <tglx@...utronix.de>,
        <torvalds@...ux-foundation.org>, <boris.ostrovsky@...cle.com>,
        <dvlasenk@...hat.com>, <jpoimboe@...hat.com>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <linux-tip-commits@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [tip:x86/mm] x86/mm: Consider effective protection
 attributes in W+X check

>>> On 26.02.18 at 11:00, <aryabinin@...tuozzo.com> wrote:
> On 02/26/2018 11:48 AM, tip-bot for Jan Beulich wrote:
>> @@ -351,7 +362,7 @@ static inline bool kasan_page_table(struct seq_file *m, struct pg_state *st,
>>  	    (pgtable_l5_enabled && __pa(pt) == __pa(kasan_zero_p4d)) ||
>>  	    __pa(pt) == __pa(kasan_zero_pud)) {
>>  		pgprotval_t prot = pte_flags(kasan_zero_pte[0]);
>> -		note_page(m, st, __pgprot(prot), 5);
>> +		note_page(m, st, __pgprot(prot), 0, 5);
> 
> Isn't this disables W+X check for kasan page table?
> Methinks it should be 'prot' here.

Might well be - I actually did ask the question before sending v3,
but didn't get any answer (yet). The kasan_zero_p?d names
suggested to me that this is a shortcut for mappings which
otherwise would be non-present anyway, but that was merely a
guess. As to W+X checks - I can't see how the result could be
any better if the protections of kasan_zero_pte[0] would be
used: Those can't possibly be applicable independent of VA.

Jan