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

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Message-ID: <702b0f08-57a6-0ee0-d2e1-1310c00351e6@virtuozzo.com>
Date:   Mon, 26 Feb 2018 13:47:23 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Jan Beulich <JBeulich@...e.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 02/26/2018 01:08 PM, Jan Beulich wrote:
>>>> 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. 

kasan_zero_p?? are used to map kasan_zero_page. That's it.

> 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.

I'm not sure I understand what do you mean.
If we somehow screw up and accidentally make kasan_zero_pte writable and executable,
note_page() should report this. With your patch, it won't work.