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

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Date:   Mon, 26 Feb 2018 03:54:22 -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 11:47, <aryabinin@...tuozzo.com> wrote:

> 
> 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.

Ah, thanks for explaining.

>> 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. 

If this is a case to care about, simply passing "prot" won't be right
though - the callers accumulated effective protections would then
need passing in here, and merging with prot.

Before I do this for a possible v4, I'd like to seek clarification
though whether this really is a case to care about.

Jan