linux-kernel - Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key faults

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Date:	Fri, 25 Sep 2015 16:18:56 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key
 faults

On 09/25/2015 12:11 AM, Ingo Molnar wrote:
>>> > > Btw., how does pkey support interact with hugepages?
>> > 
>> > Surprisingly little.  I've made sure that everything works with huge pages and 
>> > that the (huge) PTEs and VMAs get set up correctly, but I'm not sure I had to 
>> > touch the huge page code at all.  I have test code to ensure that it works the 
>> > same as with small pages, but everything worked pretty naturally.
> Yeah, so the reason I'm asking about expectations is that this code:
> 
> +       follow_ret = follow_pte(tsk->mm, address, &ptep, &ptl);
> +       if (!follow_ret) {
> +               /*
> +                * On a successful follow, make sure to
> +                * drop the lock.
> +                */
> +               pte = *ptep;
> +               pte_unmap_unlock(ptep, ptl);
> +               ret = pte_pkey(pte);
> 
> is visibly hugepage-unsafe: if a vma is hugepage mapped, there are no ptes, only 
> pmds - and the protection key index lives in the pmd. We don't seem to recover 
> that information properly.

You got me on this one.  I assumed that follow_pte() handled huge pages.
 It does not.

But, the code still worked.  Since follow_pte() fails for all huge
pages, it just falls back to pulling the protection key out of the VMA,
which _does_ work for huge pages.

I've actually removed the PTE walking and I just now use the VMA
directly.  I don't see a ton of additional value from walking the page
tables when we can get what we need from the VMA.
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