linux-kernel - Re: [PATCH 5/8] x86, pkeys: allocation/free syscalls

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Message-ID: <5499ff55-ae0f-e54c-05fd-b1e76dc05a89@gmail.com>
Date:	Thu, 2 Jun 2016 19:26:03 -0500
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Dave Hansen <dave@...1.net>
Cc:	mtk.manpages@...il.com, Jonathan Corbet <corbet@....net>,
	lkml <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Linux API <linux-api@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 5/8] x86, pkeys: allocation/free syscalls

On 06/01/2016 07:17 PM, Dave Hansen wrote:
> On 06/01/2016 05:11 PM, Michael Kerrisk (man-pages) wrote:
>>>>>>
>>>>>> If I read this right, it doesn't actually remove any pkey restrictions
>>>>>> that may have been applied while the key was allocated.  So there could be
>>>>>> pages with that key assigned that might do surprising things if the key is
>>>>>> reallocated for another use later, right?  Is that how the API is intended
>>>>>> to work?
>>>>
>>>> Yeah, that's how it works.
>>>>
>>>> It's not ideal.  It would be _best_ if we during mm_pkey_free(), we
>>>> ensured that no VMAs under that mm have that vma_pkey() set.  But, that
>>>> search would be potentially expensive (a walk over all VMAs), or would
>>>> force us to keep a data structure with a count of all the VMAs with a
>>>> given key.
>>>>
>>>> I should probably discuss this behavior in the manpages and address it
>> s/probably//
>>
>> And, did I miss it. Was there an updated man-pages patch in the latest
>> series? I did not notice it.
> 
> There have been to changes to the patches that warranted updating the
> manpages until now.  I'll send the update immediately.

Do those updated pages include discussion of the point noted above?
I could not see it mentioned there.

Just by the way, the above behavior seems to offer possibilities
for users to shoot themselves in the foot, in a way that has security
implications. (Or do I misunderstand?)

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/