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

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Date:	Wed, 1 Jun 2016 12:32:58 -0700
From:	Dave Hansen <dave@...1.net>
To:	Jonathan Corbet <corbet@....net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, dave.hansen@...ux.intel.com
Subject: Re: [PATCH 5/8] x86, pkeys: allocation/free syscalls

On 06/01/2016 11:37 AM, Jonathan Corbet wrote:
>> +static inline
>> +int mm_pkey_free(struct mm_struct *mm, int pkey)
>> +{
>> +	/*
>> +	 * pkey 0 is special, always allocated and can never
>> +	 * be freed.
>> +	 */
>> +	if (!pkey || !validate_pkey(pkey))
>> +		return -EINVAL;
>> +	if (!mm_pkey_is_allocated(mm, pkey))
>> +		return -EINVAL;
>> +
>> +	mm_set_pkey_free(mm, pkey);
>> +
>> +	return 0;
>> +}
> 
> 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
more directly in the changelog for this patch.