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

Dave Hansen
2016-06-03T17:28:23Z

On 06/02/2016 05:26 PM, Michael Kerrisk (man-pages) wrote:
> 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.

I added the following text to pkey_alloc.2. I somehow neglected to send
it out in the v3 update of the manpages RFC:

An application should not call
.BR pkey_free ()
on any protection key which has been assigned to an address
range by
.BR pkey_mprotect ()
and which is still in use. The behavior in this case is
undefined and may result in an error.

I'll add that in the version (v4) I send out shortly.

> 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?)

Protection keys has the potential to add a layer of security and
reliability to applications. But, it has not been primarily designed as
a security feature. For instance, WRPKRU is a completely unprivileged
instruction, so pkeys are useless in any case that an attacker controls
the PKRU register or can execute arbitrary instructions.

That said, this mechanism does, indeed, allow a user to shoot themselves
in the foot and in a way that could have security implications.

For instance, say the following happened:
1. A sensitive bit of data in memory was marked with a pkey
2. That pkey was set as PKEY_DISABLE_ACCESS
3. The application called pkey_free() on the pkey, without freeing
the sensitive data
4. Application calls pkey_alloc() and then clears PKEY_DISABLE_ACCESS
5. Applocation can now read the sensitive data

The application has to have basically "leaked" a reference to the pkey.
It forgot that it had sensitive data marked with that key.

The kernel _could_ enforce that no in-use pkey may have pkey_free()
called on it. But, doing that has tradeoffs which could make
pkey_free() extremely slow:

> 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.

In addition, that checking _could_ be implemented in an application by
inspecting /proc/$pid/smaps for "ProtectionKey: $foo" before calling
pkey_free($foo).

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