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

Michael Kerrisk (man-pages)
2016-06-03T19:40:54Z

On 06/03/2016 12:28 PM, Dave Hansen wrote:
> 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).

So, I think all of the above needs to be made abundantly clear in
pkeys(7).

Thanks,

Michael

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