Re: BPF hash algo (Re: [kernel-hardening] Re: [PATCH v7 3/6] random: use SipHash in place of MD5)

[Hannes Frederic Sowa]

On 23.12.2016 17:42, Andy Lutomirski wrote:
> On Fri, Dec 23, 2016 at 8:23 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Fri, Dec 23, 2016 at 3:59 AM, Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>>> On 12/23/2016 11:59 AM, Hannes Frederic Sowa wrote:
>>>>
>>>> On Fri, 2016-12-23 at 11:04 +0100, Daniel Borkmann wrote:
>>>>>
>>>>> On 12/22/2016 05:59 PM, Hannes Frederic Sowa wrote:
>>>>>>
>>>>>> On Thu, 2016-12-22 at 08:07 -0800, Andy Lutomirski wrote:
>>>
>>> [...]
>>>
>>>>>> The hashing is not a proper sha1 neither, unfortunately. I think that
>>>>>> is why it will have a custom implementation in iproute2?
>>>>>
>>>>>
>>>>> Still trying to catch up on this admittedly bit confusing thread. I
>>>>> did run automated tests over couple of days comparing the data I got
>>>>> from fdinfo with the one from af_alg and found no mismatch on the test
>>>>> cases varying from min to max possible program sizes. In the process
>>>>> of testing, as you might have seen on netdev, I found couple of other
>>>>> bugs in bpf code along the way and fixed them up as well. So my question,
>>>>> do you or Andy or anyone participating in claiming this have any
>>>>> concrete data or test cases that suggests something different? If yes,
>>>>> I'm very curious to hear about it and willing fix it up, of course.
>>>>> When I'm back from pto I'll prep and cook up my test suite to be
>>>>> included into the selftests/bpf/, should have done this initially,
>>>>> sorry about that. I'll also post something to expose the alg, that
>>>>> sounds fine to me.
>>>>
>>>>
>>>> Looking into your code closer, I noticed that you indeed seem to do the
>>>> finalization of sha-1 by hand by aligning and padding the buffer
>>>> accordingly and also patching in the necessary payload length.
>>>>
>>>> Apologies for my side for claiming that this is not correct sha1
>>>> output, I was only looking at sha_transform and its implementation and
>>>> couldn't see the padding and finalization round with embedding the data
>>>> length in there and hadn't thought of it being done manually.
>>>>
>>>> Anyway, is it difficult to get the sha finalization into some common
>>>> code library? It is not very bpf specific and crypto code reviewers
>>>> won't find it there at all.
>>>
>>>
>>> Yes, sure, I'll rework it that way (early next year when I'm back if
>>> that's fine with you).
>>
>> Can we make it SHA-256 before 4.10 comes out, though?  This really
>> looks like it will be used in situations where collisions matter and
>> it will be exposed to malicious programs, and SHA-1 should not be used
>> for new designs for this purpose because it simply isn't long enough.
>>
>> Also, a SHA-1 digest isn't a pile of u32s, so u32 digest[...] is very
>> misleading.  That should be u8 or, at the very least, __be32.
>>
>> I realize that there isn't a sha-256 implementation in lib, but would
>> it really be so bad to make the bpf digest only work (for now) when
>> crypto is enabled?  I would *love* to see the crypto core learn how to
>> export simple primitives for direct use without needing the whole
>> crypto core, and this doesn't seem particularly hard to do, but I
>> don't think that's 4.10 material.
> 
> I'm going to try to send out RFC patches for all of this today or
> tomorrow.  It doesn't look bad at all.

Factoring out sha3 to lib/ and use it as standalone and in crypto api
doesn't seem hard, yep. I also proposed this to Daniel offlist.

Bye,
Hannes