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

[Andy Lutomirski]

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.

--Andy