Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5_transform

Tom Herbert
2016-12-14T20:12:24Z

On Wed, Dec 14, 2016 at 4:53 AM, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> Hi David,
>
> On Wed, Dec 14, 2016 at 10:51 AM, David Laight <David.Laight@aculab.com> wrote:
>> From: Jason A. Donenfeld
>>> Sent: 14 December 2016 00:17
>>> This gives a clear speed and security improvement. Rather than manually
>>> filling MD5 buffers, we simply create a layout by a simple anonymous
>>> struct, for which gcc generates rather efficient code.
>> ...
>>> + const struct {
>>> + struct in6_addr saddr;
>>> + struct in6_addr daddr;
>>> + __be16 sport;
>>> + __be16 dport;
>>> + } __packed combined = {
>>> + .saddr = *(struct in6_addr *)saddr,
>>> + .daddr = *(struct in6_addr *)daddr,
>>> + .sport = sport,
>>> + .dport = dport
>>> + };
>>
>> You need to look at the effect of marking this (and the other)
>> structures 'packed' on architectures like sparc64.
>
> In all current uses of __packed in the code, I think the impact is
> precisely zero, because all structures have members in descending
> order of size, with each member being a perfect multiple of the one
> below it. The __packed is therefore just there for safety, in case
> somebody comes in and screws everything up by sticking a u8 in
> between. In that case, it wouldn't be desirable to hash the structure
> padding bits. In the worst case, I don't believe the impact would be
> worse than a byte-by-byte memcpy, which is what the old code did. But
> anyway, these structures are already naturally packed anyway, so the
> present impact is nil.
>
If you pad the data structure to 64 bits then we can call the version
of siphash that only deals in 64 bit words. Writing a zero in the
padding will be cheaper than dealing with odd lengths in siphash24.

Tom

> Jason