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

From: Andy Lutomirski <luto@amacapital.net>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	"Theodore Ts'o" <tytso@mit.edu>, Netdev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
	David Laight <David.Laight@aculab.com>,
	Eric Dumazet <edumazet@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Eric Biggers <ebiggers3@gmail.com>,
	Tom Herbert <tom@herbertland.com>,
	Andi Kleen <ak@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
Subject: Re: BPF hash algo (Re: [kernel-hardening] Re: [PATCH v7 3/6] random: use SipHash in place of MD5)
Date: Thu, 22 Dec 2016 08:53:54 -0800	[thread overview]
Message-ID: <CALCETrVoVZ3ErHXa=xw00HDuq-ThbtCr1XQbfGr8dT-Ru-KM8Q@mail.gmail.com> (raw)
In-Reply-To: <CAHmME9pvPXiBu5TUYJL7gED7b=iXKrkXu45fXstnBFe77Esv5Q@mail.gmail.com>

On Thu, Dec 22, 2016 at 8:28 AM, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> Hi all,
>
> I don't know what your design requirements are for this. It looks like
> you're generating some kind of crypto digest of a program, and you
> need to avoid collisions. If you'd like to go with a PRF (keyed hash
> function) that uses some kernel secret key, then I'd strongly suggest
> using Keyed-Blake2. Alternatively, if you need for userspace to be
> able to calculate the same hash, and don't want to use some kernel
> secret, then I'd still suggest using Blake2, which will be fast and
> secure.
>
> If you can wait until January, I'll work on a commit adding the
> primitive to the tree. I've already written it and I just need to get
> things cleaned up.
>
>> Blake2 is both less stable (didn't they slightly change it recently?)
>
> No, Blake2 is very stable. It's also extremely secure and has been
> extensively studied. Not to mention it's faster than SHA2. And if you
> want to use it as a PRF, it's obvious better suited and faster to use
> Blake2's keyed PRF mode than HMAC-SHA2.
>
> If you don't care about performance, and you don't want to use a PRF,
> then just use SHA2-256. If you're particularly concerned about certain
> types of attacks, you could go with SHA2-512 truncated to 256 bytes,
> but somehow I doubt you need this.

I don't think this cares about performance.  (Well, it cares about
performance, but the verifier will likely dominiate the cost by such a
large margin that the hash algo doesn't matter.)  And staying
FIPS-compliant-ish is worth a little bit, so I'd advocate for
something in the SHA2 family.

> If userspace hasn't landed, can we get away with changing this code
> after 4.10? Or should we just fix it before 4.10? Or should we revert
> it before 4.10? Development-policy-things like this I have zero clue
> about, so I heed to your guidance.

I think it should be fixed or reverted before 4.10.

--Andy