Date: Thu, 22 Dec 2016 09:25:09 -0800 From: Andy Lutomirski <luto@...capital.net> To: Hannes Frederic Sowa <hannes@...essinduktion.org> Cc: Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <alexei.starovoitov@...il.com>, "Jason A. Donenfeld" <Jason@...c4.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, "Theodore Ts'o" <tytso@....edu>, Netdev <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, David Laight <David.Laight@...lab.com>, Eric Dumazet <edumazet@...gle.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Eric Biggers <ebiggers3@...il.com>, Tom Herbert <tom@...bertland.com>, Andi Kleen <ak@...ux.intel.com>, "David S. Miller" <davem@...emloft.net>, Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com> Subject: Re: BPF hash algo (Re: Re: [PATCH v7 3/6] random: use SipHash in place of MD5) On Thu, Dec 22, 2016 at 8:59 AM, Hannes Frederic Sowa <hannes@...essinduktion.org> wrote: > On Thu, 2016-12-22 at 08:07 -0800, Andy Lutomirski wrote: >> On Thu, Dec 22, 2016 at 7:51 AM, Hannes Frederic Sowa >> <hannes@...essinduktion.org> wrote: >> > On Thu, 2016-12-22 at 16:41 +0100, Jason A. Donenfeld wrote: >> > > Hi Hannes, >> > > >> > > On Thu, Dec 22, 2016 at 4:33 PM, Hannes Frederic Sowa >> > > <hannes@...essinduktion.org> wrote: >> > > > IPv6 you cannot touch anymore. The hashing algorithm is part of uAPI. >> > > > You don't want to give people new IPv6 addresses with the same stable >> > > > secret (across reboots) after a kernel upgrade. Maybe they lose >> > > > connectivity then and it is extra work? >> > > >> > > Ahh, too bad. So it goes. >> > >> > If no other users survive we can put it into the ipv6 module. >> > >> > > > The bpf hash stuff can be changed during this merge window, as it is >> > > > not yet in a released kernel. Albeit I would probably have preferred >> > > > something like sha256 here, which can be easily replicated by user >> > > > space tools (minus the problem of patching out references to not >> > > > hashable data, which must be zeroed). >> > > >> > > Oh, interesting, so time is of the essence then. Do you want to handle >> > > changing the new eBPF code to something not-SHA1 before it's too late, >> > > as part of a ne >> >> w patchset that can fast track itself to David? And >> > > then I can preserve my large series for the next merge window. >> > >> > This algorithm should be a non-seeded algorithm, because the hashes >> > should be stable and verifiable by user space tooling. Thus this would >> > need a hashing algorithm that is hardened against pre-image >> > attacks/collision resistance, which siphash is not. I would prefer some >> > higher order SHA algorithm for that actually. >> > >> >> You mean: >> >> commit 7bd509e311f408f7a5132fcdde2069af65fa05ae >> Author: Daniel Borkmann <daniel@...earbox.net> >> Date: Sun Dec 4 23:19:41 2016 +0100 >> >> bpf: add prog_digest and expose it via fdinfo/netlink >> >> >> Yes, please! This actually matters for security -- imagine a >> malicious program brute-forcing a collision so that it gets loaded >> wrong. And this is IMO a use case for SHA-256 or SHA-512/256 >> (preferably the latter). Speed basically doesn't matter here and >> Blake2 is both less stable (didn't they slightly change it recently?) >> and much less well studied. > > We don't prevent ebpf programs being loaded based on the digest but > just to uniquely identify loaded programs from user space and match up > with their source. The commit log talks about using the hash to see if the program has already been compiled and JITted. If that's done, then a collision will directly cause the kernel to malfunction. >> My inclination would have been to seed them with something that isn't >> exposed to userspace for the precise reason that it would prevent user >> code from making assumptions about what's in the hash. But if there's >> a use case for why user code needs to be able to calculate the hash on >> its own, then that's fine. But perhaps the actual fdinfo string >> should be "sha256:abcd1234..." to give some flexibility down the road. >> >> Also: >> >> + result = (__force __be32 *)fp->digest; >> + for (i = 0; i < SHA_DIGEST_WORDS; i++) >> + result[i] = cpu_to_be32(fp->digest[i]); >> >> Everyone, please, please, please don't open-code crypto primitives. >> Is this and the code above it even correct? It might be but on a very >> brief glance it looks wrong to me. If you're doing this to avoid >> depending on crypto, then fix crypto so you can pull in the algorithm >> without pulling in the whole crypto core. > > The hashing is not a proper sha1 neither, unfortunately. I think that > is why it will have a custom implementation in iproute2? Putting on crypto hat: NAK NAK NAK NAK NAK. "The Linux kernel invented a new primitive in 2016 when people know better and is going to handle it by porting that new primitive to userspace" is not a particularly good argument. Okay, crypto hack back off. > > I wondered if bpf program loading should have used the module loading > infrastructure from the beginning... That would be way too complicated and would be nasty for the unprivileged cases. > >> At the very least, there should be a separate function that calculates >> the hash of a buffer and that function should explicitly run itself >> against test vectors of various lengths to make sure that it's >> calculating what it claims to be calculating. And it doesn't look >> like the userspace code has landed, so, if this thing isn't >> calculating SHA1 correctly, it's plausible that no one has noticed. > > I hope this was known from the beginning, this is not sha1 unfortunately. > > But ebpf elf programs also need preprocessing to get rid of some > embedded load-depending data, so maybe it was considered to be just > enough? I suspect it was actually an accident.
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