Date: 10 Dec 2016 10:35:17 -0500 From: "George Spelvin" <linux@...encehorizons.net> To: Jason@...c4.com, vegard.nossum@...il.com Cc: djb@...yp.to, jeanphilippe.aumasson@...il.com, kernel-hardening@...ts.openwall.com, linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, linux@...encehorizons.net, rusty@...tcorp.com.au, torvalds@...ux-foundation.org Subject: Re: [PATCH] siphash: add cryptographically secure hashtable function > There's a 32-bit secret random salt (inet_ehash_secret) which means > that in practice, inet_ehashfn() will select 1 out of 2^32 different > hash functions at random each time you boot the kernel; without > knowing which one it selected, how can a local or remote attacker can > force IPv4 connections/whatever to go into a single hash bucket? By figuring out the salt. The thing is, the timing of hash table lookups *is externally visible*. If I create connections to the target, then see which ones make responses on previous connections slightly slower, I gain information about the salt. I dont't know *where* in the hash table the collissions occur, but I know *which* inputs collide, and that's enough for me to learn something. (I need more connections than the size of the hash table, but even with just one IP source I can use 64K ports on my end times however many the target has open on its end.) With enough information (google "unicity distance") I can recover the entire salt. It's not like I care about the cryptographic strength of the hash; simply trying all 4 billion possible seeds is pretty fast on a 4 GHz processor. Once that happens, I can choose a target connection whose timing I can't observe directly and pack its hash chain without being obvious about it. > I am happy to be proven wrong, but you make it sound very easy to > exploit the current situation, so I would just like to ask whether you > have a concrete way to do that? I don't think anyone's implemented an attack on this particular hash table yet, and the reason it hasn't been urgent is that it's just a mild DoS attack it makes the computer noticeably slower withough disabling it completely. But the general style of attack is well known and has been repeatedly demonstrated. Its practicality is not in question. The only question is whether it's *more* practical that simpler techniques that don't depend on any such algorithmic subtlety like brute-force flooding. But if the history of Internet security has taught us one thing, it's that naively hoping something won't be a problem is doomed. The main issue is performance. IPv6 addresses are big, and although SipHash is fast by the standard of cryptographic hashes, it's far slower than jhash or any other non-cryptographic hash.
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