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Date: Thu, 22 Dec 2016 19:08:37 +0100
From: Hannes Frederic Sowa <hannes@...essinduktion.org>
To: Theodore Ts'o <tytso@....edu>, "Jason A. Donenfeld" <Jason@...c4.com>,
kernel-hardening@...ts.openwall.com, Andy Lutomirski <luto@...capital.net>,
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: Re: [PATCH v7 3/6] random: use SipHash in
place of MD5
On 22.12.2016 16:54, Theodore Ts'o wrote:
> On Thu, Dec 22, 2016 at 02:10:33PM +0100, Jason A. Donenfeld wrote:
>> On Thu, Dec 22, 2016 at 1:47 PM, Hannes Frederic Sowa
>> <hannes@...essinduktion.org> wrote:
>>> following up on what appears to be a random subject: ;)
>>>
>>> IIRC, ext4 code by default still uses half_md4 for hashing of filenames
>>> in the htree. siphash seems to fit this use case pretty good.
>>
>> I saw this too. I'll try to address it in v8 of this series.
>
> This is a separate issue, and this series is getting a bit too
> complex. So I'd suggest pushing this off to a separate change.
>
> Changing the htree hash algorithm is an on-disk format change, and so
> we couldn't roll it out until e2fsprogs gets updated and rolled out
> pretty broadley. In fact George sent me patches to add siphash as a
> hash algorithm for htree a while back (for both the kernel and
> e2fsprogs), but I never got around to testing and applying them,
> mainly because while it's technically faster, I had other higher
> priority issues to work on --- and see previous comments regarding
> pixel peeping. Improving the hash algorithm by tens or even hundreds
> of nanoseconds isn't really going to matter since we only do a htree
> lookup on a file creation or cold cache lookup, and the SSD or HDD I/O
> times will dominate. And from the power perspective, saving
> microwatts of CPU power isn't going to matter if you're going to be
> spinning up the storage device....
I wasn't concerned about performance but more about DoS resilience. I
wonder how safe half md4 actually is in terms of allowing users to
generate long hash chains in the filesystem (in terms of length
extension attacks against half_md4).
In ext4, is it actually possible that a "disrupter" learns about the
hashing secret in the way how the inodes are returned during getdents?
Thanks,
Hannes
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