From: Hannes Frederic Sowa <hannes@stressinduktion.org>
Subject: Re: [PATCH v2 1/4] siphash: add cryptographically secure hashtable
 function
Date: Thu, 15 Dec 2016 16:53:12 +0100
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Cc: Netdev <netdev@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com"
        <kernel-hardening@lists.openwall.com>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        "Daniel J . Bernstein" <djb@cr.yp.to>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Eric Biggers <ebiggers3@gmail.com>
To: David Laight <David.Laight@ACULAB.COM>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
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On 15.12.2016 16:41, David Laight wrote:
> Try (retyped):
> 
> echo 'struct { long a; long long b; } s; int bar { return sizeof s; }' >foo.c
> gcc [-m32] -O2 -S foo.c; cat foo.s
> 
> And look at what is generated.

I used __alignof__(unsigned long long) with -m32.

>> Right now ipv6 addresses have an alignment of 4. So we couldn't even
>> naturally pass them to siphash but would need to copy them around, which
>> I feel like a source of bugs.
> 
> That is more of a problem on systems that don't support misaligned accesses.
> Reading the 64bit values with two explicit 32bit reads would work.
> I think you can get gcc to do that by adding an aligned(4) attribute to the
> structure member.

Yes, and that is actually my fear, because we support those
architectures. I can't comment on that as I don't understand enough of this.

If someone finds a way to cause misaligned reads on a small box this
seems (maybe depending on sysctls they get fixed up or panic) to be a
much bigger issue than having a hash DoS.

Thanks,
Hannes