netdev - RE: [PATCH RFC v1 1/3] bpf: move from sha1 to blake2s in tag calculation

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Message-ID: <13d51088799746469d26a442fb3c6fd5@AcuMS.aculab.com>
Date:   Fri, 14 Jan 2022 15:59:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Jason A. Donenfeld'" <Jason@...c4.com>,
        Ard Biesheuvel <ardb@...nel.org>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "Network Development" <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: RE: [PATCH RFC v1 1/3] bpf: move from sha1 to blake2s in tag
 calculation

From: Jason A. Donenfeld
> Sent: 14 January 2022 15:21
> 
> On Fri, Jan 14, 2022 at 4:08 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > Yeah, so the issue is that, at *some* point, SHA-1 is going to have to
> > go. So it would be helpful if Alexei could clarify *why* he doesn't
> > see this as a problem. The fact that it is broken means that it is no
> > longer intractable to forge collisions, which likley means that SHA-1
> > no longer fulfills the task that you wanted it to do in the first
> > place.
> 
> I think the reason that Alexei doesn't think that the SHA-1 choice
> really matters is because the result is being truncated to 64-bits, so
> collisions are easy anyway...

Which probably means that SHA-1 is complete overkill and something
much simpler could have been used instead.
Is the buffer even big enough to have ever warranted the massive
unrolling of the sha-1 function.
(I suspect that just destroys the I-cache on most cpu.)

The IPv6 address case seems even more insane - how many bytes
are actually being hashed.
The unrolled loop is only likely to be sane for large (megabyte)
buffers.

	David

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