netdev - Re: [PATCH v2] net: ipv4: move tcp_fastopen server side code to SipHash library

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Message-ID: <20190617012253.4hgfdwvurskstz4o@gondor.apana.org.au>
Date:   Mon, 17 Jun 2019 09:22:53 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     netdev@...r.kernel.org, linux-crypto@...r.kernel.org,
        ebiggers@...nel.org, edumazet@...gle.com, davem@...emloft.net,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org, jbaron@...mai.com,
        cpaasch@...le.com, David.Laight@...lab.com
Subject: Re: [PATCH v2] net: ipv4: move tcp_fastopen server side code to
 SipHash library

On Fri, Jun 14, 2019 at 04:01:22PM +0200, Ard Biesheuvel wrote:
> Using a bare block cipher in non-crypto code is almost always a bad idea,
> not only for security reasons (and we've seen some examples of this in
> the kernel in the past), but also for performance reasons.
> 
> In the TCP fastopen case, we call into the bare AES block cipher one or
> two times (depending on whether the connection is IPv4 or IPv6). On most
> systems, this results in a call chain such as
> 
>   crypto_cipher_encrypt_one(ctx, dst, src)
>     crypto_cipher_crt(tfm)->cit_encrypt_one(crypto_cipher_tfm(tfm), ...);
>       aesni_encrypt
>         kernel_fpu_begin();
>         aesni_enc(ctx, dst, src); // asm routine
>         kernel_fpu_end();
> 
> It is highly unlikely that the use of special AES instructions has a
> benefit in this case, especially since we are doing the above twice
> for IPv6 connections, instead of using a transform which can process
> the entire input in one go.
> 
> We could switch to the cbcmac(aes) shash, which would at least get
> rid of the duplicated overhead in *some* cases (i.e., today, only
> arm64 has an accelerated implementation of cbcmac(aes), while x86 will
> end up using the generic cbcmac template wrapping the AES-NI cipher,
> which basically ends up doing exactly the above). However, in the given
> context, it makes more sense to use a light-weight MAC algorithm that
> is more suitable for the purpose at hand, such as SipHash.
> 
> Since the output size of SipHash already matches our chosen value for
> TCP_FASTOPEN_COOKIE_SIZE, and given that it accepts arbitrary input
> sizes, this greatly simplifies the code as well.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
> v2: rebase onto net-next
>     reverse order of operands in BUILD_BUG_ON() comparison expression
> 
>  include/linux/tcp.h     |  7 +-
>  include/net/tcp.h       | 10 +-
>  net/ipv4/tcp_fastopen.c | 97 +++++++-------------
>  3 files changed, 36 insertions(+), 78 deletions(-)

You should also revert commit 798b2cbf9227 in your patch:

commit 798b2cbf9227b1bd7d37ae9af4d9c750e6f4de9c
Author: David S. Miller <davem@...emloft.net>
Date:   Tue Sep 4 14:20:14 2012 -0400

    net: Add INET dependency on aes crypto for the sake of TCP fastopen.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt