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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
