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Date:   Sun, 16 Jun 2019 13:47:05 -0700 (PDT)
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To:     ard.biesheuvel@linaro.org
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Subject: Re: [PATCH v2] net: ipv4: move tcp_fastopen server side code to
 SipHash library
From:   David Miller <davem@davemloft.net>
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References: <20190614140122.20934-1-ard.biesheuvel@linaro.org>
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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date: Fri, 14 Jun 2019 16:01:22 +0200

> 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@linaro.org>

Please add some text to the commit message explaining the potential
partial deployment problems Eric mentioned.

Thank you.