RE: [RFC PATCH] net: ipv4: move tcp_fastopen server side code to SipHash library

[David Laight <David.Laight@ACULAB.COM>]

From: Ard Biesheuvel
> Sent: 14 June 2019 12:15
> (fix Eric's email address)
> 
> On Fri, 14 Jun 2019 at 13:14, Ard Biesheuvel <ard.biesheuvel@linaro.org> 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.
...
> > +       BUILD_BUG_ON(sizeof(siphash_key_t) != TCP_FASTOPEN_KEY_LENGTH);
> > +       BUILD_BUG_ON(sizeof(u64) != TCP_FASTOPEN_COOKIE_SIZE);

Those comparisons are backwards.

	David

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