On Fri, Jun 14, 2019 at 7:01 AM Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> 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> While the patch looks fine (I yet have to run our tests with it), it might cause some deployment issues for server farms. They usually share a common fastopen key, so that clients can reuse the same token for different sessions. Changing some servers in the pool will lead to inconsistencies. Probably not a too big deal, but worth mentioning.
