[MPTCP] Re: [PATCH mptcp-next] mptcp: use rightmost 64 bits in ADD_ADDR HMAC

[Christoph Paasch <cpaasch at apple.com>]

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Hello,

On 18/05/20 - 15:29:16, Todd Malsbary wrote:
> This changes the HMAC used in the ADD_ADDR option from the leftmost 64
> bits to the rightmost 64 bits as described in RFC 8684, section 3.4.1.
> 
> Signed-off-by: Todd Malsbary <todd.malsbary(a)linux.intel.com>
> ---
> 
> I found this while adding support to packetdrill for the ADD_ADDR v1
> option.  The computation I used for the HMAC in the pending
> packetdrill work gave a different result than what was being computed
> by the kernel.
> 
> It appears that the usage of the leftmost bits has been in place since
> at least 2016 in the out-of-tree code (commit de09a83186666).  Given
> that, is the issue in the implementation or the RFC?
> 
>  net/mptcp/options.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/mptcp/options.c b/net/mptcp/options.c
> index ece6f92cf7d1..0e11fb3d908f 100644
> --- a/net/mptcp/options.c
> +++ b/net/mptcp/options.c
> @@ -550,7 +550,7 @@ static u64 add_addr_generate_hmac(u64 key1, u64 key2, u8 addr_id,
>  
>  	mptcp_crypto_hmac_sha(key1, key2, msg, 7, hmac);
>  
> -	return get_unaligned_be64(hmac);
> +	return get_unaligned_be64(&hmac[MPTCP_ADDR_HMAC_LEN - sizeof(u64)]);

as I was trying to do the same change in out-of-tree MPTCP, I realize
something maybe a little bit late:

I think MPTCP_ADDR_HMAC_LEN is the wrong variable, no?

We want to take the rightmost bits of the SHA-256 output. Meaning, we need
to get the right most out of mptcp_hashed_key in mptcp_crypto_hmac_sha().

Because this latter function is already only copying the first 160bits to
hashed_out - thus we are missing the remaining bits...


Right?


Christoph



>  }
>  
>  #if IS_ENABLED(CONFIG_MPTCP_IPV6)
> @@ -567,7 +567,7 @@ static u64 add_addr6_generate_hmac(u64 key1, u64 key2, u8 addr_id,
>  
>  	mptcp_crypto_hmac_sha(key1, key2, msg, 19, hmac);
>  
> -	return get_unaligned_be64(hmac);
> +	return get_unaligned_be64(&hmac[MPTCP_ADDR_HMAC_LEN - sizeof(u64)]);
>  }
>  #endif
>  
> -- 
> 2.25.4
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