netdev - Re: [PATCH v1 net] tcp: Fix SYN option room calculation for TCP-AO.

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Message-ID: <559ac473-b771-4936-8afc-62ae39ec540b@arista.com>
Date: Thu, 2 Nov 2023 21:28:49 +0000
From: Dmitry Safonov <dima@...sta.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: Salam Noureddine <noureddine@...sta.com>,
 Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
 Francesco Ruggeri <fruggeri05@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v1 net] tcp: Fix SYN option room calculation for TCP-AO.

On 11/2/23 21:05, Kuniyuki Iwashima wrote:
> When building SYN packet in tcp_syn_options(), MSS, TS, WS, and
> SACKPERM are used without checking the remaining bytes in the
> options area.
> 
> To keep that logic as is, we limit the TCP-AO MAC length in
> tcp_ao_parse_crypto().  Currently, the limit is calculated as below.
> 
>   MAX_TCP_OPTION_SPACE - TCPOLEN_TSTAMP_ALIGNED
>                        - TCPOLEN_WSCALE_ALIGNED
>                        - TCPOLEN_SACKPERM_ALIGNED
> 
> This looks confusing as (1) we pack SACKPERM into the leading
> 2-bytes of the aligned 12-bytes of TS and (2) TCPOLEN_MSS_ALIGNED
> is not used.  Fortunately, the calculated limit is not wrong as
> TCPOLEN_SACKPERM_ALIGNED and TCPOLEN_MSS_ALIGNED are the same value.
> 
> However, we should use the proper constant in the formula.
> 
>   MAX_TCP_OPTION_SPACE - TCPOLEN_MSS_ALIGNED
>                        - TCPOLEN_TSTAMP_ALIGNED
>                        - TCPOLEN_WSCALE_ALIGNED
> 
> Fixes: 4954f17ddefc ("net/tcp: Introduce TCP_AO setsockopt()s")
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Sharp eye!
Thanks for the patch and adjusting the comment.

Reviewed-by: Dmitry Safonov <dima@...sta.com>

> ---
>  net/ipv4/tcp_ao.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
> index ef5472ed6158..7696417d0640 100644
> --- a/net/ipv4/tcp_ao.c
> +++ b/net/ipv4/tcp_ao.c
> @@ -1315,7 +1315,8 @@ static int tcp_ao_parse_crypto(struct tcp_ao_add *cmd, struct tcp_ao_key *key)
>  	key->maclen = cmd->maclen ?: 12; /* 12 is the default in RFC5925 */
>  
>  	/* Check: maclen + tcp-ao header <= (MAX_TCP_OPTION_SPACE - mss
> -	 *					- tstamp - wscale - sackperm),
> +	 *					- tstamp (including sackperm)
> +	 *					- wscale),
>  	 * see tcp_syn_options(), tcp_synack_options(), commit 33ad798c924b.
>  	 *
>  	 * In order to allow D-SACK with TCP-AO, the header size should be:
> @@ -1342,9 +1343,9 @@ static int tcp_ao_parse_crypto(struct tcp_ao_add *cmd, struct tcp_ao_key *key)
>  	 * large to leave sufficient option space.
>  	 */
>  	syn_tcp_option_space = MAX_TCP_OPTION_SPACE;
> +	syn_tcp_option_space -= TCPOLEN_MSS_ALIGNED;
>  	syn_tcp_option_space -= TCPOLEN_TSTAMP_ALIGNED;
>  	syn_tcp_option_space -= TCPOLEN_WSCALE_ALIGNED;
> -	syn_tcp_option_space -= TCPOLEN_SACKPERM_ALIGNED;
>  	if (tcp_ao_len(key) > syn_tcp_option_space) {
>  		err = -EMSGSIZE;
>  		goto err_kfree;

Thanks,
             Dmitry