linux-kernel - Re: [PATCH v2 06/25] tcp: authopt: Compute packet signatures

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Date:   Fri, 5 Nov 2021 08:09:34 +0200
From:   Leonard Crestez <cdleonard@...il.com>
To:     Dmitry Safonov <0x7f454c46@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Francesco Ruggeri <fruggeri@...sta.com>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Christoph Paasch <cpaasch@...le.com>,
        Ivan Delalande <colona@...sta.com>,
        Priyaranjan Jha <priyarjha@...gle.com>, netdev@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v2 06/25] tcp: authopt: Compute packet signatures

On 11/5/21 4:08 AM, Dmitry Safonov wrote:
> On 11/1/21 16:34, Leonard Crestez wrote:
> [..]
>> +/* Find TCP_AUTHOPT in header.
>> + *
>> + * Returns pointer to TCP_AUTHOPT or NULL if not found.
>> + */
>> +static u8 *tcp_authopt_find_option(struct tcphdr *th)
>> +{
>> +	int length = (th->doff << 2) - sizeof(*th);
>> +	u8 *ptr = (u8 *)(th + 1);
>> +
>> +	while (length >= 2) {
>> +		int opcode = *ptr++;
>> +		int opsize;
>> +
>> +		switch (opcode) {
>> +		case TCPOPT_EOL:
>> +			return NULL;
>> +		case TCPOPT_NOP:
>> +			length--;
>> +			continue;
>> +		default:
>> +			if (length < 2)
>> +				return NULL;
> 
> ^ never true, as checked by the loop condition
> 
>> +			opsize = *ptr++;
>> +			if (opsize < 2)
>> +				return NULL;
>> +			if (opsize > length)
>> +				return NULL;
>> +			if (opcode == TCPOPT_AUTHOPT)
>> +				return ptr - 2;
>> +		}
>> +		ptr += opsize - 2;
>> +		length -= opsize;
>> +	}
>> +	return NULL;
>> +}
> 
> Why copy'n'pasting tcp_parse_md5sig_option(), rather than adding a new
> argument to the function?

No good reason.

There is a requirement in RFC5925 that packets with both AO and MD5 
signatures be dropped. This currently works but the implementation is 
convoluted: after an AO signature is found an error is returned if MD5 
is also present.

A better solution would be to do a single scan for both options up 
front, for example in tcp_{v4,v6}_auth_inbound_check

--
Regards,
Leonard