netfilter: nf_ct_sip: validate Content-Length in TCP SIP messages

Verify that the message length of a single SIP message, which is calculated
based on the Content-Length field contained in the SIP message, does not
exceed the packet boundaries.
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_sip.c

@@ -1461,6 +1461,8 @@ static int sip_help_tcp(struct sk_buff *skb, unsigned int protoff,
 		end += strlen("\r\n\r\n") + clen;
 
 		msglen = origlen = end - dptr;
+		if (msglen > datalen)
+			return NF_DROP;
 
 		ret = process_sip_msg(skb, ct, dataoff, &dptr, &msglen);
 		if (ret != NF_ACCEPT)