netfilter: nf_conntrack_tcp: Fix stack out of bounds when parsing TCP options

BugLink: https://bugs.launchpad.net/bugs/1822271

commit 644c7e48 upstream.

Baozeng Ding reported a KASAN stack out of bounds issue - it uncovered that
the TCP option parsing routines in netfilter TCP connection tracking could
read one byte out of the buffer of the TCP options.  Therefore in the patch
we check that the available data length is large enough to parse both TCP
option code and size.
Reported-by: Baozeng Ding <sploving1@gmail.com>
Tested-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Zubin Mithra <zsm@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/netfilter/nf_conntrack_proto_tcp.c

@@ -410,6 +410,8 @@ static void tcp_options(const struct sk_buff *skb,
 			length--;
 			continue;
 		default:
+			if (length < 2)
+				return;
 			opsize=*ptr++;
 			if (opsize < 2) /* "silly options" */
 				return;
@@ -470,6 +472,8 @@ static void tcp_sack(const struct sk_buff *skb, unsigned int dataoff,
 			length--;
 			continue;
 		default:
+			if (length < 2)
+				return;
 			opsize = *ptr++;
 			if (opsize < 2) /* "silly options" */
 				return;