netfilter: xt_sctp: validate the flag_info count

sctp_mt_check doesn't validate the flag_count field. An attacker can
take advantage of that to trigger a OOB read and leak memory
information.

Add the field validation in the checkentry function.

Fixes: 2e4e6a1 ("[NETFILTER] x_tables: Abstraction layer for {ip,ip6,arp}_tables")
Cc: [email protected]
Reported-by: Lucas Leong <[email protected]>
Signed-off-by: Wander Lairson Costa <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>

Author: walac

net/netfilter/xt_sctp.c

@@ -149,6 +149,8 @@ static int sctp_mt_check(const struct xt_mtchk_param *par)
 {
 	const struct xt_sctp_info *info = par->matchinfo;
 
+	if (info->flag_count > ARRAY_SIZE(info->flag_info))
+		return -EINVAL;
 	if (info->flags & ~XT_SCTP_VALID_FLAGS)
 		return -EINVAL;
 	if (info->invflags & ~XT_SCTP_VALID_FLAGS)