netfilter: nf_ct_ftp: fix out of bounds read in update_nl_seq()

As noticed by Dan Carpenter <error27@gmail.com>, update_nl_seq()
currently contains an out of bounds read of the seq_aft_nl array
when looking for the oldest sequence number position.

Fix it to only compare valid positions.

Cc: stable@kernel.org
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_ftp.c

@@ -323,24 +323,24 @@ static void update_nl_seq(struct nf_conn *ct, u32 nl_seq,
 			  struct nf_ct_ftp_master *info, int dir,
 			  struct sk_buff *skb)
 {
-	unsigned int i, oldest = NUM_SEQ_TO_REMEMBER;
+	unsigned int i, oldest;
 
 	/* Look for oldest: if we find exact match, we're done. */
 	for (i = 0; i < info->seq_aft_nl_num[dir]; i++) {
 		if (info->seq_aft_nl[dir][i] == nl_seq)
 			return;
-
-		if (oldest == info->seq_aft_nl_num[dir] ||
-		    before(info->seq_aft_nl[dir][i],
-			   info->seq_aft_nl[dir][oldest]))
-			oldest = i;
 	}
 
 	if (info->seq_aft_nl_num[dir] < NUM_SEQ_TO_REMEMBER) {
 		info->seq_aft_nl[dir][info->seq_aft_nl_num[dir]++] = nl_seq;
-	} else if (oldest != NUM_SEQ_TO_REMEMBER &&
-		   after(nl_seq, info->seq_aft_nl[dir][oldest])) {
-		info->seq_aft_nl[dir][oldest] = nl_seq;
+	} else {
+		if (before(info->seq_aft_nl[dir][0], info->seq_aft_nl[dir][1]))
+			oldest = 0;
+		else
+			oldest = 1;
+
+		if (after(nl_seq, info->seq_aft_nl[dir][oldest]))
+			info->seq_aft_nl[dir][oldest] = nl_seq;
 	}
 }