ipv4: Detect rollover in specific fib table dump

Sven-Haegar reported looping on fib dumps when 255.255.255.255 route has
been added to a table. The looping is caused by the key rolling over from
FFFFFFFF to 0. When dumping a specific table only, we need a means to detect
when the table dump is done. The key and count saved to cb args are both 0
only at the start of the table dump. If key is 0 and count > 0, then we are
in the rollover case. Detect and return to avoid looping.

This only affects dumps of a specific table; for dumps of all tables
(the case prior to the change in the Fixes tag) inet_dump_fib moved
the entry counter to the next table and reset the cb args used by
fib_table_dump and fn_trie_dump_leaf, so the rollover ffffffff back
to 0 did not cause looping with the dumps.

Fixes: effe6792 ("net: Enable kernel side filtering of route dumps")
Reported-by: Sven-Haegar Koch <haegar@sdinet.de>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -2193,6 +2193,12 @@ int fib_table_dump(struct fib_table *tb, struct sk_buff *skb,
 	int count = cb->args[2];
 	t_key key = cb->args[3];
 
+	/* First time here, count and key are both always 0. Count > 0
+	 * and key == 0 means the dump has wrapped around and we are done.
+	 */
+	if (count && !key)
+		return skb->len;
+
 	while ((l = leaf_walk_rcu(&tp, key)) != NULL) {
 		int err;