netlink: Fix dump skb leak/double free

When we free cb->skb after a dump, we do it after releasing the
lock.  This means that a new dump could have started in the time
being and we'll end up freeing their skb instead of ours.

This patch saves the skb and module before we unlock so we free
the right memory.

Fixes: 16b304f3404f ("netlink: Eliminate kmalloc in netlink dump operation.")
Reported-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
int netlink_broadcast_filtered(struct sock *ssk, struct sk_buff *skb, u32 portid,
	u32 group, gfp_t allocation,
	int (*filter)(struct sock *dsk, struct sk_buff *skb, void *data),
	void *filter_data)
{
	struct net *net = sock_net(ssk);
	struct netlink_broadcast_data info;
	struct sock *sk;

	skb = netlink_trim(skb, allocation);

	info.exclude_sk = ssk;
	info.net = net;
	info.portid = portid;
	info.group = group;
	info.failure = 0;
	info.delivery_failure = 0;
	info.congested = 0;
	info.delivered = 0;
	info.allocation = allocation;
	info.skb = skb;
	info.skb2 = NULL;
	info.tx_filter = filter;
	info.tx_data = filter_data;

	/* While we sleep in clone, do not allow to change socket list */

	netlink_lock_table();

	sk_for_each_bound(sk, &nl_table[ssk->sk_protocol].mc_list)
		do_one_broadcast(sk, &info);

	consume_skb(skb);

	netlink_unlock_table();

	if (info.delivery_failure) {
		kfree_skb(info.skb2);
		return -ENOBUFS;
	}
	consume_skb(info.skb2);

	if (info.delivered) {
		if (info.congested && gfpflags_allow_blocking(allocation))
			yield();
		return 0;
	}
	return -ESRCH;
}

int netlink_broadcast_filtered(struct sock *ssk, struct sk_buff *skb, u32 portid,
	u32 group, gfp_t allocation,
	int (*filter)(struct sock *dsk, struct sk_buff *skb, void *data),
	void *filter_data)
{
	struct net *net = sock_net(ssk);
	struct netlink_broadcast_data info;
	struct sock *sk;

	skb = netlink_trim(skb, allocation);

	info.exclude_sk = ssk;
	info.net = net;
	info.portid = portid;
	info.group = group;
	info.failure = 0;
	info.delivery_failure = 0;
	info.congested = 0;
	info.delivered = 0;
	info.allocation = allocation;
	info.skb = skb;
	info.skb2 = NULL;
	info.tx_filter = filter;
	info.tx_data = filter_data;

	/* While we sleep in clone, do not allow to change socket list */

	netlink_lock_table();

	sk_for_each_bound(sk, &nl_table[ssk->sk_protocol].mc_list)
		do_one_broadcast(sk, &info);

	consume_skb(skb);

	netlink_unlock_table();

	if (info.delivery_failure) {
		kfree_skb(info.skb2);
		return -ENOBUFS;
	}
	consume_skb(info.skb2);

	if (info.delivered) {
		if (info.congested && gfpflags_allow_blocking(allocation))
			yield();
		return 0;
	}
	return -ESRCH;
}
