Code: static int netlink_dump(struct sock *sk)
{
	struct netlink_sock *nlk = nlk_sk(sk);
 	struct netlink_callback *cb;
 	struct sk_buff *skb = NULL;
 	struct nlmsghdr *nlh;
 	int len, err = -ENOBUFS;
 	int alloc_min_size;
 	int alloc_size;

	mutex_lock(nlk->cb_mutex);
	if (!nlk->cb_running) {
		err = -EINVAL;
		goto errout_skb;
	}

	if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
		goto errout_skb;

	/* NLMSG_GOODSIZE is small to avoid high order allocations being
	 * required, but it makes sense to _attempt_ a 16K bytes allocation
	 * to reduce number of system calls on dump operations, if user
	 * ever provided a big enough buffer.
	 */
	cb = &nlk->cb;
	alloc_min_size = max_t(int, cb->min_dump_alloc, NLMSG_GOODSIZE);

	if (alloc_min_size < nlk->max_recvmsg_len) {
		alloc_size = nlk->max_recvmsg_len;
		skb = alloc_skb(alloc_size, GFP_KERNEL |
					    __GFP_NOWARN | __GFP_NORETRY);
	}
	if (!skb) {
		alloc_size = alloc_min_size;
		skb = alloc_skb(alloc_size, GFP_KERNEL);
	}
	if (!skb)
		goto errout_skb;

	/* Trim skb to allocated size. User is expected to provide buffer as
	 * large as max(min_dump_alloc, 16KiB (mac_recvmsg_len capped at
	 * netlink_recvmsg())). dump will pack as many smaller messages as
	 * could fit within the allocated skb. skb is typically allocated
	 * with larger space than required (could be as much as near 2x the
	 * requested size with align to next power of 2 approach). Allowing
	 * dump to use the excess space makes it difficult for a user to have a
	 * reasonable static buffer based on the expected largest dump of a
	 * single netdev. The outcome is MSG_TRUNC error.
	 */
	skb_reserve(skb, skb_tailroom(skb) - alloc_size);
	netlink_skb_set_owner_r(skb, sk);

	len = cb->dump(skb, cb);

	if (len > 0) {
		mutex_unlock(nlk->cb_mutex);

		if (sk_filter(sk, skb))
			kfree_skb(skb);
		else
			__netlink_sendskb(sk, skb);
		return 0;
	}

	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
	if (!nlh)
		goto errout_skb;

	nl_dump_check_consistent(cb, nlh);

	memcpy(nlmsg_data(nlh), &len, sizeof(len));

	if (sk_filter(sk, skb))
		kfree_skb(skb);
	else
		__netlink_sendskb(sk, skb);

	if (cb->done)
 		cb->done(cb);
 
 	nlk->cb_running = false;
 	mutex_unlock(nlk->cb_mutex);
	module_put(cb->module);
	consume_skb(cb->skb);
 	return 0;
 
 errout_skb:
	mutex_unlock(nlk->cb_mutex);
	kfree_skb(skb);
	return err;
}

Vulnerability Type: DoS 
CWE ID: CWE-415
Summary: Race condition in the netlink_dump function in net/netlink/af_netlink.c in the Linux kernel before 4.6.3 allows local users to cause a denial of service (double free) or possibly have unspecified other impact via a crafted application that makes sendmsg system calls, leading to a free operation associated with a new dump that started earlier than anticipated.
Commit Message: 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>