Code:  static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 {
 	struct packet_sock *po = pkt_sk(sk);
 	struct packet_fanout *f, *match;
 	u8 type = type_flags & 0xff;
	u8 flags = type_flags >> 8;
	int err;

	switch (type) {
	case PACKET_FANOUT_ROLLOVER:
		if (type_flags & PACKET_FANOUT_FLAG_ROLLOVER)
			return -EINVAL;
	case PACKET_FANOUT_HASH:
	case PACKET_FANOUT_LB:
	case PACKET_FANOUT_CPU:
	case PACKET_FANOUT_RND:
	case PACKET_FANOUT_QM:
	case PACKET_FANOUT_CBPF:
	case PACKET_FANOUT_EBPF:
		break;
	default:
 		return -EINVAL;
 	}
 
 	if (!po->running)
		return -EINVAL;
 
 	if (po->fanout)
		return -EALREADY;
 
 	if (type == PACKET_FANOUT_ROLLOVER ||
 	    (type_flags & PACKET_FANOUT_FLAG_ROLLOVER)) {
		po->rollover = kzalloc(sizeof(*po->rollover), GFP_KERNEL);
		if (!po->rollover)
			return -ENOMEM;
		atomic_long_set(&po->rollover->num, 0);
		atomic_long_set(&po->rollover->num_huge, 0);
		atomic_long_set(&po->rollover->num_failed, 0);
 	}
 
	mutex_lock(&fanout_mutex);
 	match = NULL;
 	list_for_each_entry(f, &fanout_list, list) {
 		if (f->id == id &&
		    read_pnet(&f->net) == sock_net(sk)) {
			match = f;
			break;
		}
	}
	err = -EINVAL;
	if (match && match->flags != flags)
		goto out;
	if (!match) {
		err = -ENOMEM;
		match = kzalloc(sizeof(*match), GFP_KERNEL);
		if (!match)
			goto out;
		write_pnet(&match->net, sock_net(sk));
		match->id = id;
		match->type = type;
		match->flags = flags;
		INIT_LIST_HEAD(&match->list);
		spin_lock_init(&match->lock);
		atomic_set(&match->sk_ref, 0);
		fanout_init_data(match);
		match->prot_hook.type = po->prot_hook.type;
		match->prot_hook.dev = po->prot_hook.dev;
		match->prot_hook.func = packet_rcv_fanout;
		match->prot_hook.af_packet_priv = match;
		match->prot_hook.id_match = match_fanout_group;
		dev_add_pack(&match->prot_hook);
		list_add(&match->list, &fanout_list);
	}
	err = -EINVAL;
	if (match->type == type &&
	    match->prot_hook.type == po->prot_hook.type &&
	    match->prot_hook.dev == po->prot_hook.dev) {
		err = -ENOSPC;
		if (atomic_read(&match->sk_ref) < PACKET_FANOUT_MAX) {
			__dev_remove_pack(&po->prot_hook);
			po->fanout = match;
			atomic_inc(&match->sk_ref);
			__fanout_link(sk, po);
			err = 0;
 		}
 	}
 out:
	mutex_unlock(&fanout_mutex);
	if (err) {
		kfree(po->rollover);
 		po->rollover = NULL;
 	}
 	return err;
 }

Vulnerability Type: DoS 
CWE ID: CWE-416
Summary: Race condition in net/packet/af_packet.c in the Linux kernel before 4.9.13 allows local users to cause a denial of service (use-after-free) or possibly have unspecified other impact via a multithreaded application that makes PACKET_FANOUT setsockopt system calls.
Commit Message: packet: fix races in fanout_add()

Multiple threads can call fanout_add() at the same time.

We need to grab fanout_mutex earlier to avoid races that could
lead to one thread freeing po->rollover that was set by another thread.

Do the same in fanout_release(), for peace of mind, and to help us
finding lockdep issues earlier.

Fixes: dc99f600698d ("packet: Add fanout support.")
Fixes: 0648ab70afe6 ("packet: rollover prepare: per-socket state")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>