Code: static struct rds_connection *__rds_conn_create(struct net *net,
						__be32 laddr, __be32 faddr,
				       struct rds_transport *trans, gfp_t gfp,
				       int is_outgoing)
{
	struct rds_connection *conn, *parent = NULL;
	struct hlist_head *head = rds_conn_bucket(laddr, faddr);
	struct rds_transport *loop_trans;
	unsigned long flags;
	int ret;

	rcu_read_lock();
	conn = rds_conn_lookup(net, head, laddr, faddr, trans);
	if (conn && conn->c_loopback && conn->c_trans != &rds_loop_transport &&
	    laddr == faddr && !is_outgoing) {
		/* This is a looped back IB connection, and we're
		 * called by the code handling the incoming connect.
		 * We need a second connection object into which we
		 * can stick the other QP. */
		parent = conn;
		conn = parent->c_passive;
	}
	rcu_read_unlock();
	if (conn)
		goto out;

	conn = kmem_cache_zalloc(rds_conn_slab, gfp);
	if (!conn) {
		conn = ERR_PTR(-ENOMEM);
		goto out;
	}

	INIT_HLIST_NODE(&conn->c_hash_node);
	conn->c_laddr = laddr;
	conn->c_faddr = faddr;
	spin_lock_init(&conn->c_lock);
	conn->c_next_tx_seq = 1;
	rds_conn_net_set(conn, net);

	init_waitqueue_head(&conn->c_waitq);
	INIT_LIST_HEAD(&conn->c_send_queue);
	INIT_LIST_HEAD(&conn->c_retrans);

	ret = rds_cong_get_maps(conn);
	if (ret) {
		kmem_cache_free(rds_conn_slab, conn);
		conn = ERR_PTR(ret);
		goto out;
	}

	/*
	 * This is where a connection becomes loopback.  If *any* RDS sockets
	 * can bind to the destination address then we'd rather the messages
	 * flow through loopback rather than either transport.
	 */
	loop_trans = rds_trans_get_preferred(net, faddr);
	if (loop_trans) {
		rds_trans_put(loop_trans);
		conn->c_loopback = 1;
		if (is_outgoing && trans->t_prefer_loopback) {
			/* "outgoing" connection - and the transport
			 * says it wants the connection handled by the
			 * loopback transport. This is what TCP does.
			 */
			trans = &rds_loop_transport;
 		}
 	}
 
	if (trans == NULL) {
		kmem_cache_free(rds_conn_slab, conn);
		conn = ERR_PTR(-ENODEV);
		goto out;
	}
 	conn->c_trans = trans;
 
 	ret = trans->conn_alloc(conn, gfp);
	if (ret) {
		kmem_cache_free(rds_conn_slab, conn);
		conn = ERR_PTR(ret);
		goto out;
	}

	atomic_set(&conn->c_state, RDS_CONN_DOWN);
	conn->c_send_gen = 0;
	conn->c_outgoing = (is_outgoing ? 1 : 0);
	conn->c_reconnect_jiffies = 0;
	INIT_DELAYED_WORK(&conn->c_send_w, rds_send_worker);
	INIT_DELAYED_WORK(&conn->c_recv_w, rds_recv_worker);
	INIT_DELAYED_WORK(&conn->c_conn_w, rds_connect_worker);
	INIT_WORK(&conn->c_down_w, rds_shutdown_worker);
	mutex_init(&conn->c_cm_lock);
	conn->c_flags = 0;

	rdsdebug("allocated conn %p for %pI4 -> %pI4 over %s %s\n",
	  conn, &laddr, &faddr,
	  trans->t_name ? trans->t_name : "[unknown]",
	  is_outgoing ? "(outgoing)" : "");

	/*
	 * Since we ran without holding the conn lock, someone could
	 * have created the same conn (either normal or passive) in the
	 * interim. We check while holding the lock. If we won, we complete
	 * init and return our conn. If we lost, we rollback and return the
	 * other one.
	 */
	spin_lock_irqsave(&rds_conn_lock, flags);
	if (parent) {
		/* Creating passive conn */
		if (parent->c_passive) {
			trans->conn_free(conn->c_transport_data);
			kmem_cache_free(rds_conn_slab, conn);
			conn = parent->c_passive;
		} else {
			parent->c_passive = conn;
			rds_cong_add_conn(conn);
			rds_conn_count++;
		}
	} else {
		/* Creating normal conn */
		struct rds_connection *found;

		found = rds_conn_lookup(net, head, laddr, faddr, trans);
		if (found) {
			trans->conn_free(conn->c_transport_data);
			kmem_cache_free(rds_conn_slab, conn);
			conn = found;
		} else {
			hlist_add_head_rcu(&conn->c_hash_node, head);
			rds_cong_add_conn(conn);
			rds_conn_count++;
		}
	}
	spin_unlock_irqrestore(&rds_conn_lock, flags);

out:
	return conn;
}

Vulnerability Type: DoS 
CWE ID: CWE-362
Summary: Race condition in the rds_sendmsg function in net/rds/sendmsg.c in the Linux kernel before 4.3.3 allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact by using a socket that was not properly bound.  NOTE: this vulnerability exists because of an incomplete fix for CVE-2015-6937.
Commit Message: RDS: fix race condition when sending a message on unbound socket

Sasha's found a NULL pointer dereference in the RDS connection code when
sending a message to an apparently unbound socket.  The problem is caused
by the code checking if the socket is bound in rds_sendmsg(), which checks
the rs_bound_addr field without taking a lock on the socket.  This opens a
race where rs_bound_addr is temporarily set but where the transport is not
in rds_bind(), leading to a NULL pointer dereference when trying to
dereference 'trans' in __rds_conn_create().

Vegard wrote a reproducer for this issue, so kindly ask him to share if
you're interested.

I cannot reproduce the NULL pointer dereference using Vegard's reproducer
with this patch, whereas I could without.

Complete earlier incomplete fix to CVE-2015-6937:

  74e98eb08588 ("RDS: verify the underlying transport exists before creating a connection")

Cc: David S. Miller <davem@davemloft.net>
Cc: stable@vger.kernel.org

Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>