rds: fix two RCU related problems

When a rds sock is bound, it is inserted into the bind_hash_table
which is protected by RCU. But when releasing rds sock, after it
is removed from this hash table, it is freed immediately without
respecting RCU grace period. This could cause some use-after-free
as reported by syzbot.

Mark the rds sock with SOCK_RCU_FREE before inserting it into the
bind_hash_table, so that it would be always freed after a RCU grace
period.

The other problem is in rds_find_bound(), the rds sock could be
freed in between rhashtable_lookup_fast() and rds_sock_addref(),
so we need to extend RCU read lock protection in rds_find_bound()
to close this race condition.

Reported-and-tested-by:  <syzbot+8967084bcac563795dc6@syzkaller.appspotmail.com>
Reported-by:  <syzbot+93a5839deb355537440f@syzkaller.appspotmail.com>
Cc: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Cc: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Cc: rds-devel@oss.oracle.com
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oarcle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/bind.c

@@ -76,11 +76,13 @@ struct rds_sock *rds_find_bound(const struct in6_addr *addr, __be16 port,
 	struct rds_sock *rs;
 
 	__rds_create_bind_key(key, addr, port, scope_id);
-	rs = rhashtable_lookup_fast(&bind_hash_table, key, ht_parms);
+	rcu_read_lock();
+	rs = rhashtable_lookup(&bind_hash_table, key, ht_parms);
 	if (rs && !sock_flag(rds_rs_to_sk(rs), SOCK_DEAD))
 		rds_sock_addref(rs);
 	else
 		rs = NULL;
+	rcu_read_unlock();
 
 	rdsdebug("returning rs %p for %pI6c:%u\n", rs, addr,
 		 ntohs(port));
@@ -235,6 +237,7 @@ int rds_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 		goto out;
 	}
 
+	sock_set_flag(sk, SOCK_RCU_FREE);
 	ret = rds_add_bound(rs, binding_addr, &port, scope_id);
 	if (ret)
 		goto out;