[NET]: Fix module reference counts for loadable protocol modules

I have been experimenting with loadable protocol modules, and ran into
several issues with module reference counting.

The first issue was that __module_get failed at the BUG_ON check at
the top of the routine (checking that my module reference count was
not zero) when I created the first socket. When sk_alloc() is called,
my module reference count was still 0. When I looked at why sctp
didn't have this problem, I discovered that sctp creates a control
socket during module init (when the module ref count is not 0), which
keeps the reference count non-zero. This section has been updated to
address the point Stephen raised about checking the return value of
try_module_get().

The next problem arose when my socket init routine returned an error.
This resulted in my module reference count being decremented below 0.
My socket ops->release routine was also being called. The issue here
is that sock_release() calls the ops->release routine and decrements
the ref count if sock->ops is not NULL. Since the socket probably
didn't get correctly initialized, this should not be done, so we will
set sock->ops to NULL because we will not call try_module_get().

While searching for another bug, I also noticed that sys_accept() has
a possibility of doing a module_put() when it did not do an
__module_get so I re-ordered the call to security_socket_accept().

Signed-off-by: Frank Filz <ffilzlnx@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -660,16 +660,20 @@ struct sock *sk_alloc(int family, unsigned int __nocast priority,
 			sock_lock_init(sk);
 		}
 
-		if (security_sk_alloc(sk, family, priority)) {
-			if (slab != NULL)
-				kmem_cache_free(slab, sk);
-			else
-				kfree(sk);
-			sk = NULL;
-		} else
-			__module_get(prot->owner);
+		if (security_sk_alloc(sk, family, priority))
+			goto out_free;
+
+		if (!try_module_get(prot->owner))
+			goto out_free;
 	}
 	return sk;
+
+out_free:
+	if (slab != NULL)
+		kmem_cache_free(slab, sk);
+	else
+		kfree(sk);
+	return NULL;
 }
 
 void sk_free(struct sock *sk)

net/socket.c

@@ -1145,8 +1145,11 @@ static int __sock_create(int family, int type, int protocol, struct socket **res
 	if (!try_module_get(net_families[family]->owner))
 		goto out_release;
 
-	if ((err = net_families[family]->create(sock, protocol)) < 0)
+	if ((err = net_families[family]->create(sock, protocol)) < 0) {
+		sock->ops = NULL;
 		goto out_module_put;
+	}
+
 	/*
 	 * Now to bump the refcnt of the [loadable] module that owns this
 	 * socket at sock_release time we decrement its refcnt.
@@ -1360,16 +1363,16 @@ asmlinkage long sys_accept(int fd, struct sockaddr __user *upeer_sockaddr, int _
 	newsock->type = sock->type;
 	newsock->ops = sock->ops;
 
-	err = security_socket_accept(sock, newsock);
-	if (err)
-		goto out_release;
-
 	/*
 	 * We don't need try_module_get here, as the listening socket (sock)
 	 * has the protocol module (sock->ops->owner) held.
 	 */
 	__module_get(newsock->ops->owner);
 
+	err = security_socket_accept(sock, newsock);
+	if (err)
+		goto out_release;
+
 	err = sock->ops->accept(sock, newsock, sock->file->f_flags);
 	if (err < 0)
 		goto out_release;