SUNRPC: Fix races in rpcauth_create

See the FIXME: auth_flavors[] really needs a lock and module refcounting.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth.c

@@ -18,6 +18,7 @@
 # define RPCDBG_FACILITY	RPCDBG_AUTH
 #endif
 
+static DEFINE_SPINLOCK(rpc_authflavor_lock);
 static struct rpc_authops *	auth_flavors[RPC_AUTH_MAXFLAVOR] = {
 	&authnull_ops,		/* AUTH_NULL */
 	&authunix_ops,		/* AUTH_UNIX */
@@ -35,26 +36,34 @@ int
 rpcauth_register(struct rpc_authops *ops)
 {
 	rpc_authflavor_t flavor;
+	int ret = -EPERM;
 
 	if ((flavor = ops->au_flavor) >= RPC_AUTH_MAXFLAVOR)
 		return -EINVAL;
-	if (auth_flavors[flavor] != NULL)
-		return -EPERM;		/* what else? */
-	auth_flavors[flavor] = ops;
-	return 0;
+	spin_lock(&rpc_authflavor_lock);
+	if (auth_flavors[flavor] == NULL) {
+		auth_flavors[flavor] = ops;
+		ret = 0;
+	}
+	spin_unlock(&rpc_authflavor_lock);
+	return ret;
 }
 
 int
 rpcauth_unregister(struct rpc_authops *ops)
 {
 	rpc_authflavor_t flavor;
+	int ret = -EPERM;
 
 	if ((flavor = ops->au_flavor) >= RPC_AUTH_MAXFLAVOR)
 		return -EINVAL;
-	if (auth_flavors[flavor] != ops)
-		return -EPERM;		/* what else? */
-	auth_flavors[flavor] = NULL;
-	return 0;
+	spin_lock(&rpc_authflavor_lock);
+	if (auth_flavors[flavor] == ops) {
+		auth_flavors[flavor] = NULL;
+		ret = 0;
+	}
+	spin_unlock(&rpc_authflavor_lock);
+	return ret;
 }
 
 struct rpc_auth *
@@ -68,15 +77,19 @@ rpcauth_create(rpc_authflavor_t pseudoflavor, struct rpc_clnt *clnt)
 	if (flavor >= RPC_AUTH_MAXFLAVOR)
 		goto out;
 
-	/* FIXME - auth_flavors[] really needs an rw lock,
-	 * and module refcounting. */
 #ifdef CONFIG_KMOD
 	if ((ops = auth_flavors[flavor]) == NULL)
 		request_module("rpc-auth-%u", flavor);
 #endif
-	if ((ops = auth_flavors[flavor]) == NULL)
+	spin_lock(&rpc_authflavor_lock);
+	ops = auth_flavors[flavor];
+	if (ops == NULL || !try_module_get(ops->owner)) {
+		spin_unlock(&rpc_authflavor_lock);
 		goto out;
+	}
+	spin_unlock(&rpc_authflavor_lock);
 	auth = ops->create(clnt, pseudoflavor);
+	module_put(ops->owner);
 	if (IS_ERR(auth))
 		return auth;
 	if (clnt->cl_auth)