hwmon: (pmbus/q54sj108a2) Add missing NULL pointer check

When CONFIG_DEBUG_FS is enabled and per-client PMBus debugfs directory
creation fails during pmbus_do_probe(), q54sj108a2_probe() passes the NULL
pointer returned by pmbus_get_debugfs_dir() to debugfs_create_dir()
without validation, creating its debugfs directory at the debugfs root
instead of under the client directory.

If pmbus_init_debugfs() fails to create the client directory, data->debugfs
is left as NULL or an ERR_PTR, which pmbus_get_debugfs_dir() converts to
NULL. Passing a NULL parent to debugfs_create_dir() places q54sj108a2_dir
in the root of the debugfs filesystem and populates it with attribute files
whose private data pointers reference psu->debugfs_entries. Because this
root-level directory is outside the client debugfs hierarchy, PMBus core
cleanup does not remove it on device unbind, leaking the debugfs entries
and causing a use-after-free of the devm-allocated struct q54sj108a2_data
if the orphaned files are accessed via q54sj108a2_debugfs_read() or
q54sj108a2_debugfs_write() after device removal.

Fix the problem by adding the missing NULL pointer check.

Fixes: d014538aa3 ("hwmon: (pmbus) Driver for Delta power supplies Q54SJ108A2")
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/q54sj108a2.c

@@ -351,6 +351,8 @@ static int q54sj108a2_probe(struct i2c_client *client)
 	psu->client = client;
 
 	debugfs = pmbus_get_debugfs_dir(client);
+	if (!debugfs)
+		return 0;
 
 	q54sj108a2_dir = debugfs_create_dir(client->name, debugfs);