firmware loader: fix use-after-free by double abort

fw_priv->buf is accessed in both request_firmware_load() and
writing to sysfs file of 'loading' context, but not protected
by 'fw_lock' entirely. The patch makes sure that access on
'fw_priv->buf' is protected by the lock.

So fixes the double abort problem reported by nirinA raseliarison:

http://lkml.org/lkml/2013/6/14/188

Reported-and-tested-by: nirinA raseliarison <[e-mail]>
Cc: Guenter Roeck <[e-mail]>
Cc: Bjorn Helgaas <[e-mail]>
Cc: stable <[e-mail]> # 3.9
Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/firmware_class.c

···
 {
 	struct firmware_buf *buf = fw_priv->buf;
 
+	/*
+	 * There is a small window in which user can write to 'loading'
+	 * between loading done and disappearance of 'loading'
+	 */
+	if (test_bit(FW_STATUS_DONE, &buf->status))
+		return;
+
 	set_bit(FW_STATUS_ABORT, &buf->status);
 	complete_all(&buf->completion);
+
+	/* avoid user action after loading abort */
+	fw_priv->buf = NULL;
 }
 
 #define is_fw_load_aborted(buf)	\
···
 				     struct device_attribute *attr, char *buf)
 {
 	struct firmware_priv *fw_priv = to_firmware_priv(dev);
-	int loading = test_bit(FW_STATUS_LOADING, &fw_priv->buf->status);
+	int loading = 0;
+
+	mutex_lock(&fw_lock);
+	if (fw_priv->buf)
+		loading = test_bit(FW_STATUS_LOADING, &fw_priv->buf->status);
+	mutex_unlock(&fw_lock);
 
 	return sprintf(buf, "%d\n", loading);
 }
···
 				      const char *buf, size_t count)
 {
 	struct firmware_priv *fw_priv = to_firmware_priv(dev);
-	struct firmware_buf *fw_buf = fw_priv->buf;
+	struct firmware_buf *fw_buf;
 	int loading = simple_strtol(buf, NULL, 10);
 	int i;
 
 	mutex_lock(&fw_lock);
-
+	fw_buf = fw_priv->buf;
 	if (!fw_buf)
 		goto out;
 
···
 			struct firmware_priv, timeout_work.work);
 
 	mutex_lock(&fw_lock);
-	if (test_bit(FW_STATUS_DONE, &(fw_priv->buf->status))) {
-		mutex_unlock(&fw_lock);
-		return;
-	}
 	fw_load_abort(fw_priv);
 	mutex_unlock(&fw_lock);
 }
···
 	wait_for_completion(&buf->completion);
 
 	cancel_delayed_work_sync(&fw_priv->timeout_work);
-
-	fw_priv->buf = NULL;
 
 	device_remove_file(f_dev, &dev_attr_loading);
 err_del_bin_attr: