xc2028: Fix use-after-free bug properly

commit 22a1e778 upstream.

The commit 8dfbcc43 ("[media] xc2028: avoid use after free") tried
to address the reported use-after-free by clearing the reference.

However, it's clearing the wrong pointer; it sets NULL to
priv->ctrl.fname, but it's anyway overwritten by the next line
memcpy(&priv->ctrl, p, sizeof(priv->ctrl)).

OTOH, the actual code accessing the freed string is the strcmp() call
with priv->fname:
	if (!firmware_name[0] && p->fname &&
	    priv->fname && strcmp(p->fname, priv->fname))
		free_firmware(priv);

where priv->fname points to the previous file name, and this was
already freed by kfree().

For fixing the bug properly, this patch does the following:

- Keep the copy of firmware file name in only priv->fname,
  priv->ctrl.fname isn't changed;
- The allocation is done only when the firmware gets loaded;
- The kfree() is called in free_firmware() commonly

Fixes: commit 8dfbcc43 ('[media] xc2028: avoid use after free')
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/media/tuners/tuner-xc2028.c

@@ -281,6 +281,14 @@ static void free_firmware(struct xc2028_data *priv)
 	int i;
 	tuner_dbg("%s called\n", __func__);
 
+	/* free allocated f/w string */
+	if (priv->fname != firmware_name)
+		kfree(priv->fname);
+	priv->fname = NULL;
+
+	priv->state = XC2028_NO_FIRMWARE;
+	memset(&priv->cur_fw, 0, sizeof(priv->cur_fw));
+
 	if (!priv->firm)
 		return;
 
@@ -291,9 +299,6 @@ static void free_firmware(struct xc2028_data *priv)
 
 	priv->firm = NULL;
 	priv->firm_size = 0;
-	priv->state = XC2028_NO_FIRMWARE;
-
-	memset(&priv->cur_fw, 0, sizeof(priv->cur_fw));
 }
 
 static int load_all_firmwares(struct dvb_frontend *fe,
@@ -884,9 +889,8 @@ static int check_firmware(struct dvb_frontend *fe, unsigned int type,
 	return 0;
 
 fail:
-	priv->state = XC2028_NO_FIRMWARE;
+	free_firmware(priv);
 
-	memset(&priv->cur_fw, 0, sizeof(priv->cur_fw));
 	if (retry_count < 8) {
 		msleep(50);
 		retry_count++;
@@ -1332,11 +1336,8 @@ static int xc2028_dvb_release(struct dvb_frontend *fe)
 	mutex_lock(&xc2028_list_mutex);
 
 	/* only perform final cleanup if this is the last instance */
-	if (hybrid_tuner_report_instance_count(priv) == 1) {
+	if (hybrid_tuner_report_instance_count(priv) == 1)
 		free_firmware(priv);
-		kfree(priv->ctrl.fname);
-		priv->ctrl.fname = NULL;
-	}
 
 	if (priv)
 		hybrid_tuner_release_state(priv);
@@ -1399,19 +1400,8 @@ static int xc2028_set_config(struct dvb_frontend *fe, void *priv_cfg)
 
 	/*
 	 * Copy the config data.
-	 * For the firmware name, keep a local copy of the string,
-	 * in order to avoid troubles during device release.
 	 */
-	kfree(priv->ctrl.fname);
-	priv->ctrl.fname = NULL;
 	memcpy(&priv->ctrl, p, sizeof(priv->ctrl));
-	if (p->fname) {
-		priv->ctrl.fname = kstrdup(p->fname, GFP_KERNEL);
-		if (priv->ctrl.fname == NULL) {
-			rc = -ENOMEM;
-			goto unlock;
-		}
-	}
 
 	/*
 	 * If firmware name changed, frees firmware. As free_firmware will
@@ -1426,10 +1416,15 @@ static int xc2028_set_config(struct dvb_frontend *fe, void *priv_cfg)
 
 	if (priv->state == XC2028_NO_FIRMWARE) {
 		if (!firmware_name[0])
-			priv->fname = priv->ctrl.fname;
+			priv->fname = kstrdup(p->fname, GFP_KERNEL);
 		else
 			priv->fname = firmware_name;
 
+		if (!priv->fname) {
+			rc = -ENOMEM;
+			goto unlock;
+		}
+
 		rc = request_firmware_nowait(THIS_MODULE, 1,
 					     priv->fname,
 					     priv->i2c_props.adap->dev.parent,