[PATCH v2] net: hso: fix null-ptr-deref during tty device unregistration

[Anirudh Rayabharam <mail@anirudhrb.com>]

Multiple ttys try to claim the same the minor number causing a double
unregistration of the same device. The first unregistration succeeds
but the next one results in a null-ptr-deref.

The get_free_serial_index() function returns an available minor number
but doesn't assign it immediately. The assignment is done by the caller
later. But before this assignment, calls to get_free_serial_index()
would return the same minor number.

Fix this by modifying get_free_serial_index to assign the minor number
immediately after one is found to be and rename it to obtain_minor()
to better reflect what it does. Similary, rename set_serial_by_index()
to release_minor() and modify it to free up the minor number of the
given hso_serial. Every obtain_minor() should have corresponding
release_minor() call.

Fixes: 72dc1c096c705 ("HSO: add option hso driver")
Reported-by: syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com
Tested-by: syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
---

Changes in v2:
1. Fixed unused variable warning
2. Added "Fixes:" tag
3. Added the "Reviewed-by:" tags that were receieved

---
 drivers/net/usb/hso.c | 33 ++++++++++++---------------------
 1 file changed, 12 insertions(+), 21 deletions(-)

diff --git a/drivers/net/usb/hso.c b/drivers/net/usb/hso.c
index 31d51346786a..9bc58e64b5b7 100644
--- a/drivers/net/usb/hso.c
+++ b/drivers/net/usb/hso.c
@@ -611,7 +611,7 @@ static struct hso_serial *get_serial_by_index(unsigned index)
 	return serial;
 }
 
-static int get_free_serial_index(void)
+static int obtain_minor(struct hso_serial *serial)
 {
 	int index;
 	unsigned long flags;
@@ -619,8 +619,10 @@ static int get_free_serial_index(void)
 	spin_lock_irqsave(&serial_table_lock, flags);
 	for (index = 0; index < HSO_SERIAL_TTY_MINORS; index++) {
 		if (serial_table[index] == NULL) {
+			serial_table[index] = serial->parent;
+			serial->minor = index;
 			spin_unlock_irqrestore(&serial_table_lock, flags);
-			return index;
+			return 0;
 		}
 	}
 	spin_unlock_irqrestore(&serial_table_lock, flags);
@@ -629,15 +631,12 @@ static int get_free_serial_index(void)
 	return -1;
 }
 
-static void set_serial_by_index(unsigned index, struct hso_serial *serial)
+static void release_minor(struct hso_serial *serial)
 {
 	unsigned long flags;
 
 	spin_lock_irqsave(&serial_table_lock, flags);
-	if (serial)
-		serial_table[index] = serial->parent;
-	else
-		serial_table[index] = NULL;
+	serial_table[serial->minor] = NULL;
 	spin_unlock_irqrestore(&serial_table_lock, flags);
 }
 
@@ -2230,6 +2229,7 @@ static int hso_stop_serial_device(struct hso_device *hso_dev)
 static void hso_serial_tty_unregister(struct hso_serial *serial)
 {
 	tty_unregister_device(tty_drv, serial->minor);
+	release_minor(serial);
 }
 
 static void hso_serial_common_free(struct hso_serial *serial)
@@ -2253,24 +2253,22 @@ static void hso_serial_common_free(struct hso_serial *serial)
 static int hso_serial_common_create(struct hso_serial *serial, int num_urbs,
 				    int rx_size, int tx_size)
 {
-	int minor;
 	int i;
 
 	tty_port_init(&serial->port);
 
-	minor = get_free_serial_index();
-	if (minor < 0)
+	if (obtain_minor(serial))
 		goto exit2;
 
 	/* register our minor number */
 	serial->parent->dev = tty_port_register_device_attr(&serial->port,
-			tty_drv, minor, &serial->parent->interface->dev,
+			tty_drv, serial->minor, &serial->parent->interface->dev,
 			serial->parent, hso_serial_dev_groups);
-	if (IS_ERR(serial->parent->dev))
+	if (IS_ERR(serial->parent->dev)) {
+		release_minor(serial);
 		goto exit2;
+	}
 
-	/* fill in specific data for later use */
-	serial->minor = minor;
 	serial->magic = HSO_SERIAL_MAGIC;
 	spin_lock_init(&serial->serial_lock);
 	serial->num_rx_urbs = num_urbs;
@@ -2667,9 +2665,6 @@ static struct hso_device *hso_create_bulk_serial_device(
 
 	serial->write_data = hso_std_serial_write_data;
 
-	/* and record this serial */
-	set_serial_by_index(serial->minor, serial);
-
 	/* setup the proc dirs and files if needed */
 	hso_log_port(hso_dev);
 
@@ -2726,9 +2721,6 @@ struct hso_device *hso_create_mux_serial_device(struct usb_interface *interface,
 	serial->shared_int->ref_count++;
 	mutex_unlock(&serial->shared_int->shared_int_lock);
 
-	/* and record this serial */
-	set_serial_by_index(serial->minor, serial);
-
 	/* setup the proc dirs and files if needed */
 	hso_log_port(hso_dev);
 
@@ -3113,7 +3105,6 @@ static void hso_free_interface(struct usb_interface *interface)
 			cancel_work_sync(&serial_table[i]->async_get_intf);
 			hso_serial_tty_unregister(serial);
 			kref_put(&serial_table[i]->ref, hso_serial_ref_free);
-			set_serial_by_index(i, NULL);
 		}
 	}
 
-- 
2.26.2