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

[Anirudh Rayabharam]

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.

Reported-by: syzbot+c49fe6089f295a05e...@syzkaller.appspotmail.com
Tested-by: syzbot+c49fe6089f295a05e...@syzkaller.appspotmail.com

Signed-off-by: Anirudh Rayabharam <m...@anirudhrb.com>
---
 drivers/net/usb/hso.c | 32 ++++++++++++--------------------
 1 file changed, 12 insertions(+), 20 deletions(-)

diff --git a/drivers/net/usb/hso.c b/drivers/net/usb/hso.c
index 31d51346786a..295ca330e70c 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)
@@ -2258,19 +2258,18 @@ static int hso_serial_common_create(struct hso_serial 
*serial, int num_urbs,
 
        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 +2666,6 @@ 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 +2722,6 @@ 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 +3106,6 @@ 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