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

Anirudh Rayabharam
2021-04-06T13:01:03Z

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+c49fe6089f295a05e6f8@syzkaller.appspotmail.com
Tested-by: syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com

Signed-off-by: Anirudh Rayabharam <mail@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