ax25: fix UAF bugs of net_device caused by rebinding operation

The ax25_kill_by_device() will set s->ax25_dev = NULL and
call ax25_disconnect() to change states of ax25_cb and
sock, if we call ax25_bind() before ax25_kill_by_device().

However, if we call ax25_bind() again between the window of
ax25_kill_by_device() and ax25_dev_device_down(), the values
and states changed by ax25_kill_by_device() will be reassigned.

Finally, ax25_dev_device_down() will deallocate net_device.
If we dereference net_device in syscall functions such as
ax25_release(), ax25_sendmsg(), ax25_getsockopt(), ax25_getname()
and ax25_info_show(), a UAF bug will occur.

One of the possible race conditions is shown below:

(USE) | (FREE)
ax25_bind() |
| ax25_kill_by_device()
ax25_bind() |
ax25_connect() | ...
| ax25_dev_device_down()
| ...
| dev_put_track(dev, ...) //FREE
ax25_release() | ...
ax25_send_control() |
alloc_skb() //USE |

the corresponding fail log is shown below:
===============================================================
BUG: KASAN: use-after-free in ax25_send_control+0x43/0x210
...
Call Trace:
...
ax25_send_control+0x43/0x210
ax25_release+0x2db/0x3b0
__sock_release+0x6d/0x120
sock_close+0xf/0x20
__fput+0x11f/0x420
...
Allocated by task 1283:
...
__kasan_kmalloc+0x81/0xa0
alloc_netdev_mqs+0x5a/0x680
mkiss_open+0x6c/0x380
tty_ldisc_open+0x55/0x90
...
Freed by task 1969:
...
kfree+0xa3/0x2c0
device_release+0x54/0xe0
kobject_put+0xa5/0x120
tty_ldisc_kill+0x3e/0x80
...

In order to fix these UAF bugs caused by rebinding operation,
this patch adds dev_hold_track() into ax25_bind() and
corresponding dev_put_track() into ax25_kill_by_device().

Signed-off-by: Duoming Zhou <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ax25/af_ax25.c

···
 			spin_unlock_bh(&ax25_list_lock);
 			lock_sock(sk);
 			s->ax25_dev = NULL;
+			dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
 			ax25_dev_put(ax25_dev);
 			ax25_disconnect(s, ENETUNREACH);
 			release_sock(sk);
···
 		}
 	}
 
-	if (ax25_dev != NULL)
+	if (ax25_dev) {
 		ax25_fillin_cb(ax25, ax25_dev);
+		dev_hold_track(ax25_dev->dev, &ax25_dev->dev_tracker, GFP_ATOMIC);
+	}
 
 done:
 	ax25_cb_add(ax25);