ax25: Fix refcount leak caused by setting SO_BINDTODEVICE sockopt

If an AX25 device is bound to a socket by setting the SO_BINDTODEVICE
socket option, a refcount leak will occur in ax25_release().

Commit 9fd75b6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
added decrement of device refcounts in ax25_release(). In order for that
to work correctly the refcounts must already be incremented when the
device is bound to the socket. An AX25 device can be bound to a socket
by either calling ax25_bind() or setting SO_BINDTODEVICE socket option.
In both cases the refcounts should be incremented, but in fact it is done
only in ax25_bind().

This bug leads to the following issue reported by Syzkaller:

================================================================
refcount_t: decrement hit 0; leaking memory.
WARNING: CPU: 1 PID: 5932 at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31
Modules linked in:
CPU: 1 UID: 0 PID: 5932 Comm: syz-executor424 Not tainted 6.13.0-rc4-syzkaller-00110-g4099a71718b0 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31
Call Trace:
 <TASK>
 __refcount_dec include/linux/refcount.h:336 [inline]
 refcount_dec include/linux/refcount.h:351 [inline]
 ref_tracker_free+0x710/0x820 lib/ref_tracker.c:236
 netdev_tracker_free include/linux/netdevice.h:4156 [inline]
 netdev_put include/linux/netdevice.h:4173 [inline]
 netdev_put include/linux/netdevice.h:4169 [inline]
 ax25_release+0x33f/0xa10 net/ax25/af_ax25.c:1069
 __sock_release+0xb0/0x270 net/socket.c:640
 sock_close+0x1c/0x30 net/socket.c:1408
 ...
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 ...
 </TASK>
================================================================

Fix the implementation of ax25_setsockopt() by adding increment of
refcounts for the new device bound, and decrement of refcounts for
the old unbound device.

Fixes: 9fd75b6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
Reported-by: [email protected]
Signed-off-by: Murad Masimov <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Murad Masimov

net/ax25/af_ax25.c

@@ -685,13 +685,24 @@ static int ax25_setsockopt(struct socket *sock, int level, int optname,
 			break;
 		}
 
+		if (ax25->ax25_dev) {
+			if (dev == ax25->ax25_dev->dev) {
+				rcu_read_unlock();
+				break;
+			}
+			netdev_put(ax25->ax25_dev->dev, &ax25->dev_tracker);
+			ax25_dev_put(ax25->ax25_dev);
+		}
+
 		ax25->ax25_dev = ax25_dev_ax25dev(dev);
 		if (!ax25->ax25_dev) {
 			rcu_read_unlock();
 			res = -ENODEV;
 			break;
 		}
 		ax25_fillin_cb(ax25, ax25->ax25_dev);
+		netdev_hold(dev, &ax25->dev_tracker, GFP_ATOMIC);
+		ax25_dev_hold(ax25->ax25_dev);
 		rcu_read_unlock();
 		break;