CVE-2025-39925

In the Linux kernel, the following vulnerability has been resolved:

can: j1939: implement NETDEV_UNREGISTER notification handler

syzbot is reporting

unregister_netdevice: waiting for vcan0 to become free. Usage count = 2

problem, for j1939 protocol did not have NETDEV_UNREGISTER notification

handler for undoing changes made by j1939_sk_bind().

Commit 25fe97cb7620 ("can: j1939: move j1939_priv_put() into sk_destruct

callback") expects that a call to j1939_priv_put() can be unconditionally

delayed until j1939_sk_sock_destruct() is called. But we need to call

j1939_priv_put() against an extra ref held by j1939_sk_bind() call

(as a part of undoing changes made by j1939_sk_bind()) as soon as

NETDEV_UNREGISTER notification fires (i.e. before j1939_sk_sock_destruct()

is called via j1939_sk_release()). Otherwise, the extra ref on "struct

j1939_priv" held by j1939_sk_bind() call prevents "struct net_device" from

dropping the usage count to 1; making it impossible for

unregister_netdevice() to continue.

[mkl: remove space in front of label]