CVE-2025-39925can: j1939: implement NETDEV_UNREGISTER notification handler

Official description

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]

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/479d8a2aedcc1db16f14c1a8a9c74b5bdf18b9ac
• https://git.kernel.org/stable/c/2c88069fac2c792e228e6a4ae71c9b9b5b9a87a6
• https://git.kernel.org/stable/c/76957b618ce729c3bd1e782fbc9d9991af653925
• https://git.kernel.org/stable/c/4e154cb5e7681c2910e2dfa09f05e3469e333745
• https://git.kernel.org/stable/c/da9e8f429139928570407e8f90559b5d46c20262
• https://git.kernel.org/stable/c/7fcbe5b2c6a4b5407bf2241fdb71e0a390f6ab9a