CVE-2021-47459

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

can: j1939: j1939_netdev_start(): fix UAF for rx_kref of j1939_priv

It will trigger UAF for rx_kref of j1939_priv as following.

    cpu0                                    cpu1

j1939_sk_bind(socket0, ndev0, …)
j1939_netdev_start
j1939_sk_bind(socket1, ndev0, …)
j1939_netdev_start
j1939_priv_set
j1939_priv_get_by_ndev_locked
j1939_jsk_add
…
j1939_netdev_stop
kref_put_lock(&priv->rx_kref, …)
kref_get(&priv->rx_kref, …)
REFCOUNT_WARN(“addition on 0;…”)

====================================================
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 20874 at lib/refcount.c:25 refcount_warn_saturate+0x169/0x1e0
RIP: 0010:refcount_warn_saturate+0x169/0x1e0
Call Trace:
j1939_netdev_start+0x68b/0x920
j1939_sk_bind+0x426/0xeb0
? security_socket_bind+0x83/0xb0

The rx_kref’s kref_get() and kref_put() should use j1939_netdev_lock to
protect.