There are session cleanup problems in ax25_release() and
ax25_disconnect(). If we setup a session and then disconnect,
the disconnected session is still in "LISTENING" state that
is shown below.

Active AX.25 sockets
Dest       Source     Device  State        Vr/Vs    Send-Q  Recv-Q
DL9SAU-4   DL9SAU-3   ???     LISTENING    000/000  0       0
DL9SAU-3   DL9SAU-4   ???     LISTENING    000/000  0       0

The first reason is caused by del_timer_sync() in ax25_release().
The timers of ax25 are used for correct session cleanup. If we use
ax25_release() to close ax25 sessions and ax25_dev is not null,
the del_timer_sync() functions in ax25_release() will execute.
As a result, the sessions could not be cleaned up correctly,
because the timers have stopped.

In order to solve this problem, this patch adds a device_up flag
in ax25_dev in order to judge whether the device is up. If there
are sessions to be cleaned up, the del_timer_sync() in
ax25_release() will not execute. What's more, we add ax25_cb_del()
in ax25_kill_by_device(), because the timers have been stopped
and there are no functions that could delete ax25_cb if we do not
call ax25_release(). Finally, we reorder the position of
ax25_list_lock in ax25_cb_del() in order to synchronize among
different functions that call ax25_cb_del().

The second reason is caused by improper check in ax25_disconnect().
The incoming ax25 sessions which ax25->sk is null will close
heartbeat timer, because the check "if(!ax25->sk || ..)" is
satisfied. As a result, the session could not be cleaned up properly.

In order to solve this problem, this patch changes the improper
check to "if(ax25->sk && ..)" in ax25_disconnect().

What`s more, the ax25_disconnect() may be called twice, which is
not necessary. For example, ax25_kill_by_device() calls
ax25_disconnect() and sets ax25->state to AX25_STATE_0, but
ax25_release() calls ax25_disconnect() again.

In order to solve this problem, this patch add a check in
ax25_release(). If the flag of ax25->sk equals to SOCK_DEAD,
the ax25_disconnect() in ax25_release() should not be executed.

Fixes: 82e31755 ("ax25: Fix UAF bugs in ax25 timers")
Fixes: 8a367e74 ("ax25: Fix segfault after sock connection timeout")
Reported-and-tested-by: Thomas Osterried <thomas@osterried.de>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Link: https://lore.kernel.org/r/20220530152158.108619-1-duoming@zju.edu.cnSigned-off-by: Paolo Abeni <pabeni@redhat.com>