Bluetooth: RFCOMM: hold listener socket in rfcomm_connect_ind()

Bluetooth: RFCOMM: hold listener socket in rfcomm_connect_ind()

rfcomm_get_sock_by_channel() scans rfcomm_sk_list under the list lock,
but returns the selected listener after dropping that lock without
taking a reference. rfcomm_connect_ind() then locks the listener,
queues a child socket on it, and may notify it after unlocking it.

The buggy scenario involves two paths, with each column showing the
order within that path:

rfcomm_connect_ind():            listener close:
  1. Find parent in              1. close() enters
     rfcomm_get_sock_by_channel()   rfcomm_sock_release().
  2. Drop rfcomm_sk_list.lock    2. rfcomm_sock_shutdown()
     without pinning parent.        closes the listener.
  3. Call lock_sock(parent) and  3. rfcomm_sock_kill()
     bt_accept_enqueue(parent,      unlinks and puts parent.
     sk, true).
  4. Read parent flags and may   4. parent can be freed.
     call sk_state_change().

If close wins the race, parent can be freed before
rfcomm_connect_ind() reaches lock_sock(), bt_accept_enqueue(), or the
deferred-setup callback.

Take a reference on the listener before leaving rfcomm_sk_list.lock.
After lock_sock() succeeds, recheck that it is still in BT_LISTEN
before queueing a child, cache the deferred-setup bit while the parent
is locked, and drop the reference after the last parent use.

KASAN reported a slab-use-after-free in lock_sock_nested() from
rfcomm_connect_ind(), with the freeing stack going through
rfcomm_sock_kill() and rfcomm_sock_release().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>