An ESP packet could be decrypted in async mode if the input handler for
this packet returns -EINPROGRESS in xfrm_input(). At this moment the device
reference in skb is held. Later xfrm_input() will be invoked again to
resume the processing.
If the transform state is still valid it would continue to release the
device reference and there won't be a problem; however if the transform
state is not valid when async resumption happens, the packet will be
dropped while the device reference is still being held.
When the device is deleted for some reason and the reference to this
device is not properly released, the kernel will keep logging like:

unregister_netdevice: waiting for ppp2 to become free. Usage count = 1

The issue is observed when running IPsec traffic over a PPPoE device based
on a bridge interface. By terminating the PPPoE connection on the server
end for multiple times, the PPPoE device on the client side will eventually
get stuck on the above warning message.

This patch will check the async mode first and continue to release device
reference in async resumption, before it is dropped due to invalid state.

v2: Do not assign address family from outer_mode in the transform if the
state is invalid

v3: Release device reference in the error path instead of jumping to resume

Fixes: 4ce3dbe3

 ("xfrm: Fix xfrm_input() to verify state is valid when (encap_type < 0)")
Signed-off-by: Xiaodong Xu <stid.smth@gmail.com>
Reported-by: Bo Chen <chenborfc@163.com>
Tested-by: Bo Chen <chenborfc@163.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>