https://chromium-review.googlesource.com/#/c/333910/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/kernel/+/efc639722012099d21c2b71cb2571555884af207

commit efc639722012099d21c2b71cb2571555884af207
Author: Johannes Berg <johannes.berg@intel.com>
Date: Fri Jan 16 10:37:14 2015

UPSTREAM: genetlink: synchronize socket closing and family removal

In addition to the problem Jeff Layton reported, I looked at the code
and reproduced the same warning by subscribing and removing the genl
family with a socket still open. This is a fairly tricky race which
originates in the fact that generic netlink allows the family to go
away while sockets are still open - unlike regular netlink which has
a module refcount for every open socket so in general this cannot be
triggered.

Trying to resolve this issue by the obvious locking isn't possible as
it will result in deadlocks between unregistration and group unbind
notification (which incidentally lockdep doesn't find due to the home
grown locking in the netlink table.)

To really resolve this, introduce a "closing socket" reference counter
(for generic netlink only, as it's the only affected family) in the
core netlink code and use that in generic netlink to wait for all the
sockets that are being closed at the same time as a generic netlink
family is removed.

This fixes the race that when a socket is closed, it will should call
the unbind, but if the family is removed at the same time the unbind
will not find it, leading to the warning. The real problem though is
that in this case the unbind could actually find a new family that is
registered to have a multicast group with the same ID, and call its
mcast_unbind() leading to confusing.

Also remove the warning since it would still trigger, but is now no
longer a problem.

This also moves the code in af_netlink.c to before unreferencing the
module to avoid having the same problem in the normal non-genl case.

BUG=chromium:596019
TEST=run syzkaller on KASAN kernel

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit ee1c244219fd652964710a6cc3e4f922e86aa492)
Signed-off-by: Alexander Potapenko <glider@google.com>

Change-Id: I1100bcdffad45eb30f9be4a36c58dc8744579587
Reviewed-on: https://chromium-review.googlesource.com/333910
Commit-Ready: Alexander Potapenko <glider@chromium.org>
Tested-by: Alexander Potapenko <glider@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>

[modify] https://crrev.com/efc639722012099d21c2b71cb2571555884af207/include/linux/genetlink.h
[modify] https://crrev.com/efc639722012099d21c2b71cb2571555884af207/net/netlink/genetlink.c
[modify] https://crrev.com/efc639722012099d21c2b71cb2571555884af207/net/netlink/af_netlink.c
[modify] https://crrev.com/efc639722012099d21c2b71cb2571555884af207/include/net/genetlink.h
[modify] https://crrev.com/efc639722012099d21c2b71cb2571555884af207/net/netlink/af_netlink.h