[PATCH net v2 0/2] genetlink: fix joining multicast groups no family owns

[Anil Kaushik <anilkaushikwireless@gmail.com>]

genl_bind() checks the GENL_MCAST_CAP_* flags of a group only when a
family owns it. A socket can join a free group ID and stay a member when
a family with those flags gets the ID later.

Patch 1 rejects joins to group IDs that no family owns. Patch 2 clears
the members of the group IDs when a family is given them. This covers
the race Sashiko found on v1, where the family is unregistered between
genl_bind() and the point where the membership bit is set.

One case is left. If the family is unregistered and a family with
GENL_MCAST_CAP_* flags gets the ID, both inside that window, the bit is
set after the clear. Closing it needs the check and the update under one
lock in af_netlink.c. I will send that separately.

Tested as an unprivileged user, with an msleep() added after the bind
callback in netlink_setsockopt() and a test family that registers again
with GENL_MCAST_CAP_NET_ADMIN:

                                         v1      v2
  unregister in the window,
  register later                         member  not member
  unregister and register in the window  member  member

The test from v1 (join all free IDs, then load gtp and drop_monitor)
gives the same result as before, and patch 2 alone also fixes it.

Changes in v2:
- New patch 2 (Sashiko).
- Patch 1: no change, added Reviewed-by tags.

v1: https://lore.kernel.org/netdev/20261029062214.3407223-1-anilkaushikwireless@gmail.com/

Anil Kaushik (2):
  genetlink: Reject joining multicast groups no family owns
  genetlink: Clear stale members when assigning multicast group IDs

 net/netlink/genetlink.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

-- 
2.25.1