tipc: serialize udp bearer replicast list updates

tipc: serialize udp bearer replicast list updates

tipc_udp_rcast_add() and cleanup_bearer() both update ub->rcast.list with
list_add_rcu() / list_del_rcu(), but nothing serializes them. The add runs
from the encap receive softirq (via tipc_udp_rcast_disc()) without
rtnl_lock(), so it can race the cleanup delete and corrupt the list:

  list_del corruption. prev->next should be ffff8880298d7ab8,
    but was ffff88802449ad38. (prev=ffff888027e3ec98)
  kernel BUG at lib/list_debug.c:62!
  RIP: __list_del_entry_valid_or_report+0x17a/0x200
  Workqueue: events cleanup_bearer
  Call Trace:
   cleanup_bearer (net/tipc/udp_media.c:811)
   process_one_work (kernel/workqueue.c:3302)
   worker_thread (kernel/workqueue.c:3466)

The bearer can be enabled from an unprivileged user namespace, as the
TIPCv2 generic-netlink ops carry no GENL_ADMIN_PERM.

Add a spinlock to struct udp_bearer and take it around the list_add_rcu()
in tipc_udp_rcast_add() and the list_del_rcu() loop in cleanup_bearer() so
the two writers can no longer corrupt the list.

Reject a duplicate peer under the same lock before allocating, and remove
tipc_udp_is_known_peer(). The old lockless pre-check in
tipc_udp_rcast_disc() was racy: two softirqs discovering the same peer
could both find it absent and add it twice.

cleanup_bearer() runs from a workqueue after tipc_udp_disable() clears the
bearer's up bit, so an encap softirq can still reach tipc_udp_rcast_add()
and add a peer after cleanup_bearer() has already emptied the list, leaking
that entry when the bearer is freed. Mark the bearer disabled under
rcast_lock once the list is emptied and refuse further additions.

Fixes: ef20cd4dd163 ("tipc: introduce UDP replicast")
Reported-by: Xiang Mei <xmei5@asu.edu>
Suggested-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Link: https://patch.msgid.link/20260716025203.9332-2-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>