[PATCH net v2] tipc: fix NULL deref in tipc_named_node_up() on empty publication list

[Weiming Shi <bestswngs@gmail.com>]

named_distribute() builds the bulk messages for @pls into @list and then
dereferences the tail skb:

	hdr = buf_msg(skb_peek_tail(list));
	msg_set_last_bulk(hdr);

If @pls is empty no skb is enqueued, skb_peek_tail() returns NULL, and
msg_set_last_bulk() writes through buf_msg(NULL).

tipc_named_node_up() passes &nt->cluster_scope. With a node-id
configuration the TIPC_NODE_STATE name is published by tipc_net_finalize()
from a work item, which sets the node address before publishing the name.
The node accepts links once the address is set, so a link that comes up
before the publish runs named_distribute() on an empty cluster_scope:

 KASAN: null-ptr-deref in range [0x00000000000000d8-0x00000000000000df]
 RIP: 0010:tipc_named_node_up (net/tipc/name_distr.c:196)
  tipc_named_node_up (net/tipc/name_distr.c:196 net/tipc/name_distr.c:221)
  tipc_node_write_unlock (net/tipc/node.c:428)
  tipc_rcv (net/tipc/node.c:2185)
  tipc_udp_recv (net/tipc/udp_media.c:392)
 Kernel panic - not syncing: Fatal exception in interrupt

TIPC genl ops use GENL_UNS_ADMIN_PERM, so an unprivileged user can reach
this from a user+net namespace.

Return early in tipc_named_node_up() when cluster_scope is empty, so
named_distribute() is not called with an empty list.

Fixes: cad2929dc432 ("tipc: update a binding service via broadcast")
Reported-by: Xiang Mei <xmei5@asu.edu>
Suggested-by: Tung Quang Nguyen <tung.quang.nguyen@est.tech>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
v2:
- Return early in tipc_named_node_up() when cluster_scope is empty,
  instead of guarding the tail access inside named_distribute().

 net/tipc/name_distr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/tipc/name_distr.c b/net/tipc/name_distr.c
index ba4f4906e13b..60ccaa862162 100644
--- a/net/tipc/name_distr.c
+++ b/net/tipc/name_distr.c
@@ -218,6 +218,10 @@ void tipc_named_node_up(struct net *net, u32 dnode, u16 capabilities)
 	spin_unlock_bh(&tn->nametbl_lock);
 
 	read_lock_bh(&nt->cluster_scope_lock);
+	if (list_empty(&nt->cluster_scope)) {
+		read_unlock_bh(&nt->cluster_scope_lock);
+		return;
+	}
 	named_distribute(net, &head, dnode, &nt->cluster_scope, seqno);
 	tipc_node_xmit(net, &head, dnode, 0);
 	read_unlock_bh(&nt->cluster_scope_lock);
-- 
2.43.0