[PATCH net v3 2/2] tipc: fix NULL deref in tipc_named_node_up() on empty publication list

[Weiming Shi <bestswngs@gmail.com>]

named_distribute() ends by stamping the last_bulk flag on the tail skb via
buf_msg(skb_peek_tail(list)). When the publication list is empty no skb is
enqueued, skb_peek_tail() returns NULL, and buf_msg(NULL) is dereferenced.

tipc_named_node_up() runs this on &nt->cluster_scope. With a node-id
configuration cluster_scope is populated only later by tipc_net_finalize(),
so a peer link that comes up first reaches named_distribute() with an empty
list. It is reachable by an unprivileged user (TIPC genl ops use
GENL_UNS_ADMIN_PERM) over a UDP bearer in a user+net namespace:

 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

The peer holds back this node's later name updates until it sees a bulk
with the last_bulk flag, so simply skipping the empty bulk would stall it.
Emit an item-less bulk when the list is empty, and break out of the build
loop on allocation failure instead of returning, so the last_bulk flag is
applied to the last queued skb.

Fixes: cad2929dc432 ("tipc: update a binding service via broadcast")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 net/tipc/name_distr.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/net/tipc/name_distr.c b/net/tipc/name_distr.c
index ba4f4906e13b..dbcfa965de34 100644
--- a/net/tipc/name_distr.c
+++ b/net/tipc/name_distr.c
@@ -165,7 +165,7 @@ static void named_distribute(struct net *net, struct sk_buff_head *list,
 						dnode);
 			if (!skb) {
 				pr_warn("Bulk publication failure\n");
-				return;
+				break;
 			}
 			hdr = buf_msg(skb);
 			msg_set_bc_ack_invalid(hdr, true);
@@ -192,6 +192,20 @@ static void named_distribute(struct net *net, struct sk_buff_head *list,
 		skb_trim(skb, INT_H_SIZE + (msg_dsz - msg_rem));
 		__skb_queue_tail(list, skb);
 	}
+
+	if (skb_queue_empty(list)) {
+		skb = named_prepare_buf(net, PUBLICATION, 0, dnode);
+		if (!skb) {
+			pr_warn("Bulk publication failure\n");
+			return;
+		}
+		hdr = buf_msg(skb);
+		msg_set_bc_ack_invalid(hdr, true);
+		msg_set_bulk(hdr);
+		msg_set_non_legacy(hdr);
+		__skb_queue_tail(list, skb);
+	}
+
 	hdr = buf_msg(skb_peek_tail(list));
 	msg_set_last_bulk(hdr);
 	msg_set_named_seqno(hdr, seqno);
-- 
2.43.0