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

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 from named_distribute() when the list is empty, and skip
tipc_node_xmit() for an empty chain. The empty chain would otherwise hit
tipc_lxc_xmit() -> buf_msg(skb_peek(list)), the same zero-skb case fixed for
tipc_link_xmit() in commit b77413446408 ("tipc: fix NULL deref in
tipc_link_xmit()").

It needs to return earlier in tipc_named_node_up() like this:

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);