tipc: read le->link under the node lock in tipc_node_link_down()

tipc: read le->link under the node lock in tipc_node_link_down()

tipc_node_link_down() caches the link pointer before taking n->lock:

	struct tipc_link *l = le->link;		/* unlocked */

	if (!l)
		return;
	tipc_node_write_lock(n);
	if (!tipc_link_is_establishing(l)) {	/* deref l */
	...
		tipc_link_reset(l);		/* write into l */
	if (delete) {
		kfree(l);
		le->link = NULL;

The delete=true caller frees that very object under n->lock, so the lock
does not protect the cached pointer against it:

 - CPU A, delete=false: tipc_rcv() on TIPC_LINK_DOWN_EVT, or the link
   supervision timer via tipc_node_timeout(), reads l unlocked and then
   dereferences it under n->lock;
 - CPU B, delete=true: netlink TIPC_NL_BEARER_DISABLE -> bearer_disable()
   -> tipc_node_delete_links() -> tipc_node_link_down(n, bearer_id, true)
   -> kfree(l).

The link is freed with plain kfree(), not kfree_rcu(), and for UDP bearers
disable_media() only schedules the asynchronous cleanup_bearer() work, so
its synchronize_net() runs after the links are already gone.  An in-flight
CPU A that has read l therefore dereferences freed memory once B frees it:
a use-after-free read in tipc_link_is_establishing(), and a use-after-free
write via tipc_link_reset() on the establishing branch.

The following trace was captured on 7.2.0-rc5-00284-gaf39eb111ce6:

  BUG: KASAN: slab-use-after-free in tipc_link_is_establishing (net/tipc/link.c:285)
  Read of size 4 at addr ffff88802e2aa068 by task swapper/2/0
   tipc_link_is_establishing (net/tipc/link.c:285)
   tipc_node_link_down (net/tipc/node.c:1076)
   tipc_node_timeout (net/tipc/node.c:843)
  Allocated by task 9549:
   tipc_link_create (net/tipc/link.c:490)
   tipc_node_check_dest (net/tipc/node.c:1279)
   tipc_disc_rcv (net/tipc/discover.c:252)
   tipc_udp_recv (net/tipc/udp_media.c:389)
  Freed by task 9549:
   tipc_node_link_down (net/tipc/node.c:1084)
   tipc_node_delete_links (net/tipc/node.c:1320)
   bearer_disable (net/tipc/bearer.c:414)
   __tipc_nl_bearer_disable (net/tipc/bearer.c:992)

Move the le->link read inside tipc_node_write_lock(), so it is serialised
against the kfree() in the delete path.  A racing teardown now either has
not run yet, and we see a valid link, or has already run, and we see NULL.

Fixes: 73f646cec354 ("tipc: delay ESTABLISH state event when link is established")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Reviewed-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Link: https://patch.msgid.link/20260810102147.48191-1-juny24602@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>