[PATCH net v2] tipc: hold a reference to nodes found by link name

[Chengfeng Ye <nicoyip.dev@gmail.com>]

tipc_node_find_by_name() returns a node after dropping its RCU read lock
without taking a reference.  The LINK_SET, LINK_GET and LINK_RESET_STATS
handlers then lock and access the node, racing with timer-driven cleanup of
a down peer.  Generic netlink serialization does not exclude the node
timer.

The following interleaving can leave a handler using a freed node:

  CPU 0: find the node under RCU and release the node read lock
  CPU 1: tipc_node_timeout() clears the links and unlinks the down node
  CPU 1: drop the list and timer references, queuing tipc_node_free()
  CPU 0: leave the RCU read-side critical section
  CPU 1: complete the grace period and free the node
  CPU 0: acquire the node lock through the stale pointer

LINK_SET also uses the node's media address after releasing the node lock,
when passing queued packets to tipc_bearer_xmit().

KASAN on v7.3-rc5 reported:

  BUG: KASAN: slab-use-after-free in _raw_read_lock_bh
  Write of size 4 at addr ffff88807e06f008 by task poc/92
  Call Trace:
   _raw_read_lock_bh                 kernel/locking/spinlock.c:287
   tipc_nl_node_set_link             net/tipc/node.c:2475
   genl_family_rcv_msg_doit          net/netlink/genetlink.c:1114
   genl_rcv_msg                      net/netlink/genetlink.c:1209
   netlink_rcv_skb                   net/netlink/af_netlink.c:2575

  Allocated by task 0:
   tipc_node_create                  net/tipc/node.c:539
   tipc_node_check_dest              net/tipc/node.c:1196
   tipc_disc_rcv                     net/tipc/discover.c:252

  Freed by task 92:
   kfree                             mm/slub.c:6801
   rcu_core                          kernel/rcu/tree.c:2919

  Last potentially related work creation:
   __call_rcu_common.constprop.0     kernel/rcu/tree.c:3181
   tipc_node_timeout                 net/tipc/node.c:814

Acquire a reference to the selected node with kref_get_unless_zero() before
leaving RCU, returning NULL if the node has already been released.  Release
that reference on every caller exit after the last node access, including
transmission in LINK_SET.  Keep the existing link lookup order and locking
so concurrent link removal still takes the existing error paths.

Fixes: 6a939f365bdb ("tipc: Auto removal of peer down node instance")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
Changes in v2:
- Take the node reference inside the matching-node block, as suggested by
  Tung Quang Nguyen.
- Reproduce the UAF on v7.3-rc5 and include a decoded KASAN trace.

Link: https://lore.kernel.org/r/20260927064036.3691962-1-nicoyip.dev@gmail.com/ [v1]

 net/tipc/node.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/tipc/node.c b/net/tipc/node.c
index bd91378b7540..d7cbfa786c13 100644
--- a/net/tipc/node.c
+++ b/net/tipc/node.c
@@ -2421,8 +2421,11 @@ static struct tipc_node *tipc_node_find_by_name(struct net *net,
 			}
 		}
 		tipc_node_read_unlock(n);
-		if (found_node)
+		if (found_node) {
+			if (!kref_get_unless_zero(&found_node->kref))
+				found_node = NULL;
 			break;
+		}
 	}
 	rcu_read_unlock();
 
@@ -2507,6 +2510,7 @@ int tipc_nl_node_set_link(struct sk_buff *skb, struct genl_info *info)
 	tipc_node_read_unlock(node);
 	tipc_bearer_xmit(net, bearer_id, &xmitq, &node->links[bearer_id].maddr,
 			 NULL);
+	tipc_node_put(node);
 	return res;
 }
 
@@ -2558,12 +2562,14 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info)
 		link = node->links[bearer_id].link;
 		if (!link) {
 			tipc_node_read_unlock(node);
+			tipc_node_put(node);
 			err = -EINVAL;
 			goto err_free;
 		}
 
 		err = __tipc_nl_add_link(net, &msg, link, 0);
 		tipc_node_read_unlock(node);
+		tipc_node_put(node);
 		if (err)
 			goto err_free;
 	}
@@ -2634,11 +2640,13 @@ int tipc_nl_node_reset_link_stats(struct sk_buff *skb, struct genl_info *info)
 	if (!link) {
 		spin_unlock_bh(&le->lock);
 		tipc_node_read_unlock(node);
+		tipc_node_put(node);
 		return -EINVAL;
 	}
 	tipc_link_reset_stats(link);
 	spin_unlock_bh(&le->lock);
 	tipc_node_read_unlock(node);
+	tipc_node_put(node);
 	return 0;
 }
 
-- 
2.43.0