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

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 reported:

  BUG: KASAN: slab-use-after-free in _raw_read_lock_bh+0x1d/0x40
  Write of size 4 at addr ffff888112723808 by task poc/87
  Call Trace:
   _raw_read_lock_bh+0x1d/0x40
   tipc_nl_node_set_link+0x30e/0x680
   genl_family_rcv_msg_doit+0x1e0/0x2c0
   genl_rcv_msg+0x419/0x6d0
   netlink_rcv_skb+0x11f/0x350
  Allocated by task 28:
   tipc_node_create+0x9c1/0x1fa0
   tipc_node_check_dest+0x121/0x11e0
   tipc_disc_rcv+0xdbf/0x1430
  Freed by task 87:
   kfree+0x149/0x330
   rcu_core+0x50a/0x1850
  Last potentially related work creation:
   __call_rcu_common.constprop.0+0x71/0xa10
   tipc_node_timeout+0xb1b/0xe70

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 <redacted>
---
 net/tipc/node.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/tipc/node.c b/net/tipc/node.c
index bd91378b7540..2726bee3bb40 100644
--- a/net/tipc/node.c
+++ b/net/tipc/node.c

@@ -2424,6 +2424,8 @@ static struct tipc_node *tipc_node_find_by_name(struct net *net,
 		if (found_node)
 			break;
 	}
+	if (found_node && !kref_get_unless_zero(&found_node->kref))
+		found_node = NULL;
 	rcu_read_unlock();
 
 	return found_node;

@@ -2507,6 +2509,7 @@ 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 +2561,14 @@ 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 +2639,13 @@ 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