[PATCH net v2] tipc: fix use-after-free of broadcast reception link at node release

[Binbin Deng <18983559317@163.com>]

When the last reference to a tipc_node is dropped, the node itself is
deferred to the RCU callback tipc_node_free(), but the nested broadcast
reception link, n->bc_entry.link, is kfree()d synchronously in
tipc_node_kref_release(), before call_rcu(), and the pointer is never
cleared.  Readers that obtained the node from node_list and then
dereference bc_entry.link deep into the link (tipc_link_name() in
tipc_nl_node_reset_link_stats(), bcl->name/stats in
tipc_nl_add_bc_link() on the link dump path) therefore touch freed
memory although the enclosing node stays alive for a full grace period.
The sibling per-bearer links get "kfree + NULL" under the node write
lock in tipc_node_clear_links(); the broadcast entry was missed.

The race is reachable through the node cleanup timer: once a peer has
been unreachable for NODE_CLEANUP_AFTER (300 s), tipc_node_timeout() ->
tipc_node_cleanup() unlinks the node under the write lock, and the
final tipc_node_put() at the caller runs outside node_list_lock, so the
kfree executes in timer softirq context, concurrently with genl dump/
reset walkers that hold only rcu_read_lock().  (A netlink "peer remove"
cannot race those walkers itself: tipc_genl_family has no parallel_ops,
so all doit/dumpit calls serialize on the global genl lock.)

Reproduced in a QEMU/KVM guest running v7.3.0 with KASAN and
PREEMPT(full): 10 TIPC peer namespaces on veth bearers, bearer removal
staggered so the 300 s node cleanup timers expire while other tasks
loop link dumps with the broadcast filter and reset-link-stats lookups.
KASAN hit within two cleanup cycles:

  BUG: KASAN: slab-use-after-free in strlen+0x85/0xa0
  Read of size 1 at addr ffff88800b54a004 by task poc_eth/371
   strlen+0x85/0xa0
   tipc_nl_add_bc_link+0x2f9/0xd40
   __tipc_nl_add_node_links+0x10c/0x140
   tipc_nl_node_dump_link+0x8d5/0xd50
  Freed by task 0:
   kfree+0x121/0x380
   tipc_node_timeout+0xa33/0xcf0
   run_timer_softirq+0x144/0x250
   sysvec_apic_timer_interrupt

with the freed object being the kmalloc-1k tipc_link allocated by
tipc_link_bc_create() during peer discovery.  The same defect fires in
strcmp() called from tipc_nl_node_reset_link_stats().

Move the kfree of n->bc_entry.link into the tipc_node_free() RCU
callback so the broadcast link lives exactly as long as the enclosing
node.  All readers are covered: the netlink walkers are inside
rcu_read_lock(), and the packet reception path reaches the node via
tipc_node_find(), whose kref defers kref_release itself.

Found during a static-analysis-assisted audit: candidate sites were
surfaced by a Coccinelle rule, and the reader/writer paths were
confirmed by LLM-assisted (GLM-5.3) code inspection.

Fixes: d25a01257e42 ("tipc: fix crash during node removal")
Assisted-by: GLM-5.3 coccinelle
Signed-off-by: Binbin Deng <18983559317@163.com>
---
 net/tipc/node.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/tipc/node.c b/net/tipc/node.c
index bd91378b7540..fdc679fe2d91 100644
--- a/net/tipc/node.c
+++ b/net/tipc/node.c
@@ -303,6 +303,7 @@ static void tipc_node_free(struct rcu_head *rp)
 #ifdef CONFIG_TIPC_CRYPTO
 	tipc_crypto_stop(&n->crypto_rx);
 #endif
+	kfree(n->bc_entry.link);
 	kfree(n);
 }
 
@@ -310,7 +311,6 @@ static void tipc_node_kref_release(struct kref *kref)
 {
 	struct tipc_node *n = container_of(kref, struct tipc_node, kref);
 
-	kfree(n->bc_entry.link);
 	call_rcu(&n->rcu, tipc_node_free);
 }
 
-- 
2.43.0