[PATCH RFC v3] rxrpc: Fix use-after-free in rxrpc_destroy_all_peers()

During network namespace teardown, rxrpc_destroy_all_peers() iterates over
the rxnet->peer_hash table to print leaked peers. However, it does so
without holding rxnet->peer_hash_lock. This allows a race condition with
asynchronous peer destruction, where RCU callbacks concurrently remove
peers from the hash table and free them. When rxrpc_destroy_all_peers()
accesses the freed peer, it results in a KASAN slab-use-after-free.

BUG: KASAN: slab-use-after-free in rxrpc_destroy_all_peers+0xcc/0x150
net/rxrpc/peer_object.c:461
Read of size 8 at addr ffff88811089e420 by task kworker/u8:1/13
Call Trace:
 <TASK>
 rxrpc_destroy_all_peers+0xcc/0x150 net/rxrpc/peer_object.c:461
 rxrpc_exit_net+0x7f/0xc0 net/rxrpc/net_ns.c:114
 ops_exit_list net/core/net_namespace.c:199 [inline]
 ops_undo_list+0x43d/0x8d0 net/core/net_namespace.c:252
 cleanup_net+0x572/0x810 net/core/net_namespace.c:702
 process_one_work kernel/workqueue.c:3322 [inline]
 process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
 worker_thread+0x92d/0xe10 kernel/workqueue.c:3486
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

To fix the slab-use-after-free, acquire rxnet->peer_hash_lock with
spin_lock_bh() while iterating over rxnet->peer_hash in
rxrpc_destroy_all_peers().

Additionally, insert rcu_barrier() in rxrpc_exit_net() prior to destroying
peers and local endpoints. Without rcu_barrier(), rxrpc_destroy_all_peers()
races with pending RCU callbacks that drop peer and local references,
logging spurious leak warnings. Furthermore, this missing synchronization
causes a downstream panic hazard in rxrpc_destroy_all_locals() because
in-flight peer releases leave rxnet->local_endpoints populated. Calling
rcu_barrier() ensures all pending RCU callbacks that drop peer and local
references complete prior to leak checks and local endpoint destruction.

Fixes: 17226f124038 ("rxrpc: Fix leak of rxrpc_peer objects")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=c876adfab6362679008c
Link: https://syzkaller.appspot.com/ai_job?id=7fecbeb2-cd9b-4ca2-8149-48663e20b153
To: "David S. Miller" <[email protected]>
To: "David Howells" <[email protected]>
To: "Eric Dumazet" <[email protected]>
To: "Jakub Kicinski" <[email protected]>
To: <[email protected]>
To: "Marc Dionne" <[email protected]>
To: <[email protected]>
To: "Paolo Abeni" <[email protected]>
Cc: "Simon Horman" <[email protected]>
Cc: <[email protected]>

---
v3:
- Added inline comments explaining the purpose of rcu_barrier() in rxrpc_exit_net() and peer_hash_lock acquisition in rxrpc_destroy_all_peers().

v2:
- Added rcu_barrier() in rxrpc_exit_net() before peer and local endpoint teardown.
- Updated commit message to detail how rcu_barrier() prevents spurious leak warnings and downstream panics in rxrpc_destroy_all_locals().
https://lore.kernel.org/all/[email protected]/T/

v1:
https://lore.kernel.org/all/[email protected]/T/
---
diff --git a/net/rxrpc/net_ns.c b/net/rxrpc/net_ns.c
index 9a9834145..c00cddfd3 100644
--- a/net/rxrpc/net_ns.c
+++ b/net/rxrpc/net_ns.c
@@ -111,6 +111,11 @@ static __net_exit void rxrpc_exit_net(struct net *net)
 	timer_delete_sync(&rxnet->peer_keepalive_timer);
 	rxrpc_destroy_all_calls(rxnet);
 	rxrpc_destroy_all_connections(rxnet);
+	/*
+	 * Wait for pending RCU callbacks that drop peer and local
+	 * references to avoid spurious leak warnings and panics.
+	 */
+	rcu_barrier();
 	rxrpc_destroy_all_peers(rxnet);
 	rxrpc_destroy_all_locals(rxnet);
 	proc_remove(rxnet->proc_net);
diff --git a/net/rxrpc/peer_object.c b/net/rxrpc/peer_object.c
index fa9a406e1..32da76256 100644
--- a/net/rxrpc/peer_object.c
+++ b/net/rxrpc/peer_object.c
@@ -454,6 +454,12 @@ void rxrpc_destroy_all_peers(struct rxrpc_net *rxnet)
 	struct rxrpc_peer *peer;
 	int i;
 
+	/*
+	 * The lock is needed to prevent use-after-free if a peer is being
+	 * destroyed concurrently by an RCU callback.
+	 */
+	spin_lock_bh(&rxnet->peer_hash_lock);
+
 	for (i = 0; i < HASH_SIZE(rxnet->peer_hash); i++) {
 		if (hlist_empty(&rxnet->peer_hash[i]))
 			continue;
@@ -465,6 +471,8 @@ void rxrpc_destroy_all_peers(struct rxrpc_net *rxnet)
 			       &peer->srx.transport);
 		}
 	}
+
+	spin_unlock_bh(&rxnet->peer_hash_lock);
 }
 
 /**


base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
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