[PATCH net] wireguard: peer: free packets left on the per-peer queues on removal

[Andrii Pasichnyk <apasichnik9@gmail.com>]

peer_remove_after_dead() flushes the crypt workqueue and then disables
the peer's NAPI. Once a disable is pending, __napi_poll() completes the
instance after the poll returns even if it used its whole budget, so a
peer removed while more than one budget of decrypted packets waits on
rx_queue keeps the rest there. Each entry holds a keypair and a peer
reference, so the peer is never released; the WARN_ON in rcu_release()
that checks for leftovers is never reached either.

Free whatever is left on both per-peer queues once nothing can feed them
any more: receive entries are single packets, transmit entries lists.

Reproduced with the WireGuard selftest VM (x86 KVM, 1 vCPU, net-next):
remove a peer while a UDP flood keeps its receive queue busy, re-add it,
repeat, then run the selftest's created/destroyed object check. Over
2140 removals the unpatched kernel leaked a peer and its keypair 5
times ("wg0: Peer 27: merely created"); with this patch, 0 times in
another 2140.

Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Assisted-by: LLM
Signed-off-by: Andrii Pasichnyk <apasichnik9@gmail.com>
---
 drivers/net/wireguard/peer.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/net/wireguard/peer.c b/drivers/net/wireguard/peer.c
index 1cb502a..3e08898 100644
--- a/drivers/net/wireguard/peer.c
+++ b/drivers/net/wireguard/peer.c
@@ -91,6 +91,25 @@ static void peer_make_dead(struct wg_peer *peer)
 	/* The caller must now synchronize_net() for this to take effect. */
 }
 
+/* Each queue entry holds a keypair and a peer reference. Transmit entries
+ * are lists of packets, receive entries single packets.
+ */
+static void peer_purge_queue(struct wg_peer *peer, struct prev_queue *queue,
+			     bool lists)
+{
+	struct sk_buff *first;
+
+	while ((first = wg_prev_queue_peek(queue)) != NULL) {
+		wg_prev_queue_drop_peeked(queue);
+		wg_noise_keypair_put(PACKET_CB(first)->keypair, false);
+		wg_peer_put(peer);
+		if (lists)
+			kfree_skb_list(first);
+		else
+			dev_kfree_skb(first);
+	}
+}
+
 static void peer_remove_after_dead(struct wg_peer *peer)
 {
 	WARN_ON(!peer->is_dead);
@@ -122,6 +141,11 @@ static void peer_remove_after_dead(struct wg_peer *peer)
 	 * here from process context.
 	 */
 	netif_napi_del(&peer->napi);
+	/* A NAPI being disabled completes after at most one more poll, which
+	 * may leave packets on rx_queue that still hold references.
+	 */
+	peer_purge_queue(peer, &peer->rx_queue, false);
+	peer_purge_queue(peer, &peer->tx_queue, true);
 
 	/* Ensure any workstructs we own (like transmit_handshake_work or
 	 * clear_peer_work) no longer are in use.
-- 
2.53.0