veth: fix OOB txq access in veth_poll() with asymmetric queue counts

XDP redirect into a veth device (via bpf_redirect()) calls
veth_xdp_xmit(), which enqueues frames into the peer's ptr_ring using
  smp_processor_id() % peer->real_num_rx_queues
as the ring index.  With an asymmetric veth pair where the peer has
fewer TX queues than RX queues, that index can exceed
peer->real_num_tx_queues.

veth_poll() then resolves peer_txq for the ring via:

  peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : NULL;

where queue_idx = rq->xdp_rxq.queue_index.  When queue_idx exceeds
peer_dev->real_num_tx_queues this is an out-of-bounds (OOB) access
into the peer's netdev_queue array, triggering DEBUG_NET_WARN_ON_ONCE
in netdev_get_tx_queue().

The normal ndo_start_xmit path is not affected: the stack clamps
skb->queue_mapping via netdev_cap_txqueue() before invoking
ndo_start_xmit, so rxq in veth_xmit() never exceeds real_num_tx_queues.

Fix veth_poll() by clamping: only dereference peer_txq when queue_idx is
within bounds, otherwise set it to NULL.  The out-of-range rings are fed
exclusively via XDP redirect (veth_xdp_xmit), never via ndo_start_xmit
(veth_xmit), so the peer txq was never stopped and there is nothing to
wake; NULL is the correct fallback.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260502071828.616C3C19425@smtp.kernel.org/
Fixes: dc82a33 ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/20260505132159.241305-2-hawk@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: netoptimizer

drivers/net/veth.c

@@ -972,7 +972,8 @@ static int veth_poll(struct napi_struct *napi, int budget)
 
 	/* NAPI functions as RCU section */
 	peer_dev = rcu_dereference_check(priv->peer, rcu_read_lock_bh_held());
-	peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : NULL;
+	peer_txq = (peer_dev && queue_idx < peer_dev->real_num_tx_queues) ?
+		   netdev_get_tx_queue(peer_dev, queue_idx) : NULL;
 
 	xdp_set_return_frame_no_direct();
 	done = veth_xdp_rcv(rq, budget, &bq, &stats);