net: rswitch: Avoid use-after-free in rswitch_poll()

The use-after-free is actually in rswitch_tx_free(), which is inlined in
rswitch_poll(). Since `skb` and `gq->skbs[gq->dirty]` are in fact the
same pointer, the skb is first freed using dev_kfree_skb_any(), then the
value in skb->len is used to update the interface statistics.

Let's move around the instructions to use skb->len before the skb is
freed.

This bug is trivial to reproduce using KFENCE. It will trigger a splat
every few packets. A simple ARP request or ICMP echo request is enough.

Fixes: 271e015 ("net: rswitch: Add unmap_addrs instead of dma address in each desc")
Signed-off-by: Radu Rendec <[email protected]>
Reviewed-by: Yoshihiro Shimoda <[email protected]>
Reviewed-by: Niklas Söderlund <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/net/ethernet/renesas/rswitch.c

@@ -871,13 +871,13 @@ static void rswitch_tx_free(struct net_device *ndev)
 		dma_rmb();
 		skb = gq->skbs[gq->dirty];
 		if (skb) {
+			rdev->ndev->stats.tx_packets++;
+			rdev->ndev->stats.tx_bytes += skb->len;
 			dma_unmap_single(ndev->dev.parent,
 					 gq->unmap_addrs[gq->dirty],
 					 skb->len, DMA_TO_DEVICE);
 			dev_kfree_skb_any(gq->skbs[gq->dirty]);
 			gq->skbs[gq->dirty] = NULL;
-			rdev->ndev->stats.tx_packets++;
-			rdev->ndev->stats.tx_bytes += skb->len;
 		}
 		desc->desc.die_dt = DT_EEMPTY;
 	}