net: ethernet: lantiq_etop: fix memory disclosure

When applying padding, the buffer is not zeroed, which results in memory
disclosure. The mentioned data is observed on the wire. This patch uses
skb_put_padto() to pad Ethernet frames properly. The mentioned function
zeroes the expanded buffer.

In case the packet cannot be padded it is silently dropped. Statistics
are also not incremented. This driver does not support statistics in the
old 32-bit format or the new 64-bit format. These will be added in the
future. In its current form, the patch should be easily backported to
stable versions.

Ethernet MACs on Amazon-SE and Danube cannot do padding of the packets
in hardware, so software padding must be applied.

Fixes: 504d472 ("MIPS: Lantiq: Add ethernet driver")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20240923214949.231511-2-olek2@wp.pl
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/lantiq_etop.c

@@ -481,7 +481,9 @@ ltq_etop_tx(struct sk_buff *skb, struct net_device *dev)
 	unsigned long flags;
 	u32 byte_offset;
 
-	len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;
+	if (skb_put_padto(skb, ETH_ZLEN))
+		return NETDEV_TX_OK;
+	len = skb->len;
 
 	if ((desc->ctl & (LTQ_DMA_OWN | LTQ_DMA_C)) || ch->skb[ch->dma.desc]) {
 		netdev_err(dev, "tx ring full\n");