[PATCH net v2 2/5] loopback: use eth_type_trans_careful()

tc egress actions can leave an skb with fewer than ETH_HLEN bytes in
its linear part. loopback_xmit() then crashes in eth_type_trans():

  tc qdisc add dev lo clsact
  tc filter add dev lo egress matchall action vlan pop_eth
  # AF_PACKET SOCK_RAW: eth / eth / 8192 bytes of payload

  kernel BUG at include/linux/skbuff.h:2839!
  RIP: 0010:eth_type_trans (include/linux/etherdevice.h:635 net/ethernet/eth.c:163)
  Call Trace:
   loopback_xmit (drivers/net/loopback.c:87)
   dev_hard_start_xmit (include/linux/netdevice.h:5455 include/linux/netdevice.h:5464 net/core/dev.c:4057 net/core/dev.c:4073)
   __dev_queue_xmit (net/core/dev.c:5066)
   packet_sendmsg (net/packet/af_packet.c:3157 net/packet/af_packet.c:3189)

The same happens with "action ife decode" on lo egress.

Use eth_type_trans_careful(), and drop the packet if the Ethernet
header can not be pulled.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Vega <redacted>
Link: https://lore.kernel.org/netdev/20261005052249.1914367-2-weir@nebusec.ai/ (local)
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 drivers/net/loopback.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 55ecd5ebac0f21b61b64373c429a6f9ba5d15c8e..8a6cb34e6b5f02c88dfb0c6bf112068c4f51fec1 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c

@@ -70,6 +70,7 @@ EXPORT_SYMBOL(blackhole_netdev);
 static netdev_tx_t loopback_xmit(struct sk_buff *skb,
 				 struct net_device *dev)
 {
+	enum skb_drop_reason reason;
 	int len;
 
 	skb_tx_timestamp(skb);

@@ -84,7 +85,12 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
 	 */
 	skb_dst_force(skb);
 
-	skb->protocol = eth_type_trans(skb, dev);
+	reason = eth_type_trans_careful(skb, dev);
+	if (unlikely(reason)) {
+		dev_core_stats_tx_dropped_inc(dev);
+		kfree_skb_reason(skb, reason);
+		return NETDEV_TX_OK;
+	}
 
 	len = skb->len;
 	if (likely(__netif_rx(skb) == NET_RX_SUCCESS))

-- 
2.53.0