[PATCH net 3/5] net: use eth_type_trans_careful() in __dev_forward_skb2()

A tc egress BPF program can push a tiny link-layer header with
bpf_skb_change_head() on an skb whose head is almost empty, then
redirect it to an Ethernet device ingress. __bpf_redirect_common()
only checks mac_header < network_header, and __dev_forward_skb2()
crashes in eth_type_trans():

  tc filter add dev lo egress pref 1 matchall action vlan pop_eth continue
  tc filter add dev lo egress pref 2 bpf da obj prog.o
  # prog: bpf_skb_change_head(skb, 1, 0);
  #       return bpf_redirect(veth1_ifindex, BPF_F_INGRESS);
  # AF_PACKET SOCK_RAW on lo: 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:
   __dev_forward_skb2 (net/core/dev.c:2507)
   dev_forward_skb_nomtu (net/core/dev.c:2546)
   __dev_queue_xmit (net/core/dev.c:4738 net/core/dev.c:4998)
   packet_sendmsg (net/packet/af_packet.c:3157 net/packet/af_packet.c:3189)

Use eth_type_trans_careful(), and drop the packet if the Ethernet
header can not be pulled. This also covers other dev_forward_skb()
users (veth, macvlan, ipvlan, netdevsim, l2tp_eth).

Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/core/dev.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1a09e7869118059e92b53e00ed873dfc7485ffc3..ac784310a69c1d9ceecf68909334196b555871c5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -2501,14 +2501,22 @@ EXPORT_SYMBOL_GPL(is_skb_forwardable);
 static int __dev_forward_skb2(struct net_device *dev, struct sk_buff *skb,
 			      bool check_mtu)
 {
-	int ret = ____dev_forward_skb(dev, skb, check_mtu);
+	enum skb_drop_reason reason;
+	int ret;
+
+	ret = ____dev_forward_skb(dev, skb, check_mtu);
+	if (unlikely(ret))
+		return ret;
 
-	if (likely(!ret)) {
-		skb->protocol = eth_type_trans(skb, dev);
-		skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);
+	reason = eth_type_trans_careful(skb, dev);
+	if (unlikely(reason)) {
+		dev_core_stats_rx_dropped_inc(dev);
+		kfree_skb_reason(skb, reason);
+		return NET_RX_DROP;
 	}
+	skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);
 
-	return ret;
+	return 0;
 }
 
 int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)

-- 
2.53.0