Re: [PATCH net 1/5] net/sched: act_vlan: reject short linear Ethernet headers after POP_ETH

The issue:
act_vlan POP_ETH can leave a linear area shorter than ETH_HLEN; a
subsequent eth_type_trans() may then BUG in __skb_pull() when the frame
is redirected to an Ethernet device.

The Fix:
Drop the packet at the producer boundary when skb_headlen() is shorter
than ETH_HLEN after skb_eth_pop().

Conditions to recreate the bug:
- egress tc filter on lo: action vlan pop_eth.
- AF_PACKET SOCK_RAW with PACKET_VNET_HDR, hdr_len=28, and a non-linear
  eth / inner-eth frame.
- The pop leaves skb_headlen(skb) < ETH_HLEN; a following
  eth_type_trans() can BUG in __skb_pull().

Fixes: 19fbcb36a39e ("net/sched: act_vlan: Add {POP,PUSH}_ETH actions")
Reported-by: Vega <redacted>
Link: https://lore.kernel.org/r/40fe2d7dfce13411bfbe9271c0f7c54fb7e88e3b.1782548651.git.bronzed_45_vested@icloud.com/ (local)
Cc: stable@vger.kernel.org
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Victor Nogueira <redacted>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/act_vlan.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sched/act_vlan.c b/net/sched/act_vlan.c
index 6a375fdc63dd..ddf03b08bfe8 100644
--- a/net/sched/act_vlan.c
+++ b/net/sched/act_vlan.c

@@ -79,6 +79,8 @@ TC_INDIRECT_SCOPE int tcf_vlan_act(struct sk_buff *skb,
 		err = skb_eth_pop(skb);
 		if (err)
 			goto drop;
+		if (unlikely(skb_headlen(skb) < ETH_HLEN))
+			goto drop;