[PATCH net v2 5/5] net/sched: act_ife: do not consume the inner header on egress

[Eric Dumazet <edumazet@kernel.org>]

From: Jamal Hadi Salim <jhs@mojatatu.com>

The issue:
tcf_ife_decode() unconditionally calls eth_type_trans(), which pulls
ETH_HLEN.  On egress this consumes the inner Ethernet header, and the
device receives an skb whose data points to the L3 header:

- loopback_xmit() parses L3 data as Ethernet: the "ethertype" comes
  from the inner IPv4 source address, and the decapsulated packet is
  dropped or misparsed.
- a real NIC transmits the frame without its Ethernet header.

(The __skb_pull() BUG this used to trigger in loopback_xmit() when the
remaining linear area was short is handled by "loopback: use
eth_type_trans_careful()".)

The Fix:
Parse the inner Ethernet header with eth_type_trans() in both directions,
then restore the egress data position and MAC length.  On ingress the
existing behavior is unchanged.  On egress, push the inner Ethernet header
back after parsing so the xmit path sees the original frame and later
actions see a consistent network header.

Conditions to recreate the bug:
- egress tc filter on lo: action ife decode pipe;
- AF_PACKET SOCK_RAW frame: outer Ethernet with ETH_P_IFE, IFE metadata,
  inner Ethernet + a valid IPv4 ICMP echo request to a local address;
- before this patch, no echo is received; after it, the echo is
  delivered and answered.

Fixes: ef6980b6becb ("introduce IFE action")
Cc: stable@vger.kernel.org
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
[edumazet: reworded changelog, the crash is now handled in loopback]
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/sched/act_ife.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sched/act_ife.c b/net/sched/act_ife.c
index 2afd68983ece40d8cae6c96af86e2ea65be476bc..3946a9589bb8fcd296e6f1f9234c00238df4d954 100644
--- a/net/sched/act_ife.c
+++ b/net/sched/act_ife.c
@@ -769,6 +769,10 @@ static int tcf_ife_decode(struct sk_buff *skb, const struct tc_action *a,
 
 	skb->protocol = eth_type_trans(skb, skb->dev);
 	skb_reset_network_header(skb);
+	if (!skb_at_tc_ingress(skb)) {
+		skb_push(skb, ETH_HLEN);
+		skb_reset_mac_len(skb);
+	}
 
 	return action;
 }
-- 
2.53.0