[PATCH net] net/sched: act_skbmod: fix length calculations and avoid invalid header warnings

syzbot reported a warning in skb_network_header_len() triggered
by tcf_skbmod_act():

  !skb_transport_header_was_set(skb)
  WARNING: CPU: 0 PID: 14949 at include/linux/skbuff.h:3243 skb_network_header_len include/linux/skbuff.h:3243 [inline]
  WARNING: CPU: 0 PID: 14949 at net/sched/act_skbmod.c:55 tcf_skbmod_act+0xfe8/0x1810 net/sched/act_skbmod.c:55

There are a few issues in tcf_skbmod_act():

1. Calling skb_network_header_len() assumes skb->transport_header is set,
   which is not guaranteed when tcf_skbmod_act() runs at TC ingress.
2. Unconditionally calling skb_mac_header_len() at the beginning of
   tcf_skbmod_act() triggers a warning on L3 devices (e.g. TUN) where the
   MAC header is unset, evaluating to an underflowed garbage length.
3. On TC ingress, skb->data points to the network header. Adding the MAC
   header length to the IP header length causes skb_ensure_writable() to
   request more bytes than the actual IP packet length, dropping valid
   short packets (e.g. 28-byte UDP/IPv4 packets).

Fix these by:
- Using skb_network_offset(skb) + sizeof(struct iphdr/ipv6hdr) for
  SKBMOD_F_ECN so that the required length is correctly calculated on
  both ingress (offset == 0) and egress (offset == mac_len).
- Setting max_edit_len to ETH_HLEN for Ethernet header modifications
  after validating ARPHRD_ETHER.

Fixes: 56af5e749f20 ("net/sched: act_skbmod: Add SKBMOD_F_ECN option support")
Reported-by: [email protected]
Closes: https://lore.kernel.org/netdev/[email protected]/T/#u
Signed-off-by: Eric Dumazet <[email protected]>
---
 net/sched/act_skbmod.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/sched/act_skbmod.c b/net/sched/act_skbmod.c
index a464b0a3c1b81dba6c28c1141aa38c5c7cad3acb..7579cf1e0ff37c1314d112a6aaeed0a965280834 100644
--- a/net/sched/act_skbmod.c
+++ b/net/sched/act_skbmod.c
@@ -38,7 +38,6 @@ TC_INDIRECT_SCOPE int tcf_skbmod_act(struct sk_buff *skb,
 	if (unlikely(p->action == TC_ACT_SHOT))
 		goto drop;
 
-	max_edit_len = skb_mac_header_len(skb);
 	flags = p->flags;
 
 	/* tcf_skbmod_init() guarantees "flags" to be one of the following:
@@ -51,14 +50,19 @@ TC_INDIRECT_SCOPE int tcf_skbmod_act(struct sk_buff *skb,
 	if (flags == SKBMOD_F_ECN) {
 		switch (skb_protocol(skb, true)) {
 		case cpu_to_be16(ETH_P_IP):
+			max_edit_len = sizeof(struct iphdr);
+			break;
 		case cpu_to_be16(ETH_P_IPV6):
-			max_edit_len += skb_network_header_len(skb);
+			max_edit_len = sizeof(struct ipv6hdr);
 			break;
 		default:
 			goto out;
 		}
-	} else if (!skb->dev || skb->dev->type != ARPHRD_ETHER) {
-		goto out;
+		max_edit_len += skb_network_offset(skb);
+	} else {
+		if (!skb->dev || skb->dev->type != ARPHRD_ETHER)
+			goto out;
+		max_edit_len = ETH_HLEN;
 	}
 
 	err = skb_ensure_writable(skb, max_edit_len);
-- 
2.55.0.766.g2966f0265a-goog