[PATCH net v3 3/6] net/sched: act_mirred: ensure a pullable link-layer header before redirect

The issue:
tcf_mirred_to_dev() may redirect a packet that has no link-layer header to
a device that expects one.  TUN builds a valid L3 skb with mac_len == 0;
mirred then pushes zero bytes and the IP header is parsed as Ethernet by
eth_type_trans().  The same happens after a VLAN POP_ETH or MPLS POP chain
whose producer did not leave a full inner Ethernet header.

The Fix:
After the existing push/pull adjustment, when the target expects data at
the MAC header, pull dev->min_header_len bytes into the linear area and
drop the packet only when that pull fails.  This ensures that a consumer
such as eth_type_trans() can pull the target device's minimum link-layer
header from the linear area.  Do not gate the check on skb_network_offset():
tc_mpls_l2vpn.sh legitimately redirects a decapsulated packet whose MAC
header is not at the recorded network offset.

Conditions to recreate the bug:
- create a TUN device with IFF_TUN | IFF_VNET_HDR | IFF_NO_PI;
- attach a filter to the TUN device's ingress hook, with a mirred egress
  redirect action targeting an Ethernet device such as lo;
- write a virtio_net_hdr with hdr_len=1 and an 8192-byte payload.
The unfixed kernel parses the IP header as Ethernet and can BUG in
eth_type_trans() via loopback_xmit.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Link: https://lore.kernel.org/netdev/CAL4Wiiq3h_OoVo9BpBy0pqFe=AeyN25QxsU86h63qEf05uCJRA@mail.gmail.com/ (local)
Cc: <redacted>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>

---
v2 -> v3:
  - drop the skb_network_offset() condition; keep only
    pskb_may_pull(dev->min_header_len).  The offset check rejected valid
    MPLS L2VPN redirects (Eric Dumazet; tc_mpls_l2vpn.sh).
---
 net/sched/act_mirred.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
index 553342c55cf7..85163c46d652 100644
--- a/net/sched/act_mirred.c
+++ b/net/sched/act_mirred.c

@@ -316,6 +316,10 @@ static int tcf_mirred_to_dev(struct sk_buff *skb, struct tcf_mirred *m,
 		}
 	}
 
+	if (!expects_nh &&
+	    !pskb_may_pull(skb_to_send, dev->min_header_len))
+		goto err_cant_do;
+
 	skb_to_send->skb_iif = skb->dev->ifindex;
 	skb_to_send->dev = dev;
 

@@ -335,6 +339,8 @@ static int tcf_mirred_to_dev(struct sk_buff *skb, struct tcf_mirred *m,
 	return retval;
 
 err_cant_do:
+	if (skb_to_send != skb)
+		kfree_skb(skb_to_send);
 	if (is_redirect)
 		retval = TC_ACT_SHOT;
 	tcf_action_inc_overlimit_qstats(&m->common);

-- 
2.43.0