[PATCH 4.9 19/44] gso_segment: Reset skb->mac_len after modifying network header

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Toke Hød-Jøsen" <toke@xxxxxxx>

[ Upstream commit c56cae23c6b167acc68043c683c4573b80cbcc2c ]

When splitting a GSO segment that consists of encapsulated packets, the
skb->mac_len of the segments can end up being set wrong, causing packet
drops in particular when using act_mirred and ifb interfaces in
combination with a qdisc that splits GSO packets.

This happens because at the time skb_segment() is called, network_header
will point to the inner header, throwing off the calculation in
skb_reset_mac_len(). The network_header is subsequently adjust by the
outer IP gso_segment handlers, but they don't set the mac_len.

Fix this by adding skb_reset_mac_len() calls to both the IPv4 and IPv6
gso_segment handlers, after they modify the network_header.

Many thanks to Eric Dumazet for his help in identifying the cause of
the bug.

Acked-by: Dave Taht <dave.taht@xxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Toke HÃiland-JÃrgensen <toke@xxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv4/af_inet.c     |    1 +
 net/ipv6/ip6_offload.c |    1 +
 2 files changed, 2 insertions(+)

--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1277,6 +1277,7 @@ struct sk_buff *inet_gso_segment(struct
 		if (encap)
 			skb_reset_inner_headers(skb);
 		skb->network_header = (u8 *)iph - skb->head;
+		skb_reset_mac_len(skb);
 	} while ((skb = skb->next));
 
 out:
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -113,6 +113,7 @@ static struct sk_buff *ipv6_gso_segment(
 			payload_len = skb->len - nhoff - sizeof(*ipv6h);
 		ipv6h->payload_len = htons(payload_len);
 		skb->network_header = (u8 *)ipv6h - skb->head;
+		skb_reset_mac_len(skb);
 
 		if (udpfrag) {
 			int err = ip6_find_1stfragopt(skb, &prevhdr);