gso_segment: Reset skb->mac_len after modifying network header

BugLink: https://bugs.launchpad.net/bugs/1798617

[ Upstream commit c56cae23 ]

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@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv4/af_inet.c

@@ -1280,6 +1280,7 @@ static struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 		if (encap)
 			skb_reset_inner_headers(skb);
 		skb->network_header = (u8 *)iph - skb->head;
+		skb_reset_mac_len(skb);
 	} while ((skb = skb->next));
 
 out:

net/ipv6/ip6_offload.c

@@ -119,6 +119,7 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 		ipv6h = (struct ipv6hdr *)(skb_mac_header(skb) + nhoff);
 		ipv6h->payload_len = htons(skb->len - nhoff - sizeof(*ipv6h));
 		skb->network_header = (u8 *)ipv6h - skb->head;
+		skb_reset_mac_len(skb);
 
 		if (udpfrag) {
 			int err = ip6_find_1stfragopt(skb, &prevhdr);