netdev - [PATCH net v2] gso_segment: Reset skb->mac_len after modifying network header

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Message-Id: <20180913144306.9077-1-toke@toke.dk>
Date:   Thu, 13 Sep 2018 16:43:07 +0200
From:   Toke Høiland-Jørgensen <toke@...e.dk>
To:     netdev@...r.kernel.org
Cc:     Toke Høiland-Jørgensen <toke@...e.dk>,
        cake@...ts.bufferbloat.net, David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Dave Taht <dave.taht@...il.com>
Subject: [PATCH net v2] gso_segment: Reset skb->mac_len after modifying network header

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@...il.com>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...e.dk>
---
v2:
  - Properly credit Eric for his help
  - Add review and ack tags

 net/ipv4/af_inet.c     | 1 +
 net/ipv6/ip6_offload.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 20fda8fb8ffd..1fbe2f815474 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1377,6 +1377,7 @@ 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:
diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 37ff4805b20c..c7e495f12011 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -115,6 +115,7 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 			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);
-- 
2.18.0