udp6 over GRE tunnel does not work after to GRE tso changes. GRE
tso handler passes inner packet but keeps track of outer header
start in SKB_GSO_CB(skb)->mac_offset.  udp6 fragment need to
take care of outer header, which start at the mac_offset, while
adding fragment header.
This bug is introduced by commit 68c3316311 (GRE: Add TCP
segmentation offload for GRE).

Reported-by: Dmitry Kravkov <redacted>
Signed-off-by: Pravin B Shelar <redacted>
---
 include/linux/skbuff.h |   15 +++++++++++++++
 net/ipv6/udp_offload.c |   20 ++++++++++++--------
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 5f93119..f7fdf71 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -2758,6 +2758,21 @@ static inline int skb_tnl_header_len(const struct sk_buff *inner_skb)
                SKB_GSO_CB(inner_skb)->mac_offset;
 }

+static inline int gso_pskb_expand_head(struct sk_buff *skb, int extra)
+{
+       int new_headroom, headroom;
+       int ret;
+
+       headroom = skb_headroom(skb);
+       ret = pskb_expand_head(skb, extra, 0, GFP_ATOMIC);
+       if (ret)
+               return ret;
+
+       new_headroom = skb_headroom(skb);
+       SKB_GSO_CB(skb)->mac_offset += (new_headroom - headroom);
+       return 0;
+}
+
 static inline bool skb_is_gso(const struct sk_buff *skb)
 {
        return skb_shinfo(skb)->gso_size;

diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index 76d401a..5d1b8d7 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c

@@ -46,11 +46,12 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
        unsigned int mss;
        unsigned int unfrag_ip6hlen, unfrag_len;
        struct frag_hdr *fptr;
-       u8 *mac_start, *prevhdr;
+       u8 *packet_start, *prevhdr;
        u8 nexthdr;
        u8 frag_hdr_sz = sizeof(struct frag_hdr);
        int offset;
        __wsum csum;
+       int tnl_hlen;

        mss = skb_shinfo(skb)->gso_size;
        if (unlikely(skb->len <= mss))

@@ -84,9 +85,11 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
        skb->ip_summed = CHECKSUM_NONE;

        /* Check if there is enough headroom to insert fragment header. */
-       if ((skb_mac_header(skb) < skb->head + frag_hdr_sz) &&
-           pskb_expand_head(skb, frag_hdr_sz, 0, GFP_ATOMIC))
-               goto out;
+       tnl_hlen = skb_tnl_header_len(skb);
+       if (skb_headroom(skb) < (tnl_hlen + frag_hdr_sz)) {
+               if (gso_pskb_expand_head(skb, tnl_hlen + frag_hdr_sz))
+                       goto out;
+       }

        /* Find the unfragmentable header and shift it left by frag_hdr_sz
         * bytes to insert fragment header.

@@ -94,11 +97,12 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
        unfrag_ip6hlen = ip6_find_1stfragopt(skb, &prevhdr);
        nexthdr = *prevhdr;
        *prevhdr = NEXTHDR_FRAGMENT;
-       unfrag_len = skb_network_header(skb) - skb_mac_header(skb) +
-                    unfrag_ip6hlen;
-       mac_start = skb_mac_header(skb);
-       memmove(mac_start-frag_hdr_sz, mac_start, unfrag_len);
+       unfrag_len = (skb_network_header(skb) - skb_mac_header(skb)) +
+                    unfrag_ip6hlen + tnl_hlen;
+       packet_start = (u8 *) skb->head + SKB_GSO_CB(skb)->mac_offset;
+       memmove(packet_start-frag_hdr_sz, packet_start, unfrag_len);

+       SKB_GSO_CB(skb)->mac_offset -= frag_hdr_sz;
        skb->mac_header -= frag_hdr_sz;
        skb->network_header -= frag_hdr_sz;

--