[PATCH net] ipv6: Fix dangling pointer when ipv6 fragment

[hujunwei <hujunwei4@huawei.com>]

From: Junwei Hu <hujunwei4@huawei.com>

At the beginning of ip6_fragment func, the prevhdr pointer is
obtained in the ip6_find_1stfragopt func.
However, all the pointers pointing into skb header may change
when calling skb_checksum_help func with
skb->ip_summed = CHECKSUM_PARTIAL condition.
The prevhdr pointe will be dangling if it is not reloaded after
calling __skb_linearize func in skb_checksum_help func.

Here, I add a variable, nexthdr_offset, to evaluate the offset,
which does not changes even after calling __skb_linearize func.

Fixes: 405c92f7a541 ("ipv6: add defensive check for CHECKSUM_PARTIAL skbs in ip_fragment")
Signed-off-by: Junwei Hu <hujunwei4@huawei.com>
Reported-by: Wenhao Zhang <zhangwenhao8@huawei.com>
Reviewed-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>

---
 net/ipv6/ip6_output.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index edbd12067170..6db3c60b3b66 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -606,12 +606,14 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
     __be32 frag_id;
     int ptr, offset = 0, err = 0;
     u8 *prevhdr, nexthdr = 0;
+    u8 nexthdr_offset;
 
     err = ip6_find_1stfragopt(skb, &prevhdr);
     if (err < 0)
         goto fail;
     hlen = err;
     nexthdr = *prevhdr;
+    nexthdr_offset = prevhdr - skb_network_header(skb);
 
     mtu = ip6_skb_dst_mtu(skb);
 
@@ -646,6 +648,8 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
         (err = skb_checksum_help(skb)))
         goto fail;
 
+    prevhdr = skb_network_header(skb) + nexthdr_offset;
+
     hroom = LL_RESERVED_SPACE(rt->dst.dev);
     if (skb_has_frag_list(skb)) {
         unsigned int first_len = skb_pagelen(skb);
-- 
2.19.1