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

[hujunwei]

From: Junwei Hu <hujunwei4@xxxxxxxxxx>

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@xxxxxxxxxx>
Reported-by: Wenhao Zhang <zhangwenhao8@xxxxxxxxxx>
Reviewed-by: Zhiqiang Liu <liuzhiqiang26@xxxxxxxxxx>

---
Ânet/ipv6/ip6_output.c | 4 ++++
Â1 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,
ÂÂÂÂ __be32 frag_id;
ÂÂÂÂ int ptr, offset = 0, err = 0;
ÂÂÂÂ u8 *prevhdr, nexthdr = 0;
+ÂÂÂ u8 nexthdr_offset;
Â
ÂÂÂÂ err = ip6_find_1stfragopt(skb, &prevhdr);
ÂÂÂÂ if (err < 0)
ÂÂÂÂ ÂÂÂ goto fail;
ÂÂÂÂ hlen = err;
ÂÂÂÂ nexthdr = *prevhdr;
+ÂÂÂ nexthdr_offset = prevhdr - skb_network_header(skb);
Â
ÂÂÂÂ mtu = ip6_skb_dst_mtu(skb);
Â
@@ -646,6 +648,8 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
ÂÂÂÂ ÂÂÂ (err = skb_checksum_help(skb)))
ÂÂÂÂ ÂÂÂ goto fail;
Â
+ÂÂÂ prevhdr = skb_network_header(skb) + nexthdr_offset;
+
ÂÂÂÂ hroom = LL_RESERVED_SPACE(rt->dst.dev);
ÂÂÂÂ if (skb_has_frag_list(skb)) {
ÂÂÂÂ ÂÂÂ unsigned int first_len = skb_pagelen(skb);
-- 
2.19.1