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

[Eric Dumazet]

On 03/30/2019 12:48 AM, Eric Dumazet wrote:
> 
> 
> On 03/30/2019 12:29 AM, hujunwei wrote:
>> 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.
>>

...

>> 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;

Why u8 here ?

I would use "unsigned int" really.

>> 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);
>>