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Subject: Re: [PATCH net] ipv6: Fix dangling pointer when ipv6 fragment
From:   Eric Dumazet <eric.dumazet@gmail.com>
To:     hujunwei <hujunwei4@huawei.com>, davem@davemloft.net,
        kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     mingfangsen@huawei.com, liuzhiqiang26@huawei.com,
        zhangwenhao8@huawei.com
References: <cbbedd1f-3e99-85f7-86d7-d02a86d59c29@huawei.com>
 <ee0fb75b-f976-02ae-31e3-c3f957f71923@gmail.com>
Message-ID: <a277b9c7-22e0-3763-89e5-504514d2e9b0@gmail.com>
Date:   Sat, 30 Mar 2019 00:57:55 -0700
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On 03/30/2019 12:48 AM, Eric Dumazet wrote:
> 
> 
> On 03/30/2019 12:29 AM, hujunwei wrote:
>> 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.
>>

...

>> 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;

Why u8 here ?

I would use "unsigned int" really.

>>  
>>      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);
>>