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Subject: Re: [PATCH net] ipv6: Fix dangling pointer when ipv6 fragment
To:     Eric Dumazet <eric.dumazet@gmail.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>
 <a277b9c7-22e0-3763-89e5-504514d2e9b0@gmail.com>
From:   hujunwei <hujunwei4@huawei.com>
Message-ID: <30d10040-50ae-f212-271f-210972436151@huawei.com>
Date:   Sat, 30 Mar 2019 20:37:40 +0800
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Hi Eri,

Thanks for your suggestion, u8 may not enough when the packet have a lot of exthdr.

I will update the patch in v2, by the way update Report-by tag.


On 2019/3/30 15:57, Eric Dumazet wrote:
>
> 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);
>>>
> .
>