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

[hujunwei]

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@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);
>>>
> .
>