Subject: Re: kernel >= 4.0: crashes when using traceroute6 with isatap

From: Vlad Yasevich <[email-masked]>
Date: Tue, 12 May 2015 12:34:00 -0400

On 05/06/2015 06:42 PM, Eric Dumazet wrote:
> On Thu, 2015-05-07 at 00:04 +0200, Wolfgang Walter wrote:
>> Am Mittwoch, 6. Mai 2015, 12:10:00 schrieb Eric Dumazet:
>>> On Wed, 2015-05-06 at 11:15 -0700, Cong Wang wrote:
>>>> (Cc'ing netdev.)
>>>>
>>>> On Sat, May 2, 2015 at 5:29 AM, Wolfgang Walter <[email-masked]> wrote:
>>>>> Am Samstag, 2. Mai 2015, 02:16:36 schrieb Wolfgang Walter:
>>>>>> Hello,
>>>>>>
>>>>>> kernel 4.0 (and 4.0.1) crashes immediately when I use traceroute6 with
>>>>>> an
>>>>>> isatap-tunnel.
>>>>>
>>>>> I did some further tests. To trigger the crash you need
>>>>>
>>>>> * isatap-tunnel (probably any sit-tunnel will do it)
>>>>> * raw-socket
>>>>> * udp
>>>>>
>>>>> Using icmpv6 or tcp i.e. does not trigger it.
>>>>
>>>> Do you have a script to reproduce it?
>>>>
>>>>
>>>> Thanks for the bug report!
>>>> --
>>>
>>> Please Wolfgang try to revert 32dce968dd987adfb0c00946d78dad9154f64759
>>> ("ipv6: Allow for partial checksums on non-ufo packets")
>>
>> Indeed, that fixes the problem.
> 
> Yes, setting skb->csum to 0 is clearly wrong for CHECKSUM_PARTIAL
> 
> Would you try :
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 7fde1f265c90e90f16291e6c861b6e242111c25b..694ae630e1ca67e25ab1e5f6dd0b3597db3669b0 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1416,6 +1416,10 @@ alloc_new_skb:
>  			data += fragheaderlen;
>  			skb->transport_header = (skb->network_header +
>  						 fragheaderlen);
> +			if (csummode == CHECKSUM_PARTIAL) {
> +				skb->csum_start = skb_transport_header(skb) - skb->head;
> +				skb->csum_offset = offsetof(struct udphdr, check);
> +			}
>  			if (fraggap) {
>  				skb->csum = skb_copy_and_csum_bits(
>  					skb_prev, maxfraglen,
> 
> 
> 

So why is this not an issue in __ip_append_data()?

-vlad