linux-kernel - Re: [PATCH v3] [net]: Fix skb->csum update in inet_proto_csum

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Date:   Thu, 23 Jan 2020 15:20:39 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Florian Westphal <fw@...len.de>
Cc:     Praveen Chaudhary <praveen5582@...il.com>, pablo@...filter.org,
        davem@...emloft.net, kadlec@...filter.org,
        netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Zhenggen Xu <zxu@...kedin.com>,
        Andy Stracner <astracner@...kedin.com>
Subject: Re: [PATCH v3] [net]: Fix skb->csum update in
 inet_proto_csum_replace16().

On 1/23/20 9:21 AM, Florian Westphal wrote:
> Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 1/22/20 12:43 PM, Florian Westphal wrote:
>>> Daniel Borkmann <daniel@...earbox.net> wrote:
>>>>> @@ -449,9 +464,6 @@ void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
>>>>>     	if (skb->ip_summed != CHECKSUM_PARTIAL) {
>>>>>     		*sum = csum_fold(csum_partial(diff, sizeof(diff),
>>>>>     				 ~csum_unfold(*sum)));
>>>>> -		if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr)
>>>>> -			skb->csum = ~csum_partial(diff, sizeof(diff),
>>>>> -						  ~skb->csum);
>>>>
>>>> What is the technical rationale in removing this here but not in any of the
>>>> other inet_proto_csum_replace*() functions? You changelog has zero analysis
>>>> on why here but not elsewhere this change would be needed?
>>>
>>> Right, I think it could be dropped everywhere BUT there is a major caveat:
>>>
>>> At least for the nf_nat case ipv4 header manipulation (which uses the other
>>> helpers froum utils.c) will eventually also update iph->checksum field
>>> to account for the changed ip addresses.
>>>
>>> And that update doesn't touch skb->csum.
>>>
>>> So in a way the update of skb->csum in the other helpers indirectly account
>>> for later ip header checksum update.
>>>
>>> At least that was my conclusion when reviewing the earlier incarnation
>>> of the patch.
>>
>> Mainly asking because not inet_proto_csum_replace16() but the other ones are
>> exposed via BPF and they are all in no way fundamentally different to each
>> other, but my concern is that depending on how the BPF prog updates the csums
>> things could start to break. :/
> 
> I'm reasonably sure removing the skb->csum update from the other
> helpers will also break ipv4 nat :)
> 
> So, AFAIU from what you're saying above the patch seems fine as-is and
> just needs a more verbose commit message explaining why replace16()
> doesn't update skb->csum while all the other ones do.
> 
> Is that correct?

Probably better a comment in the code to avoid confusion on why it's not done in
inet_proto_csum_replace16() but all the other cases; mainly to avoid some folks
in future sending random cleanup patches w/ removal attempts.

Thanks,
Daniel