netdev - Re: [PATCH nf] netfilter: ipv6: Orphan skbs in nf_ct_frag6

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Date:	Mon, 18 Apr 2016 14:49:48 -0700
From:	Joe Stringer <joe@....org>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	Florian Westphal <fw@...len.de>,
	David Laight <David.Laight@...lab.com>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"diproiettod@...are.com" <diproiettod@...are.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH nf] netfilter: ipv6: Orphan skbs in nf_ct_frag6_gather()

On 18 April 2016 at 11:35, Pablo Neira Ayuso <pablo@...filter.org> wrote:
> On Thu, Apr 14, 2016 at 05:35:39PM -0700, Joe Stringer wrote:
>> On 14 April 2016 at 03:35, Pablo Neira Ayuso <pablo@...filter.org> wrote:
>> > On Thu, Apr 14, 2016 at 10:40:15AM +0200, Florian Westphal wrote:
>> >> David Laight <David.Laight@...LAB.COM> wrote:
>> >> > From: Joe Stringer
>> >> > > Sent: 13 April 2016 19:10
>> >> > > This is the IPv6 equivalent of commit 8282f27449bf ("inet: frag: Always
>> >> > > orphan skbs inside ip_defrag()").
>> >> > >
>> >> > > Prior to commit 029f7f3b8701 ("netfilter: ipv6: nf_defrag: avoid/free
>> >> > > clone operations"), ipv6 fragments sent to nf_ct_frag6_gather() would be
>> >> > > cloned (implicitly orphaning) prior to queueing for reassembly. As such,
>> >> > > when the IPv6 message is eventually reassembled, the skb->sk for all
>> >> > > fragments would be NULL. After that commit was introduced, rather than
>> >> > > cloning, the original skbs were queued directly without orphaning. The
>> >> > > end result is that all frags except for the first and last may have a
>> >> > > socket attached.
>> >> >
>> >> > I'd have thought that the queued fragments would still want to be
>> >> > resource-counted against the socket (I think that is what skb->sk is for).
>> >>
>> >> No, ipv4/ipv6 reasm has its own accouting.
>> >>
>> >> > Although I can't imagine why IPv6 reassembly is happening on skb
>> >> > associated with a socket.
>> >>
>> >> Right, thats a much more interesting question -- both ipv4 and
>> >> ipv6 orphan skbs before NF_HOOK prerouting trip.
>> >>
>> >> (That being said, I don't mind the patch, I'm just be curious how this
>> >>  can happen).
>> >
>> > If this change is specific to get this working in ovs and its
>> > conntrack support, then I don't think this belong to core
>> > infrastructure. This should be fixed in ovs instead.
>>
>> I admit I've only been able to reproduce it with OVS. My main reason
>> for proposing the fix this way was just because this is what the IPv4
>> code does, so I figured IPv6 should be consistent with that.
>
> You mean that this is what you did in 029f7f3b8701 to fix this, right?
>
> But we shouldn't add code to the core that is OVS specific for no
> reason. We don't need this orphan from ipv4 and ipv6 as Florian
> indicated.

That makes complete sense to me. I was wondering whether the original
IPv4 fix was the correct one from the nf core perspective - but it
seems like perhaps it is, given that ip_defrag() has a lot more users
from a lot more different paths which are likely relying on the skb to
be orphaned. In comparison, in the IPv6 path, nf_ct_frag6_gather() is
only called from OVS and the netfilter hooks; the hooks already do the
orphaning, so it's more consistent for OVS to do it as well.

> Is there any chance you can fix this from OVS and its conntrack glue
> code? Thanks.

Sure, I'll resend the patch making the fix in OVS.