netdev - Re: [PATCH net] udp: fix a skb extensions leak

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Date:   Mon, 30 Mar 2020 16:39:28 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Florian Westphal <fw@...len.de>
CC:     Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>, <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] udp: fix a skb extensions leak

On Mon, Mar 30, 2020 at 04:11:47PM +0200, Florian Westphal wrote:
> Steffen Klassert <steffen.klassert@...unet.com> wrote:
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index 621b4479fee1..7e29590482ce 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -3668,6 +3668,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> > > 
> > >                 skb_push(nskb, -skb_network_offset(nskb) + offset);
> > > 
> > > +               skb_release_head_state(nskb);
> > >                  __copy_skb_header(nskb, skb);
> > > 
> > >                 skb_headers_offset_update(nskb, skb_headroom(nskb) - skb_headroom(skb));
> > > 
> > > AFAICS we not only leak reference of extensions, but also skb->dst and skb->_nfct.
> > 
> > Would be nice if we would not need to drop the resources
> > just to add them back again in the next line. But it is ok
> > as a quick fix for the bug.
> 
> Yes, but are these the same resources?  AFAIU thats not the case, i.e.
> the skb on fraglist can have different skb->{dst,extension,_nfct} data
> than the skb head one, and we can't tell if that data is still valid
> (rerouting for example).

Some are the same, others not. Originally, I used
a subset of __copy_skb_header here. But decided then
to use __copy_skb_header to make sure I don't forget
anything. So this fix is ok for now.