netdev - Re: [PATCH net-next 4/4] act_mirred: use ACT

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Date:   Fri, 20 Jul 2018 12:16:36 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eyal Birger <eyal.birger@...il.com>
Subject: Re: [PATCH net-next 4/4] act_mirred: use ACT_REDIRECT when possible

On Thu, 2018-07-19 at 10:56 -0700, Cong Wang wrote:
> On Wed, Jul 18, 2018 at 3:05 AM Paolo Abeni <pabeni@...hat.com> wrote:
> > 
> > Hi,
> > 
> > On Tue, 2018-07-17 at 10:24 -0700, Cong Wang wrote:
> > > If you goal is to get rid of skb_clone(), why not just do the following?
> > > 
> > >         if (tcf_mirred_is_act_redirect(m_eaction)) {
> > >                 skb2 = skb;
> > >         } else {
> > >                 skb2 = skb_clone(skb, GFP_ATOMIC);
> > >                 if (!skb2)
> > >                         goto out;
> > >         }
> > > 
> > > For redirect, we return TC_ACT_SHOT, so upper layer should not
> > > touch the skb after that.
> > > 
> > > What am I missing here?
> > 
> > With ACT_SHOT caller/upper layer will free the skb, too. We will have
> > an use after free (from either the upper layer and the xmit device).
> > Similar issues with STOLEN, TRAP, etc.
> > 
> > In the past, Changli Gao attempted to avoid the clone incrementing the
> > skb usage count:
> > 
> > commit 210d6de78c5d7c785fc532556cea340e517955e1
> > Author: Changli Gao <xiaosuo@...il.com>
> > Date:   Thu Jun 24 16:25:12 2010 +0000
> > 
> >     act_mirred: don't clone skb when skb isn't shared
> > 
> > but some/many device drivers expect an skb usage count of 1, and that
> > caused ooops and was revered.
> 
> Interesting, I wasn't aware of the above commit and its revert.
> 
> First, I didn't use skb_get() above.
> 
> Second, I think the caller of dev_queue_xmit() should not
> touch the skb after it, the skb is either freed by dev_queue_xmit()
> or successfully transmitted, in either case, the ownership belongs
> to dev_queue_xmit(). So, I think we should skip the qdisc_drop()
> for this case.
> 
> Not sure about netif_receive_skb() case, given veth calls in its
> xmit too, I speculate the rule is probably same.
> 
> Not sure about other ACT_SHOT case than act_mirred...

I think any tc filter can be configured from user space to return
ACT_SHOT, so changing the ACT_SHOT handling would be quite invasive and
error prone, as all the tc filters (to free the skb on ACT_SHOT) and tc
schedulers (to avoid touching the skb) must be modified.

If there are no strong objection vs a new action value, I would opt for
such option.

Thanks,

Paolo