Subject: Re: [PATCH bpf-next V2 5/6] bpf: Add MTU check for TC-BPF packets after egress hook

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 8 Oct 2020 10:30:34 +0200

On Wed, 7 Oct 2020 23:37:00 +0200
Daniel Borkmann <[email-masked]> wrote:

> On 10/7/20 6:23 PM, Jesper Dangaard Brouer wrote:
> [...]
> >   net/core/dev.c |   24 ++++++++++++++++++++++--
> >   1 file changed, 22 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index b433098896b2..19406013f93e 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -3870,6 +3870,7 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
> >   	switch (tcf_classify(skb, miniq->filter_list, &cl_res, false)) {
> >   	case TC_ACT_OK:
> >   	case TC_ACT_RECLASSIFY:
> > +		*ret = NET_XMIT_SUCCESS;
> >   		skb->tc_index = TC_H_MIN(cl_res.classid);
> >   		break;
> >   	case TC_ACT_SHOT:
> > @@ -4064,9 +4065,12 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> >   {
> >   	struct net_device *dev = skb->dev;
> >   	struct netdev_queue *txq;
> > +#ifdef CONFIG_NET_CLS_ACT
> > +	bool mtu_check = false;
> > +#endif
> > +	bool again = false;
> >   	struct Qdisc *q;
> >   	int rc = -ENOMEM;
> > -	bool again = false;
> >   
> >   	skb_reset_mac_header(skb);
> >   
> > @@ -4082,14 +4086,28 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> >   
> >   	qdisc_pkt_len_init(skb);
> >   #ifdef CONFIG_NET_CLS_ACT
> > +	mtu_check = skb_is_redirected(skb);
> >   	skb->tc_at_ingress = 0;
> >   # ifdef CONFIG_NET_EGRESS
> >   	if (static_branch_unlikely(&egress_needed_key)) {
> > +		unsigned int len_orig = skb->len;
> > +
> >   		skb = sch_handle_egress(skb, &rc, dev);
> >   		if (!skb)
> >   			goto out;
> > +		/* BPF-prog ran and could have changed packet size beyond MTU */
> > +		if (rc == NET_XMIT_SUCCESS && skb->len > len_orig)
> > +			mtu_check = true;
> >   	}
> >   # endif
> > +	/* MTU-check only happens on "last" net_device in a redirect sequence
> > +	 * (e.g. above sch_handle_egress can steal SKB and skb_do_redirect it
> > +	 * either ingress or egress to another device).
> > +	 */  
> 
> Hmm, quite some overhead in fast path. 

Not really, normal fast-path already call is_skb_forwardable(). And it
already happens in existing code, ingress redirect code, which I remove
calling in patch 6.

(I have considered inlining is_skb_forwardable as a optimization for
general netstack dev_forward_skb)

> Also, won't this be checked multiple times on stacked devices? :(

I don't think it will be checked multiple times, because we have a
skb_reset_redirect() in ingress path (just after sch_handle_ingress()).

> Moreover, this missed the fact that 'real' qdiscs can have
> filters attached too, this would come after this check. Can't this instead be in
> driver layer for those that really need it? I would probably only drop the check
> as done in 1/6 and allow the BPF prog to do the validation if needed.

See other reply, this is likely what we will end-up with.

> > +	if (mtu_check && !is_skb_forwardable(dev, skb)) {
> > +		rc = -EMSGSIZE;
> > +		goto drop;
> > +	}
> >   #endif
> >   	/* If device/qdisc don't need skb->dst, release it right now while
> >   	 * its hot in this cpu cache.
> > @@ -4157,7 +4175,9 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> >   
> >   	rc = -ENETDOWN;
> >   	rcu_read_unlock_bh();
> > -
> > +#ifdef CONFIG_NET_CLS_ACT
> > +drop:
> > +#endif
> >   	atomic_long_inc(&dev->tx_dropped);
> >   	kfree_skb_list(skb);
> >   	return rc;
> >   
> 

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer