Subject: Re: [PATCH net-next] openvswitch: prepare for stolen verdict coming from conntrack and nat engine

From: Aaron Conole <[email-masked]>
Date: Wed, 03 Jul 2024 10:59:18 -0400

Hi Florian,

Florian Westphal <[email-masked]> writes:

> At this time, conntrack either returns NF_ACCEPT or NF_DROP.
> To improve debuging it would be nice to be able to replace NF_DROP
> verdict with NF_DROP_REASON() helper,
>
> This helper releases the skb instantly (so drop_monitor can pinpoint
> precise location) and returns NF_STOLEN.
>
> Prepare call sites to deal with this before introducing such changes
> in conntrack and nat core.
>
> Signed-off-by: Florian Westphal <[email-masked]>
> ---

AFAIU, these changes are only impacting the existing NF_DROP cases, and
won't impact how ovs + netfilter communicate about invalid packets.  One
important thing to note is that we rely on:

 * Note that if the packet is deemed invalid by conntrack, skb->_nfct will be
 * set to NULL and 0 will be returned.

Based on this, my understanding is if packet isn't part of a valid
connection, skb->_nfct is NULL and NF_ACCEPT is returned.

If this changes, those flow pipelines matching on ct_state(+inv+trk)
will no longer function as expected since we will bail early.  I think
this comment will also apply to the act_ct change as well.

>  net/openvswitch/conntrack.c | 47 +++++++++++++++++++++++++++++--------
>  1 file changed, 37 insertions(+), 10 deletions(-)
>
> diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
> index 3b980bf2770b..8eb1d644b741 100644
> --- a/net/openvswitch/conntrack.c
> +++ b/net/openvswitch/conntrack.c
> @@ -679,6 +679,8 @@ static int ovs_ct_nat(struct net *net, struct sw_flow_key *key,
>  		action |= BIT(NF_NAT_MANIP_DST);
>  
>  	err = nf_ct_nat(skb, ct, ctinfo, &action, &info->range, info->commit);
> +	if (err != NF_ACCEPT)
> +		return err;
>  
>  	if (action & BIT(NF_NAT_MANIP_SRC))
>  		ovs_nat_update_key(key, skb, NF_NAT_MANIP_SRC);
> @@ -697,6 +699,22 @@ static int ovs_ct_nat(struct net *net, struct sw_flow_key *key,
>  }
>  #endif
>  
> +static int verdict_to_errno(unsigned int verdict)
> +{
> +	switch (verdict & NF_VERDICT_MASK) {
> +	case NF_ACCEPT:
> +		return 0;
> +	case NF_DROP:
> +		return -EINVAL;
> +	case NF_STOLEN:
> +		return -EINPROGRESS;
> +	default:
> +		break;
> +	}
> +
> +	return -EINVAL;
> +}
> +
>  /* Pass 'skb' through conntrack in 'net', using zone configured in 'info', if
>   * not done already.  Update key with new CT state after passing the packet
>   * through conntrack.
> @@ -735,7 +753,7 @@ static int __ovs_ct_lookup(struct net *net, struct sw_flow_key *key,
>  
>  		err = nf_conntrack_in(skb, &state);
>  		if (err != NF_ACCEPT)
> -			return -ENOENT;
> +			return verdict_to_errno(err);
>  
>  		/* Clear CT state NAT flags to mark that we have not yet done
>  		 * NAT after the nf_conntrack_in() call.  We can actually clear
> @@ -762,9 +780,12 @@ static int __ovs_ct_lookup(struct net *net, struct sw_flow_key *key,
>  		 * the key->ct_state.
>  		 */
>  		if (info->nat && !(key->ct_state & OVS_CS_F_NAT_MASK) &&
> -		    (nf_ct_is_confirmed(ct) || info->commit) &&
> -		    ovs_ct_nat(net, key, info, skb, ct, ctinfo) != NF_ACCEPT) {
> -			return -EINVAL;
> +		    (nf_ct_is_confirmed(ct) || info->commit)) {
> +			int err = ovs_ct_nat(net, key, info, skb, ct, ctinfo);
> +
> +			err = verdict_to_errno(err);
> +			if (err)
> +				return err;
>  		}
>  
>  		/* Userspace may decide to perform a ct lookup without a helper
> @@ -795,9 +816,12 @@ static int __ovs_ct_lookup(struct net *net, struct sw_flow_key *key,
>  		 * - When committing an unconfirmed connection.
>  		 */
>  		if ((nf_ct_is_confirmed(ct) ? !cached || add_helper :
> -					      info->commit) &&
> -		    nf_ct_helper(skb, ct, ctinfo, info->family) != NF_ACCEPT) {
> -			return -EINVAL;
> +					      info->commit)) {
> +			int err = nf_ct_helper(skb, ct, ctinfo, info->family);
> +
> +			err = verdict_to_errno(err);
> +			if (err)
> +				return err;
>  		}
>  
>  		if (nf_ct_protonum(ct) == IPPROTO_TCP &&
> @@ -1001,10 +1025,9 @@ static int ovs_ct_commit(struct net *net, struct sw_flow_key *key,
>  	/* This will take care of sending queued events even if the connection
>  	 * is already confirmed.
>  	 */
> -	if (nf_conntrack_confirm(skb) != NF_ACCEPT)
> -		return -EINVAL;
> +	err = nf_conntrack_confirm(skb);
>  
> -	return 0;
> +	return verdict_to_errno(err);
>  }
>  
>  /* Returns 0 on success, -EINPROGRESS if 'skb' is stolen, or other nonzero
> @@ -1039,6 +1062,10 @@ int ovs_ct_execute(struct net *net, struct sk_buff *skb,
>  	else
>  		err = ovs_ct_lookup(net, key, info, skb);
>  
> +	/* conntrack core returned NF_STOLEN */
> +	if (err == -EINPROGRESS)
> +		return err;
> +
>  	skb_push_rcsum(skb, nh_ofs);
>  	if (err)
>  		ovs_kfree_skb_reason(skb, OVS_DROP_CONNTRACK);