netdev - Re: [PATCH net v3] sched: consistently handle layer3 header accesses in the presence of VLANs

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Message-ID: <b62fcd67-1b0a-ab7f-850d-22e62faf3a23@gmail.com>
Date:   Mon, 6 Jul 2020 13:24:42 +0900
From:   Toshiaki Makita <toshiaki.makita1@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, bpf@...r.kernel.org,
        cake@...ts.bufferbloat.net, Davide Caratti <dcaratti@...hat.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Roman Mashak <mrv@...atatu.com>,
        Lawrence Brakmo <brakmo@...com>,
        Ilya Ponetayev <i.ponetaev@...systems.com>
Subject: Re: [PATCH net v3] sched: consistently handle layer3 header accesses
 in the presence of VLANs

On 2020/07/04 20:33, Toke Høiland-Jørgensen wrote:
> Toshiaki Makita <toshiaki.makita1@...il.com> writes:
>> On 2020/07/04 5:26, Toke Høiland-Jørgensen wrote:
>> ...
>>> +/* A getter for the SKB protocol field which will handle VLAN tags consistently
>>> + * whether VLAN acceleration is enabled or not.
>>> + */
>>> +static inline __be16 skb_protocol(const struct sk_buff *skb, bool skip_vlan)
>>> +{
>>> +	unsigned int offset = skb_mac_offset(skb) + sizeof(struct ethhdr);
>>> +	__be16 proto = skb->protocol;
>>> +
>>> +	if (!skip_vlan)
>>> +		/* VLAN acceleration strips the VLAN header from the skb and
>>> +		 * moves it to skb->vlan_proto
>>> +		 */
>>> +		return skb_vlan_tag_present(skb) ? skb->vlan_proto : proto;
>>> +
>>> +	while (eth_type_vlan(proto)) {
>>> +		struct vlan_hdr vhdr, *vh;
>>> +
>>> +		vh = skb_header_pointer(skb, offset, sizeof(vhdr), &vhdr);
>>> +		if (!vh)
>>> +			break;
>>> +
>>> +		proto = vh->h_vlan_encapsulated_proto;
>>> +		offset += sizeof(vhdr);
>>> +	}
>>
>> Why don't you use __vlan_get_protocol() here? It looks quite similar.
>> Is there any problem with using that?
> 
> TBH, I completely missed that helper. It seems to have side effects,
> though (pskb_may_pull()), which is one of the things the original patch
> to sch_cake that initiated all of this was trying to avoid.

Sorry for not completely following the discussion...
Pulling data is wrong for cake or other schedulers?

> I guess I could just fix that, though, and switch __vlan_get_protocol()
> over to using skb_header_pointer(). Will send a follow-up to do that.
> 
> Any opinion on whether it's a good idea to limit the max parse depth
> while I'm at it (see Daniel's reply)?

The logic was originally introduced by skb_network_protocol() back in v3.10,
and I have never heard of security report about that. But yes, I guess it
potentially can be used for DoS attack.

Toshiaki Makita