Re: [PATCH net] net: parse the eighth nested VLAN tag

[netdev-bot+sashiko@kernel.org]

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- [High] The patch fixes the counter only when mac_len is 0 or covers the
  first VLAN header.

--

net: parse the eighth nested VLAN tag

This commit changes the parse_depth check in __vlan_get_protocol_offset()
from a pre-decrement to a post-decrement. The goal is for a packet with
eight nested VLAN tags to return the encapsulated EtherType instead of 0,
while a ninth tag is still rejected.

> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 48f6764117243..b10ce1d46674b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -7703,7 +7703,7 @@ struct vlan_type_depth __vlan_get_protocol_offset(const struct sk_buff *skb,
>  
>  		vh = skb_header_pointer(skb, mac_offset + vlan_depth,
>  					sizeof(vhdr), &vhdr);
> -		if (unlikely(!vh || !--parse_depth))
> +		if (unlikely(!vh || !parse_depth--))
>  			return (struct vlan_type_depth) { 0 };

[Severity: High]
Does this allow eight nested tags for every value of skb->mac_len?

parse_depth goes down once per loop pass, not once per VLAN header that is
actually parsed. What the first pass reads depends on where the loop
starts:

net/core/skbuff.c:__vlan_get_protocol_offset() {
    ...
	if (vlan_depth) {
		if (WARN_ON_ONCE(vlan_depth < VLAN_HLEN))
			return (struct vlan_type_depth) { 0 };
		vlan_depth -= VLAN_HLEN;
	} else {
		vlan_depth = ETH_HLEN;
	}
    ...
}

With mac_len == 0 or mac_len == ETH_HLEN + VLAN_HLEN, the first pass
parses the first tag, and eight tags now work.

With mac_len == ETH_HLEN and the first tag still in-band, vlan_depth
starts at 10. The first pass reads the last two bytes of the source MAC as
the TCI and the outer TPID as h_vlan_encapsulated_proto. That only re-reads
the type that was passed in.

Eight in-band tags then take nine passes. The ninth pass sees
parse_depth == 0 and returns { 0 }, which is the result the commit message
says is fixed.

The egress path on a device without VLAN TX offload seems to produce this
state:

validate_xmit_vlan()
  __vlan_hwaccel_push_inside()
    vlan_insert_tag_set_proto()
      __vlan_insert_inner_tag()

__vlan_insert_inner_tag() pushes the hwaccel tag inline and moves
skb->mac_header, but it does not update skb->mac_len:

	skb_push(skb, VLAN_HLEN);
	...
	if (skb_mac_header_was_set(skb))
		skb->mac_header -= VLAN_HLEN;

A forwarded skb that arrived with mac_len == ETH_HLEN keeps that value. A
later vlan_get_protocol() call, such as one in a driver's xmit checksum
path, would then still get 0 for a frame with eight tags.

On the plain RX path (skb_vlan_untag() followed by skb_reset_mac_len()),
the extra pass happens to line up with the stripped hwaccel tag, so eight
tags still work there.

Could parse_depth be decremented only for passes that parse a real VLAN
header? Alternatively, the start offset could be chosen so that every pass
parses one.

If the current behaviour is kept, should the commit message and the
VLAN_MAX_DEPTH comment mention that the result depends on mac_len? The two
texts in question are "allowing eight nested tags while continuing to
reject a ninth" and "Max. number of nested VLAN tags parsed".

>  
>  		type = vh->h_vlan_encapsulated_proto;

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261031135507.3970551-2-thnkslprpt%40gmail.com