[PATCH nf v2 2/2] netfilter: br_netfilter: clear stale VLAN tag on refragmented packets

[Andrea Parri]

Bridged packets that conntrack reassembles and br_netfilter refragments
can leave a port that is an untagged member of their VLAN with all but
the first fragment still carrying the ingress VLAN tag.

With bridge-nf-filter-vlan-tagged enabled, conntrack defrag keeps the
original fragments on the frag_list of the reassembled skb, and each of
them keeps the tag it had on ingress. br_handle_vlan() only strips the
tag of the head skb. When that skb is not cloned, ip_do_fragment() and
ip6_fragment() send the frag_list skbs as the fragments, and
br_nf_push_frag_xmit() only ever sets a tag.

This was triggered with a reproducer that sends fragmented IPv4 and
IPv6 packets in VLAN 10 to a port that is untagged in VLAN 10, through
a static FDB entry. The first fragment of each packet left untagged and
the others left tagged:

  ... ethertype 802.1Q (0x8100), length 1514: vlan 10, p 0,
      ethertype IPv6 (0x86dd), fd00:10::1 > fd00:10::4:
      frag (1448|1448)

IPv6 became affected when commit efb6de9b4ba0 ("netfilter: bridge:
forward IPv6 fragmented packets") added IPv6 refragmentation.

IPv4 has been affected since commit 7885198861fc ("bridge: Implement
vlan ingress/egress policy with PVID.") made the bridge strip the tag on
untagged egress, when IPv4 defragmentation retained fragments on
frag_list. Commit 14fe22e33462 ("Revert "ipv4: use skb coalescing in
defragmentation"") later restored unconditional frag_list reuse.

Clear the tag when no tag was saved, as nf_ct_bridge_frag_restore()
does on the nf_conntrack_bridge path.

Fixes: efb6de9b4ba0 ("netfilter: bridge: forward IPv6 fragmented packets")
Fixes: 7885198861fc ("bridge: Implement vlan ingress/egress policy with PVID.")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
---
 net/bridge/br_netfilter_hooks.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
index fe8d2910dae21..2519c3820fb00 100644
--- a/net/bridge/br_netfilter_hooks.c
+++ b/net/bridge/br_netfilter_hooks.c
@@ -795,8 +795,12 @@ static int br_nf_push_frag_xmit(struct net *net, struct sock *sk, struct sk_buff
 		return 0;
 	}
 
-	if (data->vlan_proto)
+	if (data->vlan_proto) {
 		__vlan_hwaccel_put_tag(skb, data->vlan_proto, data->vlan_tci);
+	} else if (skb_vlan_tag_present(skb)) {
+		/* Fragments reused from frag_list keep their ingress tag. */
+		__vlan_hwaccel_clear_tag(skb);
+	}
 
 	skb_copy_to_linear_data_offset(skb, -data->size, data->mac, data->size);
 	__skb_push(skb, data->encap_size);
-- 
2.53.0