linux-kernel - [PATCH net] net: dsa: tag_brcm: legacy: fix untagged rx on unbridged ports for bcm63xx

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Message-ID: <20251015070854.36281-1-jonas.gorski@gmail.com>
Date: Wed, 15 Oct 2025 09:08:54 +0200
From: Jonas Gorski <jonas.gorski@...il.com>
To: Andrew Lunn <andrew@...n.ch>,
	Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Álvaro Fernández Rojas <noltari@...il.com>
Cc: netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH net] net: dsa: tag_brcm: legacy: fix untagged rx on unbridged ports for bcm63xx

The internal switch on BCM63XX SoCs will unconditionally add 802.1Q VLAN
tags on egress to CPU when 802.1Q mode is enabled. We do this
unconditionally since commit ed409f3bbaa5 ("net: dsa: b53: Configure
VLANs while not filtering").

This is fine for VLAN aware bridges, but for standalone ports and vlan
unaware bridges this means all packets are tagged with the default VID,
which is 0.

While the kernel will treat that like untagged, this can break userspace
applications processing raw packets, expecting untagged traffic, like
STP daemons.

This also breaks several bridge tests, where the tcpdump output then
does not match the expected output anymore.

Since 0 isn't a valid VID, just strip out the VLAN tag if we encounter
it, unless the priority field is set, since that would be a valid tag
again.

Fixes: 964dbf186eaa ("net: dsa: tag_brcm: add support for legacy tags")
Signed-off-by: Jonas Gorski <jonas.gorski@...il.com>
---
 net/dsa/tag_brcm.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/net/dsa/tag_brcm.c b/net/dsa/tag_brcm.c
index 26bb657ceac3..32879d1b908b 100644
--- a/net/dsa/tag_brcm.c
+++ b/net/dsa/tag_brcm.c
@@ -224,12 +224,14 @@ static struct sk_buff *brcm_leg_tag_rcv(struct sk_buff *skb,
 {
 	int len = BRCM_LEG_TAG_LEN;
 	int source_port;
+	__be16 *proto;
 	u8 *brcm_tag;
 
 	if (unlikely(!pskb_may_pull(skb, BRCM_LEG_TAG_LEN + VLAN_HLEN)))
 		return NULL;
 
 	brcm_tag = dsa_etype_header_pos_rx(skb);
+	proto = (__be16 *)(brcm_tag + BRCM_LEG_TAG_LEN);
 
 	source_port = brcm_tag[5] & BRCM_LEG_PORT_ID;
 
@@ -237,8 +239,14 @@ static struct sk_buff *brcm_leg_tag_rcv(struct sk_buff *skb,
 	if (!skb->dev)
 		return NULL;
 
-	/* VLAN tag is added by BCM63xx internal switch */
-	if (netdev_uses_dsa(skb->dev))
+	/* The internal switch in BCM63XX SoCs will add a 802.1Q VLAN tag on
+	 * egress to the CPU port for all packets, regardless of the untag bit
+	 * in the VLAN table.  VID 0 is used for untagged traffic on unbridged
+	 * ports and vlan unaware bridges. If we encounter a VID 0 tagged
+	 * packet, we know it is supposed to be untagged, so strip the VLAN
+	 * tag as well in that case.
+	 */
+	if (proto[0] == htons(ETH_P_8021Q) && proto[1] == 0)
 		len += VLAN_HLEN;
 
 	/* Remove Broadcom tag and update checksum */

base-commit: 7f0fddd817ba6daebea1445ae9fab4b6d2294fa8
-- 
2.43.0