[PATCH net-next v2 3/4] net: dsa: qca8k: check the length of MIB autocast frames

[Stanislaw Pal <kuncy7@gmail.com>]

qca_tag_rcv() makes only the 2 byte tag linear before it hands a MIB
frame to qca8k_mib_autocast_handler(). The handler then reads the
counters that follow the Ethernet header, up to 164 bytes on QCA833x,
straight from skb->data, without checking the frame length. A short or
non-linear frame makes it read past the packet data.

Pull the counters into the linear area before reading them. A frame
that is too short is left uncounted, so the request times out and
the caller reads the counters from the registers instead.

An LLM assistant (Claude, claude-fable-5-1) helped write the change
and this description; I reviewed both and built them with W=1 on
arm64.

Fixes: 5c957c7ca78c ("net: dsa: qca8k: add support for mib autocast in Ethernet packet")
Signed-off-by: Stanislaw Pal <kuncy7@gmail.com>
---
 drivers/net/dsa/qca/qca8k-8xxx.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/dsa/qca/qca8k-8xxx.c b/drivers/net/dsa/qca/qca8k-8xxx.c
index 86b20bbc43..bc77dd45b6 100644
--- a/drivers/net/dsa/qca/qca8k-8xxx.c
+++ b/drivers/net/dsa/qca/qca8k-8xxx.c
@@ -1669,6 +1669,7 @@ static void qca8k_mib_autocast_handler(struct dsa_switch *ds, struct sk_buff *sk
 	struct qca8k_priv *priv = ds->priv;
 	const struct qca8k_mib_desc *mib;
 	struct mib_ethhdr *mib_ethhdr;
+	unsigned int len = 0;
 	__le32 *data2;
 	u8 port;
 	int i;
@@ -1691,6 +1692,16 @@ static void qca8k_mib_autocast_handler(struct dsa_switch *ds, struct sk_buff *sk
 	if (port != mib_eth_data->req_port)
 		goto exit;
 
+	/* Past the first 3, the counters follow the header. Leave a short
+	 * frame uncounted, so that the request times out and the caller
+	 * reads the registers instead.
+	 */
+	for (i = 3; i < priv->info->mib_count; i++)
+		len += ar8327_mib[i].size * sizeof(u32);
+	if (!pskb_may_pull(skb, len))
+		goto unlock;
+
+	mib_ethhdr = (struct mib_ethhdr *)skb_mac_header(skb);
 	data2 = (__le32 *)skb->data;
 
 	for (i = 0; i < priv->info->mib_count; i++) {
-- 
2.43.0