[PATCH net] net: lan966x: do not inject uninitialised bytes past skb->len

[Daniel Machon <daniel.machon@microchip.com>]

lan966x_port_ifh_xmit() writes the frame to the injection FIFO one
32-bit word at a time, and pads frames shorter than 60 bytes with zero
words.

Before this patch, the last word is loaded whole even when skb->len is
not a multiple of four, so 1 to 3 bytes past the end of the frame are
read. For a frame shorter than 60 bytes those bytes are also
transmitted, as VLD_BYTES marks the whole last word valid and the
padding starts at the next word.

Write the trailing bytes from a zeroed word. The number of words written
and VLD_BYTES are unchanged.

Found by an AI code review agent. Reproduced on a lan9668 board with
register injection, by sending a
57 byte frame and capturing it on the link partner: byte 57 of the
padded frame was not zero. With the fix all padding bytes are zero.

Fixes: d28d6d2e37d1 ("net: lan966x: add port module support")
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
---
 drivers/net/ethernet/microchip/lan966x/lan966x_main.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_main.c b/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
index e4c9c5812ccab..c276fec2f9d6f 100644
--- a/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
+++ b/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
@@ -263,9 +263,8 @@ static int lan966x_port_ifh_xmit(struct sk_buff *skb,
 		lan_wr((__force u32)ifh[i], lan966x, QS_INJ_WR(grp));
 	}
 
-	/* Write frame */
-	count = DIV_ROUND_UP(skb->len, 4);
-	last = skb->len % 4;
+	/* Write the whole words of the frame */
+	count = skb->len / 4;
 	for (i = 0; i < count; ++i) {
 		/* Wait until the fifo is ready */
 		err = lan966x_port_inj_ready(lan966x, grp);
@@ -275,6 +274,20 @@ static int lan966x_port_ifh_xmit(struct sk_buff *skb,
 		lan_wr(((u32 *)skb->data)[i], lan966x, QS_INJ_WR(grp));
 	}
 
+	/* Write the last 1 to 3 bytes, zero padded to a whole word */
+	last = skb->len % 4;
+	if (last) {
+		/* Wait until the fifo is ready */
+		err = lan966x_port_inj_ready(lan966x, grp);
+		if (err)
+			goto err;
+
+		val = 0;
+		memcpy(&val, skb->data + count * 4, last);
+		lan_wr(val, lan966x, QS_INJ_WR(grp));
+		++i;
+	}
+
 	/* Add padding */
 	while (i < (LAN966X_BUFFER_MIN_SZ / 4)) {
 		/* Wait until the fifo is ready */

---
base-commit: 6430fb7f1cc4e329e386521b7b208df443de92f2
change-id: 20261023-lan966x-inject-tail-bytes-9bba9e6cc081

Best regards,
-- 
Daniel Machon <daniel.machon@microchip.com>