[PATCH 2/2] net: usb: cdc_ncm: prevent silent u16 truncation in min_tx_pkt_store()

[Omer Zalman <omerzalman42@gmail.com>]

min_tx_pkt is a u16 field but the sysfs store handler accepted any
unsigned long and assigned it directly. Values that do not fit in 16
bits silently truncate: writing 65537 wraps to 1, turning a
"never pad" intent into "always pad".

ctx->tx_max can itself reach 65537 because cdc_ncm_update_rxtx_max()
increments the negotiated NTB size by one pad byte when it is an exact
multiple of the USB maxpacket size.

Clamp the stored value to min(tx_max, U16_MAX). Any value above tx_max
is already semantically equivalent to tx_max since frame lengths never
exceed it; the U16_MAX cap ensures the result is always representable
in the u16 field without wrapping.

Fixes: 39eb7e0e8c88 ("net: cdc_ncm: allow tuning min_tx_pkt")
Signed-off-by: Omer Zalman <omerzalman42@gmail.com>
---
 drivers/net/usb/cdc_ncm.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 6fcb4d711a64..edd69f4e1596 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -242,11 +242,17 @@ static ssize_t min_tx_pkt_store(struct device *d,
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 	unsigned long val;
 
-	/* no need to restrict values - anything from 0 to infinity is OK */
 	if (kstrtoul(buf, 0, &val))
 		return -EINVAL;
 
-	ctx->min_tx_pkt = val;
+	/* Clamp to tx_max: the frame length can never exceed tx_max,
+	 * so any threshold above it has the same effect (padding is
+	 * never applied).  Also cap at U16_MAX since min_tx_pkt is
+	 * a u16 - without this, values like 65537 silently wrap to 1
+	 * and invert the intended behavior.
+	 */
+	ctx->min_tx_pkt = min_t(unsigned long, val,
+				min_t(u32, ctx->tx_max, U16_MAX));
 	return len;
 }
 
-- 
2.39.5 (Apple Git-154)