[PATCH v2] USB: serial: cypress_m8: validate interrupt packet headers

[Zhang Cen <rollkingzzc@gmail.com>]

cypress_read_int_callback() parses the interrupt-in buffer according to
the selected Cypress packet format. Format 1 has a two-byte status/count
header and format 2 has a one-byte combined status/count header. The
usb-serial core sizes the interrupt-in buffer from the endpoint
descriptor's wMaxPacketSize, and successful interrupt transfers can
complete short when URB_SHORT_NOT_OK is not set.

Check that the completed packet contains the selected header before
reading it. Malformed short reports are ignored and the interrupt URB is
resubmitted through the existing retry path, preventing out-of-bounds
header-byte reads.

KASAN report as below:
KASAN slab-out-of-bounds in cypress_read_int_callback+0x240/0x7f0
Read of size 1
Call trace:
  cypress_read_int_callback() (drivers/usb/serial/cypress_m8.c:1009)
  __usb_hcd_giveback_urb()
  dummy_timer()

Fixes: 3416eaa1f8f8 ("USB: cypress_m8: Packet format is separate from characteristic size")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
---
v2:
Check only urb->actual_length before reading the packet-format header.
Reuse the existing i header-length variable instead of adding a new one.
Shorten the KASAN trace in the commit message.

 drivers/usb/serial/cypress_m8.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/serial/cypress_m8.c b/drivers/usb/serial/cypress_m8.c
index afff1a0f4298b..49c0f3e379bd0 100644
--- a/drivers/usb/serial/cypress_m8.c
+++ b/drivers/usb/serial/cypress_m8.c
@@ -1060,18 +1060,27 @@ static void cypress_read_int_callback(struct urb *urb)
 	default:
 	case packet_format_1:
 		/* This is for the CY7C64013... */
-		priv->current_status = data[0] & 0xF8;
-		bytes = data[1] + 2;
 		i = 2;
+		if (result < i)
+			break;
+		priv->current_status = data[0] & 0xF8;
+		bytes = data[1] + i;
 		break;
 	case packet_format_2:
 		/* This is for the CY7C63743... */
+		i = 1;
+		if (result < i)
+			break;
 		priv->current_status = data[0] & 0xF8;
 		bytes = (data[0] & 0x07) + 1;
-		i = 1;
 		break;
 	}
 	spin_unlock_irqrestore(&priv->lock, flags);
+	if (result < i) {
+		dev_dbg(dev, "%s - short packet received: %d bytes\n",
+			__func__, result);
+		goto continue_read;
+	}
 	if (result < bytes) {
 		dev_dbg(dev,
 			"%s - wrong packet size - received %d bytes but packet said %d bytes\n",
-- 
2.43.0