USB: serial: option: fix slab OOB read in interrupt URB callback

commit 885d802 upstream.

The interrupt URB buffer is allocated in setup_port_interrupt_in() based
on the endpoint's wMaxPacketSize:

    buffer_size = usb_endpoint_maxp(epd);
    port->interrupt_in_buffer = kmalloc(buffer_size, GFP_KERNEL);

When a USB device declares wMaxPacketSize = 8 on its interrupt IN
endpoint, the buffer is allocated from kmalloc-8 cache (exactly
8 bytes).

If the device sends a short packet (actual_length < wMaxPacketSize),
the URB completes with status == 0 and the callback proceeds to read:

    data[sizeof(struct usb_ctrlrequest)]

which evaluates to data[8], accessing 1 byte beyond the allocated 8-byte
buffer. This results in a slab out-of-bounds read.

Fix this by adding the missing bounds check: first verify that the
actual length is large enough to contain the struct usb_ctrlrequest
header before accessing req_pkt->bRequestType and req_pkt->bRequest,
and then verify that there is an additional byte for the modem signal
state before reading data[sizeof(struct usb_ctrlrequest)] inside the
conditional.  Use sizeof(*req_pkt) instead of sizeof(struct
usb_ctrlrequest) for consistency.

Assisted-by: Claude:deepseek-v4-pro
Signed-off-by: Jiale Yao <yaojiale02@163.com>
Fixes: 58cfe91 ("[PATCH] USB: add Option Card driver")
Cc: stable@vger.kernel.org	# v2.6.12
[ johan: use dev_err(); split signals declaration and initialisation ]
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Jiale Yao

drivers/usb/serial/option.c

@@ -2691,12 +2691,26 @@ static void option_instat_callback(struct urb *urb)
 			dev_dbg(dev, "%s: NULL req_pkt\n", __func__);
 			return;
 		}
+
+		if (urb->actual_length < sizeof(*req_pkt)) {
+			dev_err(dev, "%s: short packet: %u bytes\n", __func__,
+				urb->actual_length);
+			return;
+		}
+
 		if ((req_pkt->bRequestType == 0xA1) &&
 				(req_pkt->bRequest == 0x20)) {
+			unsigned char signals;
 			int old_dcd_state;
-			unsigned char signals = *((unsigned char *)
-					urb->transfer_buffer +
-					sizeof(struct usb_ctrlrequest));
+
+			if (urb->actual_length < sizeof(*req_pkt) + 1) {
+				dev_err(dev, "%s: short interrupt transfer: %u bytes\n",
+					__func__, urb->actual_length);
+				return;
+			}
+
+			signals = *((unsigned char *)urb->transfer_buffer +
+					sizeof(*req_pkt));
 
 			dev_dbg(dev, "%s: signal x%x\n", __func__, signals);