comedi: me4000: Fix potential overrun of firmware buffer

`me4000_xilinx_download()` loads the firmware that was requested by
`request_firmware()`.  It is possible for it to overrun the source
buffer because it blindly trusts the file format.  It reads a data
stream length from the first 4 bytes into variable `file_length` and
reads the data stream contents of length `file_length` from offset 16
onwards.

Add a test to ensure that the supplied firmware is long enough to
contain the header and the data stream.  On failure, log an error and
return `-EINVAL`.

Note: The firmware loading was totally broken before commit ac584af
("staging: comedi: me4000: fix firmware downloading"), but that is the
most sensible target for this fix.

Fixes: ac584af ("staging: comedi: me4000: fix firmware downloading")
Cc: stable <stable@kernel.org>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20260205133949.71722-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ian-abbott

drivers/comedi/drivers/me4000.c

@@ -315,6 +315,18 @@ static int me4000_xilinx_download(struct comedi_device *dev,
 	unsigned int val;
 	unsigned int i;
 
+	/* Get data stream length from header. */
+	if (size >= 4) {
+		file_length = (((unsigned int)data[0] & 0xff) << 24) +
+			      (((unsigned int)data[1] & 0xff) << 16) +
+			      (((unsigned int)data[2] & 0xff) << 8) +
+			      ((unsigned int)data[3] & 0xff);
+	}
+	if (size < 16 || file_length > size - 16) {
+		dev_err(dev->class_dev, "Firmware length inconsistency\n");
+		return -EINVAL;
+	}
+
 	if (!xilinx_iobase)
 		return -ENODEV;
 
@@ -346,10 +358,6 @@ static int me4000_xilinx_download(struct comedi_device *dev,
 	outl(val, devpriv->plx_regbase + PLX9052_CNTRL);
 
 	/* Download Xilinx firmware */
-	file_length = (((unsigned int)data[0] & 0xff) << 24) +
-		      (((unsigned int)data[1] & 0xff) << 16) +
-		      (((unsigned int)data[2] & 0xff) << 8) +
-		      ((unsigned int)data[3] & 0xff);
 	usleep_range(10, 1000);
 
 	for (i = 0; i < file_length; i++) {