platform/chrome: fix memory corruption in ioctl

If "s_mem.bytes" is larger than the buffer size it leads to memory
corruption.

Fixes: eda2e30c ("mfd / platform: cros_ec: Miscellaneous character device to talk with the EC")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/Yv8dpCFZJdbUT5ye@kili

drivers/platform/chrome/cros_ec_chardev.c

@@ -327,6 +327,9 @@ static long cros_ec_chardev_ioctl_readmem(struct cros_ec_dev *ec,
 	if (copy_from_user(&s_mem, arg, sizeof(s_mem)))
 		return -EFAULT;
 
+	if (s_mem.bytes > sizeof(s_mem.buffer))
+		return -EINVAL;
+
 	num = ec_dev->cmd_readmem(ec_dev, s_mem.offset, s_mem.bytes,
 				  s_mem.buffer);
 	if (num <= 0)