USB: yurex: fix out-of-bounds uaccess in read handler

In general, accessing userspace memory beyond the length of the supplied
buffer in VFS read/write handlers can lead to both kernel memory corruption
(via kernel_read()/kernel_write(), which can e.g. be triggered via
sys_splice()) and privilege escalation inside userspace.

Fix it by using simple_read_from_buffer() instead of custom logic.

Fixes: 6bc235a2 ("USB: add driver for Meywa-Denki & Kayac YUREX")
Signed-off-by: Jann Horn <jannh@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/yurex.c

@@ -396,8 +396,7 @@ static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count,
 			  loff_t *ppos)
 {
 	struct usb_yurex *dev;
-	int retval = 0;
-	int bytes_read = 0;
+	int len = 0;
 	char in_buffer[20];
 	unsigned long flags;
 
@@ -405,26 +404,16 @@ static ssize_t yurex_read(struct file *file, char __user *buffer, size_t count,
 
 	mutex_lock(&dev->io_mutex);
 	if (!dev->interface) {		/* already disconnected */
-		retval = -ENODEV;
-		goto exit;
+		mutex_unlock(&dev->io_mutex);
+		return -ENODEV;
 	}
 
 	spin_lock_irqsave(&dev->lock, flags);
-	bytes_read = snprintf(in_buffer, 20, "%lld\n", dev->bbu);
+	len = snprintf(in_buffer, 20, "%lld\n", dev->bbu);
 	spin_unlock_irqrestore(&dev->lock, flags);
-
-	if (*ppos < bytes_read) {
-		if (copy_to_user(buffer, in_buffer + *ppos, bytes_read - *ppos))
-			retval = -EFAULT;
-		else {
-			retval = bytes_read - *ppos;
-			*ppos += bytes_read;
-		}
-	}
-
-exit:
 	mutex_unlock(&dev->io_mutex);
-	return retval;
+
+	return simple_read_from_buffer(buffer, count, ppos, in_buffer, len);
 }
 
 static ssize_t yurex_write(struct file *file, const char __user *user_buffer,