USB: ldusb: fix read info leaks

Fix broken read implementation, which could be used to trigger slab info
leaks.

The driver failed to check if the custom ring buffer was still empty
when waking up after having waited for more data. This would happen on
every interrupt-in completion, even if no data had been added to the
ring buffer (e.g. on disconnect events).

Due to missing sanity checks and uninitialised (kmalloced) ring-buffer
entries, this meant that huge slab info leaks could easily be triggered.

Note that the empty-buffer check after wakeup is enough to fix the info
leak on disconnect, but let's clear the buffer on allocation and add a
sanity check to read() to prevent further leaks.

Fixes: 2824bd25 ("[PATCH] USB: add ldusb driver")
Cc: stable <stable@vger.kernel.org>     # 2.6.13
Reported-by: syzbot+6fe95b826644f7f12b0b@syzkaller.appspotmail.com
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191018151955.25135-2-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/ldusb.c

@@ -464,7 +464,7 @@ static ssize_t ld_usb_read(struct file *file, char __user *buffer, size_t count,
 
 	/* wait for data */
 	spin_lock_irq(&dev->rbsl);
-	if (dev->ring_head == dev->ring_tail) {
+	while (dev->ring_head == dev->ring_tail) {
 		dev->interrupt_in_done = 0;
 		spin_unlock_irq(&dev->rbsl);
 		if (file->f_flags & O_NONBLOCK) {
@@ -474,12 +474,17 @@ static ssize_t ld_usb_read(struct file *file, char __user *buffer, size_t count,
 		retval = wait_event_interruptible(dev->read_wait, dev->interrupt_in_done);
 		if (retval < 0)
 			goto unlock_exit;
-	} else {
-		spin_unlock_irq(&dev->rbsl);
+
+		spin_lock_irq(&dev->rbsl);
 	}
+	spin_unlock_irq(&dev->rbsl);
 
 	/* actual_buffer contains actual_length + interrupt_in_buffer */
 	actual_buffer = (size_t *)(dev->ring_buffer + dev->ring_tail * (sizeof(size_t)+dev->interrupt_in_endpoint_size));
+	if (*actual_buffer > dev->interrupt_in_endpoint_size) {
+		retval = -EIO;
+		goto unlock_exit;
+	}
 	bytes_to_read = min(count, *actual_buffer);
 	if (bytes_to_read < *actual_buffer)
 		dev_warn(&dev->intf->dev, "Read buffer overflow, %zd bytes dropped\n",
@@ -690,8 +695,7 @@ static int ld_usb_probe(struct usb_interface *intf, const struct usb_device_id *
 		dev_warn(&intf->dev, "Interrupt out endpoint not found (using control endpoint instead)\n");
 
 	dev->interrupt_in_endpoint_size = usb_endpoint_maxp(dev->interrupt_in_endpoint);
-	dev->ring_buffer =
-		kmalloc_array(ring_buffer_size,
+	dev->ring_buffer = kcalloc(ring_buffer_size,
 			sizeof(size_t) + dev->interrupt_in_endpoint_size,
 			GFP_KERNEL);
 	if (!dev->ring_buffer)