USB: core: fix out-of-bounds access bug in usb_get_bos_descriptor()

commit 1c0edc3 upstream.

Andrey used the syzkaller fuzzer to find an out-of-bounds memory access
in usb_get_bos_descriptor().  The code wasn't checking that the next
usb_dev_cap_header structure could fit into the remaining buffer space.

This patch fixes the error and also reduces the bNumDeviceCaps field in
the header to match the actual number of capabilities found, in cases
where there are fewer than expected.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/core/config.c

@@ -913,10 +913,12 @@ int usb_get_bos_descriptor(struct usb_device *dev)
 	for (i = 0; i < num; i++) {
 		buffer += length;
 		cap = (struct usb_dev_cap_header *)buffer;
-		length = cap->bLength;
 
-		if (total_len < length)
+		if (total_len < sizeof(*cap) || total_len < cap->bLength) {
+			dev->bos->desc->bNumDeviceCaps = i;
 			break;
+		}
+		length = cap->bLength;
 		total_len -= length;
 
 		if (cap->bDescriptorType != USB_DT_DEVICE_CAPABILITY) {