[PATCH 3.16 102/136] USB: gadget: Reject endpoints with 0 maxpacket value

Ben Hutchings
2019-12-17T00:56:44Z

From: Alan Stern <stern@rowland.harvard.edu>

commit 54f83b8c8ea9b22082a496deadf90447a326954e upstream.

Endpoints with a maxpacket length of 0 are probably useless. They
can't transfer any data, and it's not at all unlikely that a UDC will
crash or hang when trying to handle a non-zero-length usb_request for
such an endpoint. Indeed, dummy-hcd gets a divide error when trying
to calculate the remainder of a transfer length by the maxpacket
value, as discovered by the syzbot fuzzer.

Currently the gadget core does not check for endpoints having a
maxpacket value of 0. This patch adds a check to usb_ep_enable(),
preventing such endpoints from being used.

As far as I know, none of the gadget drivers in the kernel tries to
create an endpoint with maxpacket = 0, but until now there has been
nothing to prevent userspace programs under gadgetfs or configfs from
doing it.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: syzbot+8ab8bf161038a8768553@syzkaller.appspotmail.com
Acked-by: Felipe Balbi <balbi@kernel.org>

Link: https://lore.kernel.org/r/Pine.LNX.4.44L0.1910281052370.1485-100000@iolanthe.rowland.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
include/linux/usb/gadget.h | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/include/linux/usb/gadget.h
+++ b/include/linux/usb/gadget.h
@@ -222,6 +222,16 @@ static inline void usb_ep_set_maxpacket_
*/
static inline int usb_ep_enable(struct usb_ep *ep)
{
+ /* UDC drivers can't handle endpoints with maxpacket size 0 */
+ if (usb_endpoint_maxp(ep->desc) == 0) {
+ /*
+ * We should log an error message here, but we can't call
+ * dev_err() because there's no way to find the gadget
+ * given only ep.
+ */
+ return -EINVAL;
+ }
+
return ep->ops->enable(ep, ep->desc);
}