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

[Ben Hutchings]

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxx>
Reported-and-tested-by: syzbot+8ab8bf161038a8768553@xxxxxxxxxxxxxxxxxxxxxxxxx
Acked-by: Felipe Balbi <balbi@xxxxxxxxxx>

Link: https://lore.kernel.org/r/Pine.LNX.4.44L0.1910281052370.1485-100000@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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);
 }