USB: gadget: Reject endpoints with 0 maxpacket value

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>
CC: <stable@vger.kernel.org>
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>