USB: cdc-wdm: Avoid hanging on interface with no USB_CDC_DMM_TYPE

The probe does not strictly require the USB_CDC_DMM_TYPE
descriptor, which is a good thing as it makes the driver
usable on non-conforming interfaces.  A user could e.g.
bind to it to a CDC ECM interface by using the new_id and
bind sysfs files.  But this would fail with a 0 buffer length
due to the missing descriptor.

Fix by defining a reasonable fallback size: The minimum
device receive buffer size required by the CDC WMC standard,
revision 1.1

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index bb8208a13a53..c0197af22fd8 100644
--- a/drivers/usb/class/cdc-wdm.c
+++ b/drivers/usb/class/cdc-wdm.c
@@ -57,6 +57,8 @@ MODULE_DEVICE_TABLE (usb, wdm_ids);
 
 #define WDM_MAX                 16
 
+/* CDC-WMC r1.1 requires wMaxCommand to be "at least 256 decimal (0x100)" */
+#define WDM_DEFAULT_BUFSIZE     256
 
 static DEFINE_MUTEX(wdm_mutex);
 
@@ -602,7 +604,7 @@ static int wdm_probe(struct usb_interface *intf, const struct usb_device_id *id)
         struct usb_cdc_dmm_desc *dmhd;
         u8 *buffer = intf->altsetting->extra;
         int buflen = intf->altsetting->extralen;
-        u16 maxcom = 0;
+        u16 maxcom = WDM_DEFAULT_BUFSIZE;
 
         if (!buffer)
                 goto out;