usb: cdc-wdm: resp_count can be 0 even if WDM_READ is set

Do not decrement resp_count if it's already 0.

We set resp_count to 0 when the device is closed.  The next open and
read will try to clear the WDM_READ flag if there was leftover data
in the read buffer. This fix is necessary to prevent resubmitting
the read URB in a tight loop because resp_count becomes negative.

The bug can easily be triggered from userspace by not reading all
data in the read buffer, and then closing and reopening the chardev.

Fixes: 8dd5cd5395b9 ("usb: cdc-wdm: avoid hanging on zero length reads")
Cc: <stable@vger.kernel.org> # 3.13
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index 590ff8b..a051a7a 100644 (file)
--- a/drivers/usb/class/cdc-wdm.c
+++ b/drivers/usb/class/cdc-wdm.c
@@ -445,7 +445,7 @@ static int clear_wdm_read_flag(struct wdm_device *desc)
        clear_bit(WDM_READ, &desc->flags);
 
        /* submit read urb only if the device is waiting for it */
-       if (!--desc->resp_count)
+       if (!desc->resp_count || !--desc->resp_count)
                goto out;
 
        set_bit(WDM_RESPONDING, &desc->flags);