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

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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@xxxxxxxxxxxxxxx>
Signed-off-by: Bjørn Mork <bjorn@xxxxxxx>
---
A fix for the fix...  Sorry about that.

Bjørn

 drivers/usb/class/cdc-wdm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index 590ff8b5aa20..a051a7a2b1bd 100644
--- 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);
-- 
1.8.5.2

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