[PATCH v5] usb: dwc3: gadget: Move null pinter check to proper place

[Albert Wang <albertccwang@google.com>]

When dwc3_gadget_ep_cleanup_completed_requests() called to
dwc3_gadget_giveback() where the dwc3 lock is released, other thread is
able to execute. In this situation, usb_ep_disable() gets the chance to
clear endpoint descriptor pointer which leds to the null pointer
dereference problem. So needs to move the null pointer check to a proper
place.

Example call stack:

Thread#1:
dwc3_thread_interrupt()
  spin_lock
  -> dwc3_process_event_buf()
   -> dwc3_process_event_entry()
    -> dwc3_endpoint_interrupt()
     -> dwc3_gadget_endpoint_trbs_complete()
      -> dwc3_gadget_ep_cleanup_completed_requests()
       ...
       -> dwc3_giveback()
          spin_unlock
          Thread#2 executes

Thread#2:
configfs_composite_disconnect()
  -> __composite_disconnect()
   -> ffs_func_disable()
    -> ffs_func_set_alt()
     -> ffs_func_eps_disable()
      -> usb_ep_disable()
         wait for dwc3 spin_lock
         Thread#1 released lock
         clear endpoint.desc

Fixes: 26288448120b ("usb: dwc3: gadget: Fix null pointer exception")
Signed-off-by: Albert Wang <albertccwang@google.com>
---
 v5: Supplement the change log
 v4: Modify the change log text
 v3: Add change log to be compliant with the canonical patch format
 v2: Remove redundant 'else' and add additional comments and more
     descriptive commit text

 drivers/usb/dwc3/gadget.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index 19477f4bbf54..7edd4a5dc6af 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -3366,14 +3366,14 @@ static bool dwc3_gadget_endpoint_trbs_complete(struct dwc3_ep *dep,
 	struct dwc3		*dwc = dep->dwc;
 	bool			no_started_trb = true;
 
-	if (!dep->endpoint.desc)
-		return no_started_trb;
-
 	dwc3_gadget_ep_cleanup_completed_requests(dep, event, status);
 
 	if (dep->flags & DWC3_EP_END_TRANSFER_PENDING)
 		goto out;
 
+	if (!dep->endpoint.desc)
+		return no_started_trb;
+
 	if (usb_endpoint_xfer_isoc(dep->endpoint.desc) &&
 		list_empty(&dep->started_list) &&
 		(list_empty(&dep->pending_list) || status == -EXDEV))
-- 
2.36.1.124.g0e6072fb45-goog