ath10k: Fix a NULL-ptr-deref bug in ath10k_usb_alloc_urb_from_pipe

The `ar_usb` field of `ath10k_usb_pipe_usb_pipe` objects
are initialized to point to the containing `ath10k_usb` object
according to endpoint descriptors read from the device side, as shown
below in `ath10k_usb_setup_pipe_resources`:

for (i = 0; i < iface_desc->desc.bNumEndpoints; ++i) {
endpoint = &iface_desc->endpoint[i].desc;

// get the address from endpoint descriptor
pipe_num = ath10k_usb_get_logical_pipe_num(ar_usb,
endpoint->bEndpointAddress,
&urbcount);
......
// select the pipe object
pipe = &ar_usb->pipes[pipe_num];

// initialize the ar_usb field
pipe->ar_usb = ar_usb;
}

The driver assumes that the addresses reported in endpoint
descriptors from device side to be complete. If a device is
malicious and does not report complete addresses, it may trigger
NULL-ptr-deref `ath10k_usb_alloc_urb_from_pipe` and
`ath10k_usb_free_urb_to_pipe`.

This patch fixes the bug by preventing potential NULL-ptr-deref.

Signed-off-by: Hui Peng <[e-mail]>
Reported-by: Hui Peng <[e-mail]>
Reported-by: Mathias Payer <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
[groeck: Add driver tag to subject, fix build warning]
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>

drivers/net/wireless/ath/ath10k/usb.c

···
 	struct ath10k_urb_context *urb_context = NULL;
 	unsigned long flags;
 
+	/* bail if this pipe is not initialized */
+	if (!pipe->ar_usb)
+		return NULL;
+
 	spin_lock_irqsave(&pipe->ar_usb->cs_lock, flags);
 	if (!list_empty(&pipe->urb_list_head)) {
 		urb_context = list_first_entry(&pipe->urb_list_head,
···
 					struct ath10k_urb_context *urb_context)
 {
 	unsigned long flags;
+
+	/* bail if this pipe is not initialized */
+	if (!pipe->ar_usb)
+		return;
 
 	spin_lock_irqsave(&pipe->ar_usb->cs_lock, flags);