xhci: Fix USB3 NULL pointer dereference at logical disconnect.

Hub driver will try to disable a USB3 device twice at logical disconnect,
racing with xhci_free_dev() callback from the first port disable.

This can be triggered with "udisksctl power-off --block-device <disk>"
or by writing "1" to the "remove" sysfs file for a USB3 device
in 4.17-rc4.

USB3 devices don't have a similar disabled link state as USB2 devices,
and use a U3 suspended link state instead. In this state the port
is still enabled and connected.

hub_port_connect() first disconnects the device, then later it notices
that device is still enabled (due to U3 states) it will try to disable
the port again (set to U3).

The xhci_free_dev() called during device disable is async, so checking
for existing xhci->devs[i] when setting link state to U3 the second time
was successful, even if device was being freed.

The regression was caused by, and whole thing revealed by,
Commit 44a182b9 ("xhci: Fix use-after-free in xhci_free_virt_device")
which sets xhci->devs[i]->udev to NULL before xhci_virt_dev() returned.
and causes a NULL pointer dereference the second time we try to set U3.

Fix this by checking xhci->devs[i]->udev exists before setting link state.

The original patch went to stable so this fix needs to be applied there as
well.

Fixes: 44a182b9 ("xhci: Fix use-after-free in xhci_free_virt_device")
Cc: <stable@vger.kernel.org>
Reported-by: Jordan Glover <Golden_Miller83@protonmail.ch>
Tested-by: Jordan Glover <Golden_Miller83@protonmail.ch>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-hub.c

@@ -354,7 +354,7 @@ int xhci_find_slot_id_by_port(struct usb_hcd *hcd, struct xhci_hcd *xhci,
 
 	slot_id = 0;
 	for (i = 0; i < MAX_HC_SLOTS; i++) {
-		if (!xhci->devs[i])
+		if (!xhci->devs[i] || !xhci->devs[i]->udev)
 			continue;
 		speed = xhci->devs[i]->udev->speed;
 		if (((speed >= USB_SPEED_SUPER) == (hcd->speed >= HCD_USB3))