usb-host: use correct altsetting in usb_host_ep_update

In order to keep track of the alternate setting that should be used for
a given interface, the USBDevice struct keeps an array of alternate
setting values, which is indexed by the interface number. In
usb_host_set_interface, when this array is updated, usb_host_ep_update
is called as a result. However, when usb_host_ep_update accesses the
active libusb_config_descriptor, it indexes udev->altsetting with the
loop variable, rather than the interface number.

With the simple trace backend enable, this behavior can be seen:

  [...]

  usb_xhci_xfer_start 0.440 pid=1215 xfer=0x5596a4b85930 slotid=0x1 epid=0x1 streamid=0x0
  usb_packet_state_change 1.703 pid=1215 bus=0x1 port=b'1' ep=0x0 p=0x5596a4b85938 o=b'undef' n=b'setup'
  usb_host_req_control 2.269 pid=1215 bus=0x1 addr=0x5 p=0x5596a4b85938 req=0x10b value=0x1 index=0xd
  usb_host_set_interface 0.449 pid=1215 bus=0x1 addr=0x5 interface=0xd alt=0x1
  usb_host_parse_config 2542.648 pid=1215 bus=0x1 addr=0x5 value=0x2 active=0x1
  usb_host_parse_interface 1.804 pid=1215 bus=0x1 addr=0x5 num=0xc alt=0x0 active=0x1
  usb_host_parse_endpoint 2.012 pid=1215 bus=0x1 addr=0x5 ep=0x2 dir=b'in' type=b'int' active=0x1
  usb_host_parse_interface 1.598 pid=1215 bus=0x1 addr=0x5 num=0xd alt=0x0 active=0x1
  usb_host_req_emulated 3.593 pid=1215 bus=0x1 addr=0x5 p=0x5596a4b85938 status=0x0
  usb_packet_state_change 2.550 pid=1215 bus=0x1 port=b'1' ep=0x0 p=0x5596a4b85938 o=b'setup' n=b'complete'
  usb_xhci_xfer_success 4.298 pid=1215 xfer=0x5596a4b85930 bytes=0x0

  [...]

In particular, it is seen that although usb_host_set_interface sets the
alternate setting of interface 0xd to 0x1, usb_host_ep_update uses 0x0
as the alternate setting due to using the incorrect index to
udev->altsetting.

Fix this problem by getting the interface number from the active
libusb_config_descriptor, and then using that as the index to
udev->altsetting.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Message-Id: <20210201213021.500277-1-rosbrookn@ainfosec.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/host-libusb.c

@@ -836,7 +836,7 @@ static void usb_host_ep_update(USBHostDevice *s)
     struct libusb_ss_endpoint_companion_descriptor *endp_ss_comp;
 #endif
     uint8_t devep, type;
-    int pid, ep;
+    int pid, ep, alt;
     int rc, i, e;
 
     usb_ep_reset(udev);
@@ -848,8 +848,20 @@ static void usb_host_ep_update(USBHostDevice *s)
                                 conf->bConfigurationValue, true);
 
     for (i = 0; i < conf->bNumInterfaces; i++) {
-        assert(udev->altsetting[i] < conf->interface[i].num_altsetting);
-        intf = &conf->interface[i].altsetting[udev->altsetting[i]];
+        /*
+         * The udev->altsetting array indexes alternate settings
+         * by the interface number. Get the 0th alternate setting
+         * first so that we can grab the interface number, and
+         * then correct the alternate setting value if necessary.
+         */
+        intf = &conf->interface[i].altsetting[0];
+        alt = udev->altsetting[intf->bInterfaceNumber];
+
+        if (alt != 0) {
+            assert(alt < conf->interface[i].num_altsetting);
+            intf = &conf->interface[i].altsetting[alt];
+        }
+
         trace_usb_host_parse_interface(s->bus_num, s->addr,
                                        intf->bInterfaceNumber,
                                        intf->bAlternateSetting, true);