usb: gadget: f_ncm: fix potential NULL ptr deref in ncm_bitrate()

In Google internal bug 265639009 we've received an (as yet) unreproducible
crash report from an aarch64 GKI 5.10.149-android13 running device.

AFAICT the source code is at:
  https://android.googlesource.com/kernel/common/+/refs/tags/ASB-2022-12-05_13-5.10

The call stack is:
  ncm_close() -> ncm_notify() -> ncm_do_notify()
with the crash at:
  ncm_do_notify+0x98/0x270
Code: 79000d0b b9000a6c f940012a f9400269 (b9405d4b)

Which I believe disassembles to (I don't know ARM assembly, but it looks sane enough to me...):

  // halfword (16-bit) store presumably to event->wLength (at offset 6 of struct usb_cdc_notification)
  0B 0D 00 79    strh w11, [x8, #6]

  // word (32-bit) store presumably to req->Length (at offset 8 of struct usb_request)
  6C 0A 00 B9    str  w12, [x19, #8]

  // x10 (NULL) was read here from offset 0 of valid pointer x9
  // IMHO we're reading 'cdev->gadget' and getting NULL
  // gadget is indeed at offset 0 of struct usb_composite_dev
  2A 01 40 F9    ldr  x10, [x9]

  // loading req->buf pointer, which is at offset 0 of struct usb_request
  69 02 40 F9    ldr  x9, [x19]

  // x10 is null, crash, appears to be attempt to read cdev->gadget->max_speed
  4B 5D 40 B9    ldr  w11, [x10, #0x5c]

which seems to line up with ncm_do_notify() case NCM_NOTIFY_SPEED code fragment:

  event->wLength = cpu_to_le16(8);
  req->length = NCM_STATUS_BYTECOUNT;

  /* SPEED_CHANGE data is up/down speeds in bits/sec */
  data = req->buf + sizeof *event;
  data[0] = cpu_to_le32(ncm_bitrate(cdev->gadget));

My analysis of registers and NULL ptr deref crash offset
  (Unable to handle kernel NULL pointer dereference at virtual address 000000000000005c)
heavily suggests that the crash is due to 'cdev->gadget' being NULL when executing:
  data[0] = cpu_to_le32(ncm_bitrate(cdev->gadget));
which calls:
  ncm_bitrate(NULL)
which then calls:
  gadget_is_superspeed(NULL)
which reads
  ((struct usb_gadget *)NULL)->max_speed
and hits a panic.

AFAICT, if I'm counting right, the offset of max_speed is indeed 0x5C.
(remember there's a GKI KABI reservation of 16 bytes in struct work_struct)

It's not at all clear to me how this is all supposed to work...
but returning 0 seems much better than panic-ing...

Cc: Felipe Balbi <balbi@kernel.org>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/20230117131839.1138208-1-maze@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_ncm.c

@@ -83,7 +83,9 @@ static inline struct f_ncm *func_to_ncm(struct usb_function *f)
 /* peak (theoretical) bulk transfer rate in bits-per-second */
 static inline unsigned ncm_bitrate(struct usb_gadget *g)
 {
-	if (gadget_is_superspeed(g) && g->speed >= USB_SPEED_SUPER_PLUS)
+	if (!g)
+		return 0;
+	else if (gadget_is_superspeed(g) && g->speed >= USB_SPEED_SUPER_PLUS)
 		return 4250000000U;
 	else if (gadget_is_superspeed(g) && g->speed == USB_SPEED_SUPER)
 		return 3750000000U;