Re: [Qemu-devel] [PULL 1/3] usb-mtp: Add some NULL checks for issues pointed out by coverity

[Peter Maydell]

On 7 May 2018 at 10:44, Gerd Hoffmann <address@hidden> wrote:
> From: Bandan Das <address@hidden>
>
> CID 1390578: In usb_mtp_write_metadata, parent can never be NULL but
> just in case, add an assert
> CID 1390592: Check for o->format only if o !=NULL
> CID 1390604: Check s->data_out != NULL in usb_mtp_handle_data

> @@ -1838,7 +1838,7 @@ static void usb_mtp_handle_data(USBDevice *dev, 
> USBPacket *p)
>              p->status = USB_RET_STALL;
>              return;
>          }
> -        if (s->data_out && !s->data_out->first) {
> +        if ((s->data_out != NULL) && !s->data_out->first) {
>              container_type = TYPE_DATA;
>          } else {
>              usb_packet_copy(p, &container, sizeof(container));
> --

I just noticed this, but this isn't actually changing the logic
in this function: "s->data_out" and "s->data_out != NULL" are
exactly the same test. So this won't change CID 1390604.

Looking back at my previous email on this, it looks like I
managed to completely misinterpret the code and/or what
coverity is talking about, which is probably the source of
the confusion. Let me try again...

In the function usb_mtp_handle_data() EP_DATA_OUT case, we have:
 (1) a check for whether s->data_out is NULL, which implies
     that it might be NULL sometimes
 (2) some time later, a call to usb_mtp_get_data() which is
     not protected by a test for whether s->data_out is NULL

and if s->data_out is NULL at point (2) then usb_mtp_get_data()
will crash.

Obviously the code path that goes through "containe_type = TYPE_DATA"
must have s->data_out being non-NULL. But in the else branch of
that, can the container_type we get via usb_packet_copy() ever
be TYPE_DATA ? (It's not clear to me whether that comes from
a trusted or untrusted source.)

If this is a "can't happen" situation we can mark it as a false
positive in coverity.

thanks
-- PMM