Re: [PATCH][next] media: usb: siano: Fix undefined behavior bug in struct smsusb_urb_t

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Sep 29, 2023 at 06:20:10PM +0200, Jann Horn wrote:
> On Fri, Sep 29, 2023 at 5:42 PM Gustavo A. R. Silva
> <gustavoars@kernel.org> wrote:
> > `struct urb` is a flexible structure, which means that it contains a
> > flexible-array member at the bottom. This could potentially lead to an
> > overwrite of the object `wq` at run-time with the contents of `urb`.
> >
> > Fix this by placing object `urb` at the end of `struct smsusb_urb_t`.
> 
> Does this really change the situation? "struct smsusb_device_t"
> contains an array of "struct smsusb_urb_t", so it seems to be like
> you're just shifting the "VLA inside a non-final member of a struct"
> thing around so that there is one more layer of abstraction in
> between.
> 
> Comments on "struct urb" say:
> 
>  * Isochronous URBs have a different data transfer model, in part because
>  * the quality of service is only "best effort".  Callers provide specially
>  * allocated URBs, with number_of_packets worth of iso_frame_desc structures
>  * at the end.
> 
> and:
> 
> /* (in) ISO ONLY */
> 
> And it looks like smsusb only uses that URB as a bulk URB, so the flex
> array is unused and we can't have an overflow here?
> 
> If this is intended to make it possible to enable some kinda compiler
> warning, it might be worth talking to the USB folks to figure out the
> right approach here.
> 
> > Fixes: dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > ---
> >  drivers/media/usb/siano/smsusb.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/media/usb/siano/smsusb.c b/drivers/media/usb/siano/smsusb.c
> > index 9d9e14c858e6..2c048f8e8371 100644
> > --- a/drivers/media/usb/siano/smsusb.c
> > +++ b/drivers/media/usb/siano/smsusb.c
> > @@ -40,10 +40,10 @@ struct smsusb_urb_t {
> >         struct smscore_buffer_t *cb;
> >         struct smsusb_device_t *dev;
> >
> > -       struct urb urb;
> > -
> >         /* For the bottom half */
> >         struct work_struct wq;
> > +
> > +       struct urb urb;
> >  };

Yeah, this is going to get messy.  Ideally, just dynamically create the
urb and change this to a "struct urb *urb;" instead.

thanks,

greg k-h