Re: [PATCH v3] staging: comedi: check validity of wMaxPacketSize of usb endpoints found
From: Greg Kroah-Hartman
Date: Sat Oct 10 2020 - 19:03:26 EST
On Sat, Oct 10, 2020 at 11:35:19AM +0200, Greg Kroah-Hartman wrote:
> On Sat, Oct 10, 2020 at 01:59:32PM +0530, Anant Thazhemadam wrote:
> > While finding usb endpoints in vmk80xx_find_usb_endpoints(), check if
> > wMaxPacketSize = 0 for the endpoints found.
> >
> > Some devices have isochronous endpoints that have wMaxPacketSize = 0
> > (as required by the USB-2 spec).
> > However, since this doesn't apply here, wMaxPacketSize = 0 can be
> > considered to be invalid.
> >
> > Reported-by: syzbot+009f546aa1370056b1c2@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Tested-by: syzbot+009f546aa1370056b1c2@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Anant Thazhemadam <anant.thazhemadam@xxxxxxxxx>
> > ---
> > Changes in v3:
> > * Correctly list version information
> >
> > Changes in v2:
> > * Fix coding style issue
> >
> > The error (as detected by syzbot) is generated in
> > vmk80xx_write_packet() (which is called in vmk80xx_reset_device()) when
> > it tries to assign devpriv->usb_tx_buf[0] = cmd.
> >
> > This NULL pointer dereference issue arises because
> > size = usb_endpoint_maxp(devpriv->ep_tx) = 0.
> >
> > This can be traced back to vmk80xx_find_usb_endpoints(), where the usb
> > endpoints are found, and assigned accordingly.
> > (For some more insight, in vmk80xx_find_usb_endpoints(),
> > if one of intf->cur_altsetting->iface_desc->endpoints' desc value = 0,
> > and consequently this endpoint is assigned to devpriv->ep_tx,
> > this issue gets triggered.)
> >
> > Checking if the wMaxPacketSize of an endpoint is invalid and returning
> > an error value accordingly, seems to fix the error.
> >
> > We could also alternatively perform this checking (if the size is 0 or not)
> > in vmk80xx_reset_device() itself, but it only seemed like covering up the issue
> > at that place, rather than fixing it, so I wasn't sure that was any better.
> >
> > However, if I'm not wrong, this might end up causing the probe to fail, and I'm
> > not sure if that's the right thing to do in cases like this, and if it isn't I'd
> > like some input on what exactly is the required course of action in cases like this.
> >
> > drivers/staging/comedi/drivers/vmk80xx.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/staging/comedi/drivers/vmk80xx.c b/drivers/staging/comedi/drivers/vmk80xx.c
> > index 65dc6c51037e..cb0a965d3c37 100644
> > --- a/drivers/staging/comedi/drivers/vmk80xx.c
> > +++ b/drivers/staging/comedi/drivers/vmk80xx.c
> > @@ -667,6 +667,9 @@ static int vmk80xx_find_usb_endpoints(struct comedi_device *dev)
> > if (!devpriv->ep_rx || !devpriv->ep_tx)
> > return -ENODEV;
> >
> > + if (!usb_endpoint_maxp(devpriv->ep_rx) || !usb_endpoint_maxp(devpriv->ep_tx))
> > + return -EINVAL;
> > +
> > return 0;
> > }
>
> Why not just rewrite vmk80xx_find_usb_endpoints() to use the
> usb_find_common_endpoints() or other helper functions like
> usb_find_bulk_in_endpoint() or others, so that this type of thing is
> checked there?
>
> Ah, wait, no, the packet size is not checked there, sorry, maybe that
> will not help out here. Is a bulk urb allowed to have a 0 size? If
> not, maybe we should just forbid that in the core? Time to go read the
> USB spec...
That being said, this patch is correct, I'll go queue it up now so that
it gets into 5.10-rc1.
thanks,
greg k-h