Re: [Linux-kernel-mentees] [PATCH v3] staging: comedi: check validity of wMaxPacketSize of usb endpoints found

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

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@syzkaller.appspotmail.com
> > Tested-by: syzbot+009f546aa1370056b1c2@syzkaller.appspotmail.com
> > Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
> > ---
> > 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
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