Re: [PATCH v3] staging: comedi: check validity of wMaxPacketSize of usb endpoints found

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

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...

thanks,

greg k-h