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Subject: Re: [PATCH] staging: comedi: check validity of wMaxPacketSize of usb
 endpoints found
Cc:     linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+009f546aa1370056b1c2@syzkaller.appspotmail.com,
        Ian Abbott <abbotti@mev.co.uk>,
        H Hartley Sweeten <hsweeten@visionengravers.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org
References: <20201009161646.286286-1-anant.thazhemadam@gmail.com>
From:   Anant Thazhemadam <anant.thazhemadam@gmail.com>
Message-ID: <21e614cc-abf7-9332-81be-c969b0fd29b6@gmail.com>
Date:   Fri, 9 Oct 2020 21:54:48 +0530
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On 09/10/20 9:46 pm, 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>
> ---
> 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;
>  }
>  

The patch in this mail has a coding style issue (that I thought I had fixed), and was sent out by
mistake.
Please ignore this mail. Apologies.

Thanks,
Anant