linux-kernel - Re: [PATCH] usb: cdns3: fix possible buffer overflow caused by bad DMA value

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Date:   Wed, 1 Jul 2020 08:31:51 +0000
From:   Peter Chen <peter.chen@....com>
To:     Felipe Balbi <balbi@...nel.org>
CC:     Jia-Ju Bai <baijiaju1990@...il.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "pawell@...ence.com" <pawell@...ence.com>,
        "rogerq@...com" <rogerq@...com>,
        "colin.king@...onical.com" <colin.king@...onical.com>,
        "yuehaibing@...wei.com" <yuehaibing@...wei.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: cdns3: fix possible buffer overflow caused by bad
 DMA value

On 20-07-01 09:52:43, Felipe Balbi wrote:
> Peter Chen <peter.chen@....com> writes:
> 
> > On 20-05-30 11:24:00, Jia-Ju Bai wrote:
> >> In cdns3_ep0_setup_phase():
> >>   struct usb_ctrlrequest *ctrl = priv_dev->setup_buf;
> >> 
> >> Because priv_dev->setup_buf (allocated in cdns3_gadget_start) is stored 
> >> in DMA memory, and thus ctrl is a DMA value.
> >> 
> >> cdns3_ep0_setup_phase()
> >>   cdns3_ep0_standard_request(priv_dev, ctrl)
> >>     cdns3_req_ep0_get_status(priv_dev, ctrl)
> >>       index = cdns3_ep_addr_to_index(ctrl->wIndex);
> >>       priv_ep = priv_dev->eps[index];
> >> 
> >> cdns3_ep0_setup_phase()
> >>   cdns3_ep0_standard_request(priv_dev, ctrl)
> >>     cdns3_req_ep0_handle_feature(priv_dev, ctrl_req, 0)
> >>       cdns3_ep0_feature_handle_endpoint(priv_dev, ctrl, set)
> >>         index = cdns3_ep_addr_to_index(ctrl->wIndex);
> >>         priv_ep = priv_dev->eps[index];
> >> 
> >> In these cases, ctrl->wIndex can be be modified at anytime by malicious
> >> hardware, and thus a buffer overflow can occur when the code
> >> "priv_dev->eps[index]" is executed.
> >> 
> >
> > Did you see the setup buffer is overwritten before the setup handling is
> > finished?
> >
> >> To fix these possible bugs, index is checked before being used.
> >
> > I think the better fix is to use one additional buffer for struct
> > usb_ctrlrequest, and copy the dma_buf to it after setup packet
> > has received, then use the value stored in this buffer for later
> > operation, it could avoid quitting the code which is useful in fact.
> 
> Why is this a better fix? If you don't have that endpoint index, you
> shouldn't try to access it. However, I think the problem here is
> slightly easier to solve :-)

The possible problem here is: it is a correct setup packet, the memory
it uses may be modified by controller wrongly (eg, try to get next setup
packet) before it finishes using. So, I suggest adding a setup buf for
struct cdns3 to store every setup packet after it receives to avoid
the original setup buffer corrupted.

Peter

> 
> >> diff --git a/drivers/usb/cdns3/ep0.c b/drivers/usb/cdns3/ep0.c
> >> index e71240b386b4..0a80c7ade613 100644
> >> --- a/drivers/usb/cdns3/ep0.c
> >> +++ b/drivers/usb/cdns3/ep0.c
> >> @@ -265,6 +265,8 @@ static int cdns3_req_ep0_get_status(struct cdns3_device *priv_dev,
> >>  		return cdns3_ep0_delegate_req(priv_dev, ctrl);
> >>  	case USB_RECIP_ENDPOINT:
> >>  		index = cdns3_ep_addr_to_index(ctrl->wIndex);
> 
> diff --git a/drivers/usb/cdns3/gadget.c b/drivers/usb/cdns3/gadget.c
> index 5e24c2e57c0d..96ba3eec805c 100644
> --- a/drivers/usb/cdns3/gadget.c
> +++ b/drivers/usb/cdns3/gadget.c
> @@ -107,7 +107,10 @@ void cdns3_set_register_bit(void __iomem *ptr, u32 mask)
>   */
>  u8 cdns3_ep_addr_to_index(u8 ep_addr)
>  {
> -       return (((ep_addr & 0x7F)) + ((ep_addr & USB_DIR_IN) ? 16 : 0));
> +       u8 num = ep_addr & USB_ENDPOINT_NUMBER_MASK;
> +       u8 dir = ep_addr & USB_ENDPOINT_DIR_MASK;
> +
> +       return num + dir ? 16 : 0;
>  }
>  
>  static int cdns3_get_dma_pos(struct cdns3_device *priv_dev,
> 
> This will guarantee that the number is never over the limit.
> 
> -- 
> balbi



-- 

Thanks,
Peter Chen