[PATCH] Prevent buffer overflow on USB control endpoint

[Szymon Heidrich]

On 20/11/2022 15:43, Marek Vasut wrote:
> On 11/17/22 12:50, Fabio Estevam wrote:
>> [Adding Lukasz and Marek]
>>
>> On Thu, Nov 17, 2022 at 6:50 AM Szymon Heidrich
>> <szymon.heidrich at gmail.com> wrote:
>>>
>>> Assure that the control endpoint buffer of size USB_BUFSIZ (4096)
>>> can not be overflown during handling of USB control transfer
>>> requests with wLength greater than USB_BUFSIZ.
>>>
>>> Signed-off-by: Szymon Heidrich <szymon.heidrich at gmail.com>
>>> ---
>>>   drivers/usb/gadget/composite.c | 11 +++++++++++
>>>   1 file changed, 11 insertions(+)
>>>
>>> diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
>>> index 2a309e624e..cb89f6dca9 100644
>>> --- a/drivers/usb/gadget/composite.c
>>> +++ b/drivers/usb/gadget/composite.c
>>> @@ -1019,6 +1019,17 @@ composite_setup(struct usb_gadget *gadget, const struct usb_ctrlrequest *ctrl)
>>>          u8                              endp;
>>>          struct usb_configuration        *c;
>>>
>>> +       if (w_length > USB_BUFSIZ) {
>>> +               if (ctrl->bRequestType & USB_DIR_IN) {
>>> +                       /* Cast away the const, we are going to overwrite on purpose. */
>>> +                       __le16 *temp = (__le16 *)&ctrl->wLength;
>>> +                       *temp = cpu_to_le16(USB_BUFSIZ);
>>> +                       w_length = USB_BUFSIZ;
> 
> Won't this end up sending corrupted packets in case they are longer than USB_BUFSIZ ?
> 
> Where do such long packets come from ?
> 
> What is the test-case ?

The USB host will not attempt to retrieve more than wLenght bytes during transfer phase.
If the device would erroneously attempt to provide more data it would result in an unexpected state.
In case of most implementations the buffer for endpoint 0 along with max control transfer is limited to 4096 bytes (USB_BUFSIZ for U-Boot and Linux kernel).
Still according to the USB specification wLength is two bytes an the device may receive requests with wLength larger than 4096 bytes e.g. in case of a custom/malicious USB host.
For example one may build libusb with MAX_CTRL_BUFFER_LENGTH altered to 0xffff and this will allow the host to send requests with wLength up to 0xffff.
In this case the original implementation may result in buffer overflows as in multiple locations a value directly derived from wLength is set as the transfer phase length.
With the change applied IN requests with wLength larger than USB_BUFSIZ will be trimmed to USB_BUFSIZ, otherwise the host would read wLength-USB_BUFSIZ past cdev->req->buf.
I am not aware of any cases where more than USB_BUFSIZ would be provided from a buffer other than cdev->req->buf. In case I missed such case please let me know.