[PATCH] Prevent buffer overflow on USB control endpoint

[Marek Vasut]

On 11/20/22 18:42, Szymon Heidrich wrote:
> On 20/11/2022 18:25, Marek Vasut wrote:
>> On 11/20/22 16:29, Szymon Heidrich wrote:
>>> 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.
>>
>> Why shouldn't the patch do simple
>>
>> w_length = min(w_length, USB_BUFSIZ);
>>
>> ?
> 
> The composite_setup function in composite.c uses w_length but function specific setup functions are passed the struct usb_ctrlrequest *ctrl so both must be updated.
> If only w_length is updated using min(w_length, USB_BUFSIZ) than function specific setup will still receive the original request including unaltered wLength so it
> may again return a number of bytes greater than buffer size resulting in an overflow.

Can you fill this sanitization into the setup wrapper here ?

https://patchwork.ozlabs.org/project/uboot/patch/20221216032047.536441-1-marex@denx.de/

That way, it would apply for all the gadget drivers and you won't have 
to deal with the const overwrite , right ?