[PATCH] Prevent buffer overflow on USB control endpoint

[Szymon Heidrich]

On 16/12/2022 04:22, Marek Vasut wrote:
> 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 ?

Sure, that should be fine. 
I'll provide a patch when when this is merged, OK?