Re: [PATCH][next] media: usb: siano: Fix undefined behavior bug in struct smsusb_urb_t

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 9/30/23 09:01, Greg Kroah-Hartman wrote:
> On Fri, Sep 29, 2023 at 06:20:10PM +0200, Jann Horn wrote:
>> On Fri, Sep 29, 2023 at 5:42 PM Gustavo A. R. Silva
>> <gustavoars@kernel.org> wrote:
>>> `struct urb` is a flexible structure, which means that it contains a
>>> flexible-array member at the bottom. This could potentially lead to an
>>> overwrite of the object `wq` at run-time with the contents of `urb`.
>>>
>>> Fix this by placing object `urb` at the end of `struct smsusb_urb_t`.
>>
>> Does this really change the situation? "struct smsusb_device_t"
>> contains an array of "struct smsusb_urb_t", so it seems to be like

Yeah. I noticed that too.

Probably what Greg suggests (dynamically create the urb) can fix this, too.

I haven't taken a deep dive into this particular case. So, let me go and
figure something out.

>> you're just shifting the "VLA inside a non-final member of a struct"
>> thing around so that there is one more layer of abstraction in
>> between.
>>
>> Comments on "struct urb" say:
>>
>>   * Isochronous URBs have a different data transfer model, in part because
>>   * the quality of service is only "best effort".  Callers provide specially
>>   * allocated URBs, with number_of_packets worth of iso_frame_desc structures
>>   * at the end.
>>
>> and:
>>
>> /* (in) ISO ONLY */
>>
>> And it looks like smsusb only uses that URB as a bulk URB, so the flex
>> array is unused and we can't have an overflow here?
>>
>> If this is intended to make it possible to enable some kinda compiler
>> warning, it might be worth talking to the USB folks to figure out the
>> right approach here.
>>
>>> Fixes: dd47fbd40e6e ("[media] smsusb: don't sleep while atomic")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>>> ---
>>>   drivers/media/usb/siano/smsusb.c | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/media/usb/siano/smsusb.c b/drivers/media/usb/siano/smsusb.c
>>> index 9d9e14c858e6..2c048f8e8371 100644
>>> --- a/drivers/media/usb/siano/smsusb.c
>>> +++ b/drivers/media/usb/siano/smsusb.c
>>> @@ -40,10 +40,10 @@ struct smsusb_urb_t {
>>>          struct smscore_buffer_t *cb;
>>>          struct smsusb_device_t *dev;
>>>
>>> -       struct urb urb;
>>> -
>>>          /* For the bottom half */
>>>          struct work_struct wq;
>>> +
>>> +       struct urb urb;
>>>   };
> 
> Yeah, this is going to get messy.  Ideally, just dynamically create the
> urb and change this to a "struct urb *urb;" instead.

Probably, yes.

Thanks
--
Gustavo