[FFmpeg-devel] [PATCH] IFF: Add error checking to byterun1 decoder

[Sebastian Vater]

Ronald S. Bultje a ?crit :
> Hi,
>
> On Wed, May 26, 2010 at 4:34 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>   
>> On Wed, May 26, 2010 at 4:29 PM, Sebastian Vater
>> <cdgs.basty at googlemail.com> wrote:
>>     
>>> Ronald S. Bultje a ?crit :
>>>       
>>>> On Wed, May 26, 2010 at 4:18 PM, Sebastian Vater
>>>> <cdgs.basty at googlemail.com> wrote:
>>>>         
>>>>> Ronald S. Bultje a ?crit :
>>>>>           
>>>>>> On Wed, May 26, 2010 at 3:20 PM, Sebastian Vater
>>>>>> <cdgs.basty at googlemail.com> wrote:
>>>>>>
>>>>>>             
>>>>>>> +            if (value >= 0) {
>>>>>>> +                const int length = (unsigned) value + 1;
>>>>>>> +                if (length > dst_size || length > (int) (buf_end - buf)) // overflow?
>>>>>>> +                    break;
>>>>>>> +                memcpy(dst, buf, length);
>>>>>>> +                dst_size -= length;
>>>>>>> +                dst += length;
>>>>>>> +                buf += length;
>>>>>>> +            } else if (value > -128) {
>>>>>>> +                const int length = (unsigned) -value + 1;
>>>>>>> +                if (length > dst_size || buf >= buf_end) // overflow?
>>>>>>> +                    break;
>>>>>>> +                memset(dst, *buf++, length);
>>>>>>> +                dst_size -= length;
>>>>>>> +                dst += length;
>>>>>>> +            } else if (buf >= buf_end) { // noop, return error on overflow, though
>>>>>>> +                break;
>>>>>>> +            }
>>>>>>> +        } while (dst_size);
>>>>>>> +    }
>>>>>>>
>>>>>>>               
>>>>>> This can overread with a specially crafted buffer because you're not
>>>>>> checking that buf < buf_end at the start of each loop iteration, which
>>>>>> the original code did do.
>>>>>>
>>>>>>             
>>>>> As already said, I do more approciate checks in these if's...at the end
>>>>> it's ensured that it doesn't overread.
>>>>> Or do you mean sth. else which I missed out? Could you give an example
>>>>> for overreading then?
>>>>>
>>>>>           
>>>> Imagine that buf_size=0, so buf==buf_end directly. The topmost if will
>>>> catch that. Now imagine that buf is exactly 1 byte + length long.
>>>> After one loop iteration, length won't be checked (because that's only
>>>> done on entry into the loop, and not before every iteration) and const
>>>> int8_t value = *buf++ will overread beyond the buffer bounds.
>>>>
>>>>         
>>> Ahh, but this isn't actually a problem, because AVPacket->data has
>>> FF_INPUT_BUFFER_PADDING_SIZE.
>>> So it will still read in valid memory area...and regardless of what the
>>> contents of that byte is, the if's will
>>> catch that, since they all check buf >= buf_end. ;-)
>>>
>>> I have adressed that issue from the very beginning by taking a look at
>>> the allocation routine for AVPacket->data. ;-)
>>>
>>> But maybe I could add a comment about this?
>>>       
>> I'm not terribly comfortable with it, but if others think it's OK then
>> I won't object.
>>     
>
> I haven't heard anyone else, so I'd prefer if the check was kept in
> place. Rest of patch looks OK on a rough first check.
>   

pong

-- 

Best regards,
                   :-) Basty/CDGS (-: