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

[Ronald S. Bultje]

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.

Ronald