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

[Ronald S. Bultje]

Hi,

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.

Ronald