[FFmpeg-devel] [PATCH] interplayacm: increase bitstream buffer size by AV_INPUT_BUFFER_PADDING_SIZE

[Andreas Cadhalpun]

On 30.10.2016 22:18, Paul B Mahol wrote:
> On 10/30/16, Andreas Cadhalpun <andreas.cadhalpun at googlemail.com> wrote:
>> This fixes out-of-bounds reads by the bitstream reader.
>>
>> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
>> ---
>>  libavcodec/interplayacm.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libavcodec/interplayacm.c b/libavcodec/interplayacm.c
>> index 0486e00..f4a3446 100644
>> --- a/libavcodec/interplayacm.c
>> +++ b/libavcodec/interplayacm.c
>> @@ -72,7 +72,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
>>      s->block   = av_calloc(s->block_len, sizeof(int));
>>      s->wrapbuf = av_calloc(s->wrapbuf_len, sizeof(int));
>>      s->ampbuf  = av_calloc(0x10000, sizeof(int));
>> -    s->bitstream = av_calloc(s->max_framesize, sizeof(*s->bitstream));
>> +    s->bitstream = av_calloc(s->max_framesize +
>> AV_INPUT_BUFFER_PADDING_SIZE / sizeof(*s->bitstream) + 1,
> 
> How did you came up with this fix?
> Little background would help.

The out-of-bounds read happens in get_bits called from linear.
The buffer passed to init_get_bits8 is &s->bitstream[s->bitstream_index].
The get_bits documentation says:
/**
 * Initialize GetBitContext.
 * @param buffer bitstream buffer, must be AV_INPUT_BUFFER_PADDING_SIZE bytes
 *        larger than the actual read bits because some optimized bitstream
 *        readers read 32 or 64 bit at once and could read over the end
 * @param byte_size the size of the buffer in bytes
 * @return 0 on success, AVERROR_INVALIDDATA if the buffer_size would overflow.
 */
static inline int init_get_bits8(GetBitContext *s, const uint8_t *buffer,
                                 int byte_size)

Increasing the buffer size fixed the problem, so the case seems quite clear.

Best regards,
Andreas