[FFmpeg-devel] [PATCH v2 2/6] avcodec/cbs: Fix potential overflow

[Mark Thompson]

On 17/11/2019 07:34, Andreas Rheinhardt wrote:
> The number of bits in a PutBitContext must fit into an int, yet nothing
> guaranteed the size argument cbs_write_unit_data() uses in init_put_bits()
> to be in the range 0..INT_MAX / 8. This has been changed.
> 
> Furthermore, the check 8 * data_size > data_bit_start that there is
> data beyond the initial padding when writing mpeg2 or H.264/5 slices
> could also overflow, so divide it by 8 to get an equivalent check
> without this problem.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
>  libavcodec/cbs.c       | 4 +++-
>  libavcodec/cbs_h2645.c | 2 +-
>  libavcodec/cbs_mpeg2.c | 2 +-
>  3 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/libavcodec/cbs.c b/libavcodec/cbs.c
> index ab3eadb534..0badb192d9 100644
> --- a/libavcodec/cbs.c
> +++ b/libavcodec/cbs.c
> @@ -309,7 +309,9 @@ static int cbs_write_unit_data(CodedBitstreamContext *ctx,
>      if (ret < 0) {
>          if (ret == AVERROR(ENOSPC)) {
>              // Overflow.
> -            ctx->write_buffer_size *= 2;
> +            if (ctx->write_buffer_size == INT_MAX / 8)
> +                return AVERROR(ENOMEM);
> +            ctx->write_buffer_size = FFMIN(2 * ctx->write_buffer_size, INT_MAX / 8);
>              goto reallocate_and_try_again;
>          }
>          // Write failed for some other reason.
> diff --git a/libavcodec/cbs_h2645.c b/libavcodec/cbs_h2645.c
> index 923f77dcb4..88fa0029cd 100644
> --- a/libavcodec/cbs_h2645.c
> +++ b/libavcodec/cbs_h2645.c
> @@ -1101,7 +1101,7 @@ static int cbs_h2645_write_slice_data(CodedBitstreamContext *ctx,
>      const uint8_t *pos = data + data_bit_start / 8;
>  
>      av_assert0(data_bit_start >= 0 &&
> -               8 * data_size > data_bit_start);
> +               data_size > data_bit_start / 8);
>  
>      if (data_size * 8 + 8 > put_bits_left(pbc))
>          return AVERROR(ENOSPC);
> diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c
> index a9cc4a4cf8..13d871cc89 100644
> --- a/libavcodec/cbs_mpeg2.c
> +++ b/libavcodec/cbs_mpeg2.c
> @@ -337,7 +337,7 @@ static int cbs_mpeg2_write_slice(CodedBitstreamContext *ctx,
>          uint8_t *pos = slice->data + slice->data_bit_start / 8;
>  
>          av_assert0(slice->data_bit_start >= 0 &&
> -                   8 * slice->data_size > slice->data_bit_start);
> +                   slice->data_size > slice->data_bit_start / 8);
>  
>          if (slice->data_size * 8 + 8 > put_bits_left(pbc))
>              return AVERROR(ENOSPC);
> 

Yep, applied.

Thanks,

- Mark