[FFmpeg-devel] [PATCH] avformat/mov: validate chunk_count vs stsc_data

[Chris Cunningham]

Some extra context: this remedies some bad math that otherwise triggers an
abort near the bottom of mov_seek_stream().

My first thought on seeing this was we should probably be
returning AVERROR_INVALIDDATA somewhere. The stsc and stco boxes aren't
agreeing (stco says no chunks, stsc.first points to chunk 135). stsc and
stco aren't gauranteed to come in a particular order, but we could try to
validate that they agree whenever both arrive. One challenge is
that mov_read_stco is already pretty relaxed about setting sc->chunk_count
(it sets to some lower number if we hit EOF during parsing).

Feedback welcome.

On Thu, Jan 31, 2019 at 5:26 PM chcunningham <chcunningham at chromium.org>
wrote:

> Bad content may contain stsc boxes with a first_chunk index that
> exceeds stco.entries (chunk_count).
>
> mov_get_stsc_samples now checks for this and returns 0 when
> values are invalid.
>
> Also updates MOVStsc to use unsigned ints, per spec.
> ---
>  libavformat/isom.h | 6 +++---
>  libavformat/mov.c  | 4 ++--
>  2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/libavformat/isom.h b/libavformat/isom.h
> index e629663949..8e0d8355b3 100644
> --- a/libavformat/isom.h
> +++ b/libavformat/isom.h
> @@ -59,9 +59,9 @@ typedef struct MOVStts {
>  } MOVStts;
>
>  typedef struct MOVStsc {
> -    int first;
> -    int count;
> -    int id;
> +    unsigned int first;
> +    unsigned int count;
> +    unsigned int id;
>  } MOVStsc;
>
>  typedef struct MOVElst {
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 9b9739f788..dcf4ee8dc1 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -2690,11 +2690,11 @@ static inline int mov_stsc_index_valid(unsigned
> int index, unsigned int count)
>  /* Compute the samples value for the stsc entry at the given index. */
>  static inline int64_t mov_get_stsc_samples(MOVStreamContext *sc, unsigned
> int index)
>  {
> -    int chunk_count;
> +    unsigned int chunk_count = 0;
>
>      if (mov_stsc_index_valid(index, sc->stsc_count))
>          chunk_count = sc->stsc_data[index + 1].first -
> sc->stsc_data[index].first;
> -    else
> +    else if (sc->chunk_count >= sc->stsc_data[index].first)
>          chunk_count = sc->chunk_count - (sc->stsc_data[index].first - 1);
>
>      return sc->stsc_data[index].count * (int64_t)chunk_count;
> --
> 2.20.1.611.gfbb209baf1-goog
>
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