[FFmpeg-devel] [PATCH 1/7] avcodec/cbs: Check for overflow when reading

[Mark Thompson]

On 09/12/2019 22:25, Andreas Rheinhardt wrote:
> While CBS itself uses size_t for sizes, it relies on other APIs that use
> int for their sizes; in particular, AVBuffer uses int for their size
> parameters and so does GetBitContext with their number of bits. While
> CBS aims to be a safe API, the checks it employed were not sufficient to
> prevent overflows: E.g. if the size of a unit was > UINT_MAX / 8, 8 *
> said size may be truncated to a positive integer before being passed to
> init_get_bits() in which case its return value would not indicate an
> error.
> 
> These checks have been improved to really capture these kinds of errors;
> furthermore, although the sizes are still size_t, they are now de-facto
> restricted to 0..INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
> The check in cbs_insert_unit() can currently not be triggered, because
> av_malloc_array makes sure that it doesn't allocate more than INT_MAX;
> so the allocation will fail long before we get even close to INT_MAX.
> 
> av1 and H.26x have not been changed, because their split functions
> already check the size (in case of H.264 and H.265 this happens in
> ff_h2645_packet_split()).
> 
> It would btw be possible to open the GetBitContext generically. The
> read_unit function would then get the already opened GetBitContext
> as a parameter.
> 
>   libavcodec/cbs.c       | 6 ++++++
>   libavcodec/cbs_jpeg.c  | 2 +-
>   libavcodec/cbs_mpeg2.c | 2 +-
>   libavcodec/cbs_vp9.c   | 2 +-
>   4 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/libavcodec/cbs.c b/libavcodec/cbs.c
> index 0badb192d9..805049404b 100644
> --- a/libavcodec/cbs.c
> +++ b/libavcodec/cbs.c
> @@ -206,6 +206,9 @@ static int cbs_fill_fragment_data(CodedBitstreamContext *ctx,
>   {
>       av_assert0(!frag->data && !frag->data_ref);
>   
> +    if (size > INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE)
> +        return AVERROR(ERANGE);

For this and the following patch I wonder if it would be nicer to pick a sensible upper bounds for fragments (something like 2^30B total in at most 2^20 units?), name them (CBS_MAX_DATA_SIZE, 
CBS_MAX_UNITS?) and then use those in all checks?

No real case should get anywhere near these bounds, and indeed anything which even gets close was likely crafted by a malicious adversary specifically to do so.

> +
>       frag->data_ref =
>           av_buffer_alloc(size + AV_INPUT_BUFFER_PADDING_SIZE);
>       if (!frag->data_ref)
> @@ -693,6 +696,9 @@ static int cbs_insert_unit(CodedBitstreamContext *ctx,
>               memmove(units + position + 1, units + position,
>                       (frag->nb_units - position) * sizeof(*units));
>       } else {
> +        if (frag->nb_units == INT_MAX)
> +            return AVERROR(ERANGE);
> +
>           units = av_malloc_array(frag->nb_units + 1, sizeof(*units));
>           if (!units)
>               return AVERROR(ENOMEM);
> diff --git a/libavcodec/cbs_jpeg.c b/libavcodec/cbs_jpeg.c
> index b189cbd9b7..2bb6c8d18c 100644
> --- a/libavcodec/cbs_jpeg.c
> +++ b/libavcodec/cbs_jpeg.c
> @@ -246,7 +246,7 @@ static int cbs_jpeg_read_unit(CodedBitstreamContext *ctx,
>       GetBitContext gbc;
>       int err;
>   
> -    err = init_get_bits(&gbc, unit->data, 8 * unit->data_size);
> +    err = init_get_bits8(&gbc, unit->data, unit->data_size);
>       if (err < 0)
>           return err;
>   
> diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c
> index 13d871cc89..255f033734 100644
> --- a/libavcodec/cbs_mpeg2.c
> +++ b/libavcodec/cbs_mpeg2.c
> @@ -227,7 +227,7 @@ static int cbs_mpeg2_read_unit(CodedBitstreamContext *ctx,
>       GetBitContext gbc;
>       int err;
>   
> -    err = init_get_bits(&gbc, unit->data, 8 * unit->data_size);
> +    err = init_get_bits8(&gbc, unit->data, unit->data_size);
>       if (err < 0)
>           return err;
>   
> diff --git a/libavcodec/cbs_vp9.c b/libavcodec/cbs_vp9.c
> index 42e4dcf5ac..f6cfaa3b36 100644
> --- a/libavcodec/cbs_vp9.c
> +++ b/libavcodec/cbs_vp9.c
> @@ -488,7 +488,7 @@ static int cbs_vp9_read_unit(CodedBitstreamContext *ctx,
>       GetBitContext gbc;
>       int err, pos;
>   
> -    err = init_get_bits(&gbc, unit->data, 8 * unit->data_size);
> +    err = init_get_bits8(&gbc, unit->data, unit->data_size);
>       if (err < 0)
>           return err;
>   
> 

There are more of these init_get_bits(..., 8 * something) hanging around.  Perhaps change all of them as one patch, even if there isn't any danger of overflow?

Thanks,

- Mark