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

[Andreas Rheinhardt]

On Tue, Dec 10, 2019 at 11:12 PM Andriy Gelman <andriy.gelman at gmail.com>
wrote:

> On Mon, 09. Dec 23:25, 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);
> > +
> >      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);
> > +
>
> Why did you decide to include this? As you say in your comments it cannot
> be
> triggered.
>
> It can currently not be triggered; but the parameters of av_malloc_array
are size_t and a future version of said function might remove the 2 GB
allocation limit.

- Andreas