[FFmpeg-devel] [PATCH 1/2] lavc/hevc_mp4toannexb: Fix interger overflow

[Andreas Rheinhardt]

On Tue, Dec 3, 2019 at 10:41 PM Andriy Gelman <andriy.gelman at gmail.com>
wrote:

> From: Andriy Gelman <andriy.gelman at gmail.com>
>
> Check packet grow size against INT_MAX instead of SIZE_MAX.
>
> Found with libFuzzer:
> 4294967044 cannot be represented as int.
>
> Signed-off-by: Andriy Gelman <andriy.gelman at gmail.com>
> ---
>  libavcodec/hevc_mp4toannexb_bsf.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/hevc_mp4toannexb_bsf.c
> b/libavcodec/hevc_mp4toannexb_bsf.c
> index 09bce5b34c2..36fd6f0b15c 100644
> --- a/libavcodec/hevc_mp4toannexb_bsf.c
> +++ b/libavcodec/hevc_mp4toannexb_bsf.c
> @@ -152,8 +152,8 @@ static int hevc_mp4toannexb_filter(AVBSFContext *ctx,
> AVPacket *out)
>          extra_size    = add_extradata * ctx->par_out->extradata_size;
>          got_irap     |= is_irap;
>
> -        if (SIZE_MAX - nalu_size < 4 ||
> -            SIZE_MAX - 4 - nalu_size < extra_size) {
> +        if (INT_MAX < 4 + nalu_size ||
> +            INT_MAX - 4 < extra_size + nalu_size) {
>              ret = AVERROR_INVALIDDATA;
>              goto fail;
>          }
> --
>
> 1. Typo in "Interger".
2. You did not really fix the overflow problem: If nalu_size is (say)
UINT32_MAX, 4 + nalu_size is 3 (no undefined behaviour involved here) and
if extra_size is > 0 and <= INT_MAX - 4, then we don't go to fail here.
Your check merely ensures that (after possible (defined) overflow) 4 +
nalu_size + extra_size is >= 0 and <= INT_MAX, so that the automatic
conversion to int (the type of the parameter of av_grow_packet()) is
lossless. But in a scenario outlined here, the memcpy operations that
follow will surely segfault.
3. There is another problem here: There is no check that the NAL unit is
actually contained in the packet. A packet of size 10 bytes could contain a
NAL unit with a size field indicating a size of  (say) 1 GB and it would be
allocated; it would not segfault, though, because the safe version of the
bytestream2 API is used. (I think I already told you this, didn't I?)

- Andreas