Re: [FFmpeg-devel] [PATCH 2/2] avformat/matroskadec: avoid integer overflows in SAR computation

From: Anton Khirnov <anton@khirnov.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 2/2] avformat/matroskadec: avoid integer overflows in SAR computation
Date: Tue, 05 Apr 2022 11:59:44 +0200
Message-ID: <164915278455.21047.416796164909893337@lain.red.khirnov.net> (raw)
In-Reply-To: <20220401104608.10193-2-michael@niedermayer.cc>

Quoting Michael Niedermayer (2022-04-01 12:46:08)
> This ignores >64bit
> Alternatively we could support that if it occurs in reality
> 
> Fixes: negation of -9223372036854775808
> Fixes: integer overflows
> Fixes: 46072/clusterfuzz-testcase-minimized-ffmpeg_dem_MATROSKA_fuzzer-5029840966778880
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavformat/matroskadec.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
> index d97fc33d44..069fba6cf7 100644
> --- a/libavformat/matroskadec.c
> +++ b/libavformat/matroskadec.c
> @@ -2886,11 +2886,14 @@ static int matroska_parse_tracks(AVFormatContext *s)
>                  mkv_stereo_mode_display_mul(track->video.stereo_mode, &display_width_mul, &display_height_mul);
>  
>              if (track->video.display_unit < MATROSKA_VIDEO_DISPLAYUNIT_UNKNOWN) {
> -                av_reduce(&st->sample_aspect_ratio.num,
> -                          &st->sample_aspect_ratio.den,
> -                          st->codecpar->height * track->video.display_width  * display_width_mul,
> -                          st->codecpar->width  * track->video.display_height * display_height_mul,
> -                          INT_MAX);
> +                if (track->video.display_width && track->video.display_height &&
> +                    st->codecpar->height * (int64_t)display_width_mul  < INT64_MAX / track->video.display_width &&
> +                    st->codecpar->width  * (int64_t)display_height_mul < INT64_MAX / track->video.display_height)

Why not move display_{width,height}_mul to the other side of the
comparison and avoid wordsize assumptions? This is header parsing, so
division performance impact should be negligible.

-- 
Anton Khirnov
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