[FFmpeg-devel] [PATCH] avutil/channel_layout: don't error out on truncated strings

[James Almer]

On 7/3/2022 7:00 AM, Nicolas George wrote:
> Andreas Rheinhardt (12022-07-03):
>>>               if (!av_bprint_is_complete(bp))
>>> -                return AVERROR(ENOMEM);
>>> +                break;
>>>   
> 
>> Isn't this actually still against the API? av_channel_layout_describe()
>> will not return the correct number of bytes necessary to write the
>> string for the channel layout.
> 
> You are both right.
> 
> BPrint-based APIs are not supposed to check for truncation, because
> printing into a bounded buffer to determine the necessary size is a
> valid use (see AV_BPRINT_SIZE_COUNT_ONLY).
> 
> What is wrong is Michael's original fix:
> 
>>> commit 8154cb7c2ff2afcb1a0842de8c215b7714c814d0
>>> Author: Michael Niedermayer <michael at niedermayer.cc>
>>> Date:   2022-06-30 00:00:32 +0200
>>>
>>>      avutil/channel_layout: av_channel_layout_describe_bprint: Check for buffer end
>>>
>>>      Fixes: Timeout printing a billion channels
>>>      Fixes: 48099/clusterfuzz-testcase-minimized-ffmpeg_dem_MATROSKA_fuzzer-6754782204788736
>>>
>>>      Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>>>      Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>>
>>> diff --git a/libavutil/channel_layout.c b/libavutil/channel_layout.c
>>> index 21b70173b7..1887987789 100644
>>> --- a/libavutil/channel_layout.c
>>> +++ b/libavutil/channel_layout.c
>>> @@ -757,6 +757,10 @@ int av_channel_layout_describe_bprint(const AVChannelLayout *channel_layout,
>>>               if (channel_layout->order == AV_CHANNEL_ORDER_CUSTOM &&
> 
>>>                   channel_layout->u.map[i].name[0])
> 
> If the channel count is insanely high, then this will cause invalid
> reads.
> 
>>>                   av_bprintf(bp, "@%s", channel_layout->u.map[i].name);
>>> +
>>> +            if (!av_bprint_is_complete(bp))
>>> +                return AVERROR(ENOMEM);
>>> +
>>>           }
>>>           if (channel_layout->nb_channels) {
>>>               av_bprintf(bp, ")");
> 
> Obviously, this fuzzer found a case where a demuxer or a decoder
> constructs an invalid channel layout in memory without proper
> validation. There is a bug, possibly an security-related one, and this
> only hides it from the test suite.

The Matroska demuxer could in theory generate a native layout with more 
than 64 channels where popcnt(mask) != channels, and nothing seems to 
validate what a demuxer's read_header() callback returns if you just 
call lavf API functions like target_dem_fuzzer.c seems to do.

Maybe:

> diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
> index 73ded761fd..ad7ee390a2 100644
> --- a/libavformat/matroskadec.c
> +++ b/libavformat/matroskadec.c
> @@ -2950,10 +2950,10 @@ static int matroska_parse_tracks(AVFormatContext *s)
>              st->codecpar->codec_tag   = fourcc;
>              st->codecpar->sample_rate = track->audio.out_samplerate;
>              // channel layout may be already set by codec private checks above
> -            if (st->codecpar->ch_layout.order == AV_CHANNEL_ORDER_NATIVE &&
> -                !st->codecpar->ch_layout.u.mask)
> +            if (!av_channel_layout_check(&st->codecpar->ch_layout)) {
>                  st->codecpar->ch_layout.order = AV_CHANNEL_ORDER_UNSPEC;
> -            st->codecpar->ch_layout.nb_channels = track->audio.channels;
> +                st->codecpar->ch_layout.nb_channels = track->audio.channels;
> +            }
>              if (!st->codecpar->bits_per_coded_sample)
>                  st->codecpar->bits_per_coded_sample = track->audio.bitdepth;
>              if (st->codecpar->codec_id == AV_CODEC_ID_MP3 ||

is enough to ensure a valid layout is propagated. This assumes 
parameters set by codec private parsing are correct (only FLAC seem to 
be present right now, and it is).

Assuming i got this right, in this fuzzing sample's case it would still 
have a billion channels (since that's what track->audio.channels 
contains, as read from the container), but using the unspec layout, 
which is technically valid even if nothing will really handle it, and 
av_channel_layout_describe() will print a small string.

Still, i think a check in avformat_open_input() might also be a good 
idea, especially once (and if) demuxers start propagating custom 
layouts, where the map array will be allocated.

> 
> The proper fix is to find where the input is improperly validated, and
> then revert this change.
> 
> I do not know how to exploit the
> "48099/clusterfuzz-testcase-minimized-ffmpeg_dem_MATROSKA_fuzzer-6754782204788736"
> information to reproduce the bug and investigate.

Michael can share the sample, but you need to compile 
tools/target_dem_fuzzer.c to read it.

> 
> Regards,
> 
> 
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