Re: [FFmpeg-devel] [PATCH 2/3] avcodec/wmalosslessdec: Check channel mask against num channels

[Michael Niedermayer <michael@niedermayer.cc>]

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On Sat, Mar 19, 2022 at 06:52:34PM +0100, Michael Niedermayer wrote:
> On Fri, Mar 18, 2022 at 08:38:28AM -0300, James Almer wrote:
> > 
> > 
> > On 3/18/2022 8:27 AM, Michael Niedermayer wrote:
> > > On Thu, Mar 17, 2022 at 10:00:18PM -0300, James Almer wrote:
> > > > 
> > > > 
> > > > On 3/17/2022 9:07 PM, James Almer wrote:
> > > > > 
> > > > > 
> > > > > On 3/17/2022 8:52 PM, James Almer wrote:
> > > > > > On 3/17/2022 8:30 PM, Michael Niedermayer wrote:
> > > > > > > Fixes: Out of array write
> > > > > > > Fixes: 45613/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-4539073606320128
> > > > > > > 
> > > > > > > 
> > > > > > > Found-by: continuous fuzzing process
> > > > > > > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > > > > > > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > > > > > > ---
> > > > > > >    libavcodec/wmalosslessdec.c | 3 +++
> > > > > > >    1 file changed, 3 insertions(+)
> > > > > > > 
> > > > > > > diff --git a/libavcodec/wmalosslessdec.c b/libavcodec/wmalosslessdec.c
> > > > > > > index cd05b22689..1728920729 100644
> > > > > > > --- a/libavcodec/wmalosslessdec.c
> > > > > > > +++ b/libavcodec/wmalosslessdec.c
> > > > > > > @@ -281,6 +281,9 @@ static av_cold int
> > > > > > > decode_init(AVCodecContext *avctx)
> > > > > > >        av_channel_layout_uninit(&avctx->ch_layout);
> > > > > > >        av_channel_layout_from_mask(&avctx->ch_layout, channel_mask);
> > > > > > > +    if (s->num_channels != avctx->ch_layout.nb_channels)
> > > > > > > +        return AVERROR_PATCHWELCOME; //are there non fuzzed
> > > > > > > files with this or is it an error ?
> > > > > > 
> > > > > > s->num_channels at this point is set to the channels count the user
> > > > > > set before calling avcodec_open2() (Normally from lavf), but it
> > > > > > could be anything.
> > > > > > If channel_mask is taken from extradata, maybe it should be used to
> > > > > > set s->num_channels instead of aborting because the user set value
> > > > > > and extradata disagreed.
> > > > > > 
> > > > > > Also, can you reproduce this crash before 3c933af493?
> > > > > > s->num_channels was being set to the user set channel count too,
> > > > > > same as now.
> > > > > 
> > > > > Right, before that commit s->num_channels and avctx->channels were
> > > > > always the same, but avctx->channel_layout was whatever came from
> > > > > extradata, and its popcnt could be != avctx->channels.
> > > > > After it, avctx->ch_layout.nb_channels is always the same as
> > > > > popcnt(avctx->ch_layout.u.mask), which can be different than
> > > > > s->num_channels.
> > > > > 
> > > > > I think my suggestion above to use the extradata channel mask and
> > > > > ignoring the user set channel count is the best approach for this.
> > > > 
> > > > Like this maybe (channel_mask could in theory be zero, so in that case the
> > > > user set value should be used).
> > > > 
> > > > > diff --git a/libavcodec/wmalosslessdec.c b/libavcodec/wmalosslessdec.c
> > > > > index cd05b22689..915add1962 100644
> > > > > --- a/libavcodec/wmalosslessdec.c
> > > > > +++ b/libavcodec/wmalosslessdec.c
> > > > > @@ -197,15 +197,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
> > > > >           return AVERROR_PATCHWELCOME;
> > > > >       }
> > > > > 
> > > > > -    s->max_frame_size = MAX_FRAMESIZE * avctx->ch_layout.nb_channels;
> > > > > -    s->frame_data = av_mallocz(s->max_frame_size + AV_INPUT_BUFFER_PADDING_SIZE);
> > > > > -    if (!s->frame_data)
> > > > > -        return AVERROR(ENOMEM);
> > > > > -
> > > > > -    s->avctx = avctx;
> > > > > -    ff_llauddsp_init(&s->dsp);
> > > > > -    init_put_bits(&s->pb, s->frame_data, s->max_frame_size);
> > > > > -
> > > > >       if (avctx->extradata_size >= 18) {
> > > > >           s->decode_flags    = AV_RL16(edata_ptr + 14);
> > > > >           channel_mask       = AV_RL32(edata_ptr +  2);
> > > > > @@ -230,6 +221,33 @@ static av_cold int decode_init(AVCodecContext *avctx)
> > > > >           return AVERROR_PATCHWELCOME;
> > > > >       }
> > > > > 
> > > > > +    if (channel_mask) {
> > > > > +        av_channel_layout_uninit(&avctx->ch_layout);
> > > > > +        av_channel_layout_from_mask(&avctx->ch_layout, channel_mask);
> > > > > +    } else
> > > > > +        avctx->ch_layout.order = AV_CHANNEL_ORDER_UNSPEC;
> > > > > +
> > > > > +    s->num_channels = avctx->ch_layout.nb_channels;
> > > > > +
> > > > > +    /* extract lfe channel position */
> > > > > +    s->lfe_channel = -1;
> > > > > +
> > > > > +    if (channel_mask & 8) {
> > > > > +        unsigned int mask;
> > > > > +        for (mask = 1; mask < 16; mask <<= 1)
> > > > > +            if (channel_mask & mask)
> > > > > +                ++s->lfe_channel;
> > > > > +    }
> > > > > +
> > > > > +    s->max_frame_size = MAX_FRAMESIZE * avctx->ch_layout.nb_channels;
> > > > > +    s->frame_data = av_mallocz(s->max_frame_size + AV_INPUT_BUFFER_PADDING_SIZE);
> > > > > +    if (!s->frame_data)
> > > > > +        return AVERROR(ENOMEM);
> > > > > +
> > > > > +    s->avctx = avctx;
> > > > > +    ff_llauddsp_init(&s->dsp);
> > > > > +    init_put_bits(&s->pb, s->frame_data, s->max_frame_size);
> > > > > +
> > > > >       /* generic init */
> > > > >       s->log2_frame_size = av_log2(avctx->block_align) + 4;
> > > > > 
> > > > > @@ -263,24 +281,10 @@ static av_cold int decode_init(AVCodecContext *avctx)
> > > > >           return AVERROR_INVALIDDATA;
> > > > >       }
> > > > > 
> > > > > -    s->num_channels = avctx->ch_layout.nb_channels;
> > > > > -
> > > > > -    /* extract lfe channel position */
> > > > > -    s->lfe_channel = -1;
> > > > > -
> > > > > -    if (channel_mask & 8) {
> > > > > -        unsigned int mask;
> > > > > -        for (mask = 1; mask < 16; mask <<= 1)
> > > > > -            if (channel_mask & mask)
> > > > > -                ++s->lfe_channel;
> > > > > -    }
> > > > > -
> > > > >       s->frame = av_frame_alloc();
> > > > >       if (!s->frame)
> > > > >           return AVERROR(ENOMEM);
> > > > > 
> > > > > -    av_channel_layout_uninit(&avctx->ch_layout);
> > > > > -    av_channel_layout_from_mask(&avctx->ch_layout, channel_mask);
> > > > >       return 0;
> > > > >   }
> > > 
> > > this will change the output of the "lossless" decoder if this case ever
> > > occurs besides this
> > 
> > As i interpret it, you could initialize the decoder using any arbitrary
> > number of channels in avctx. And if channel_mask in extradata is not zero,
> > then it must be considered to be the valid channel count for the stream, and
> > the value passed in avctx should be ignored.
> 
> without a specification and no non fuzzed testfile its really not possible
> to say for sure what this combination is intended to produce on the output
> 
> What you say sounds reasonable but it can also be something else

ping, i think your patch or something similar should be applied
The fuzzer found
46008/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-4681245747970048
which too crashes because of the channel number

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you fake or manipulate statistics in a paper in physics you will never
get a job again.
If you fake or manipulate statistics in a paper in medicin you will get
a job for life at the pharma industry.

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