[FFmpeg-devel,v5] libavformat/flacdec: Workaround for truncated metadata picture size

Some flac muxers write truncated metadata picture size if the picture
data do not fit in 24 bits. Detect this by truncting the size found inside
the picture block and if it matches the block size use it and read rest
of picture data.

This workaround is only for flac files and not ogg files with flac
METADATA_BLOCK_PICTURE comments and it can be disabled with strict level
above normal. Currently there is a 500MB limit on truncate size to protect
from large memory allocations.

The truncation bug in lavf flacenc was fixed in e447a4d112bcfee10126c54eb4481fa8712957c8
but based on existing broken files other unknown flac muxers seems to truncate also.
Before the fix a broken flac file for reproduction could be generated with:
ffmpeg -f lavfi -i sine -f lavfi -i color=red:size=2400x2400 -map 0:0 -map 1:0 -c:v:0 bmp -disposition:1 attached_pic -t 1 test.flac

Fixes ticket 6333
---
 libavformat/flac_picture.c   | 47 ++++++++++++++++++++++++++++++------
 libavformat/flac_picture.h   |  2 +-
 libavformat/flacdec.c        |  2 +-
 libavformat/oggparsevorbis.c |  2 +-
 4 files changed, 42 insertions(+), 11 deletions(-)

Hello, ok to merge?

On Tue, May 19, 2020 at 11:27 AM Mattias Wadman
<mattias.wadman@gmail.com> wrote:
>
> Some flac muxers write truncated metadata picture size if the picture
> data do not fit in 24 bits. Detect this by truncting the size found inside
> the picture block and if it matches the block size use it and read rest
> of picture data.
>
> This workaround is only for flac files and not ogg files with flac
> METADATA_BLOCK_PICTURE comments and it can be disabled with strict level
> above normal. Currently there is a 500MB limit on truncate size to protect
> from large memory allocations.
>
> The truncation bug in lavf flacenc was fixed in e447a4d112bcfee10126c54eb4481fa8712957c8
> but based on existing broken files other unknown flac muxers seems to truncate also.
> Before the fix a broken flac file for reproduction could be generated with:
> ffmpeg -f lavfi -i sine -f lavfi -i color=red:size=2400x2400 -map 0:0 -map 1:0 -c:v:0 bmp -disposition:1 attached_pic -t 1 test.flac
>
> Fixes ticket 6333
> ---
>  libavformat/flac_picture.c   | 47 ++++++++++++++++++++++++++++++------
>  libavformat/flac_picture.h   |  2 +-
>  libavformat/flacdec.c        |  2 +-
>  libavformat/oggparsevorbis.c |  2 +-
>  4 files changed, 42 insertions(+), 11 deletions(-)
>
> diff --git a/libavformat/flac_picture.c b/libavformat/flac_picture.c
> index 81ddf80465..53e24b28b7 100644
> --- a/libavformat/flac_picture.c
> +++ b/libavformat/flac_picture.c
> @@ -27,7 +27,9 @@
>  #include "id3v2.h"
>  #include "internal.h"
>
> -int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size)
> +#define MAX_TRUNC_PICTURE_SIZE (500 * 1024 * 1024)
> +
> +int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size, int truncate_workaround)
>  {
>      const CodecMime *mime = ff_id3v2_mime_tags;
>      enum AVCodecID id = AV_CODEC_ID_NONE;
> @@ -36,7 +38,8 @@ int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size)
>      GetByteContext g;
>      AVStream *st;
>      int width, height, ret = 0;
> -    unsigned int len, type;
> +    unsigned int type;
> +    uint32_t len, left, trunclen = 0;
>
>      if (buf_size < 34) {
>          av_log(s, AV_LOG_ERROR, "Attached picture metadata block too short\n");
> @@ -114,16 +117,44 @@ int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size)
>
>      /* picture data */
>      len = bytestream2_get_be32u(&g);
> -    if (len <= 0 || len > bytestream2_get_bytes_left(&g)) {
> -        av_log(s, AV_LOG_ERROR, "Attached picture metadata block too short\n");
> -        if (s->error_recognition & AV_EF_EXPLODE)
> -            ret = AVERROR_INVALIDDATA;
> -        goto fail;
> +
> +    left = bytestream2_get_bytes_left(&g);
> +    if (len <= 0 || len > left) {
> +        if (len > MAX_TRUNC_PICTURE_SIZE || len >= INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE) {
> +            av_log(s, AV_LOG_ERROR, "Attached picture metadata block too big %u\n", len);
> +            if (s->error_recognition & AV_EF_EXPLODE)
> +                ret = AVERROR_INVALIDDATA;
> +            goto fail;
> +        }
> +
> +        // Workaround bug for flac muxers that writs truncated metadata picture block size if
> +        // the picture size do not fit in 24 bits. lavf flacenc used to have the issue and based
> +        // on existing broken files other unknown flac muxers seems to truncate also.
> +        if (truncate_workaround &&
> +            s->strict_std_compliance <= FF_COMPLIANCE_NORMAL &&
> +            len > left && (len & 0xffffff) == left) {
> +            av_log(s, AV_LOG_INFO, "Correcting truncated metadata picture size from %u to %u\n", left, len);
> +            trunclen = len - left;
> +        } else {
> +            av_log(s, AV_LOG_ERROR, "Attached picture metadata block too short\n");
> +            if (s->error_recognition & AV_EF_EXPLODE)
> +                ret = AVERROR_INVALIDDATA;
> +            goto fail;
> +        }
>      }
>      if (!(data = av_buffer_alloc(len + AV_INPUT_BUFFER_PADDING_SIZE))) {
>          RETURN_ERROR(AVERROR(ENOMEM));
>      }
> -    bytestream2_get_bufferu(&g, data->data, len);
> +
> +    if (trunclen == 0) {
> +        bytestream2_get_bufferu(&g, data->data, len);
> +    } else {
> +        // If truncation was detected copy all data from block and read missing bytes
> +        // not included in the block size
> +        bytestream2_get_bufferu(&g, data->data, left);
> +        if (avio_read(s->pb, data->data + len - trunclen, trunclen) < trunclen)
> +            RETURN_ERROR(AVERROR_INVALIDDATA);
> +    }
>      memset(data->data + len, 0, AV_INPUT_BUFFER_PADDING_SIZE);
>
>      if (AV_RB64(data->data) == PNGSIG)
> diff --git a/libavformat/flac_picture.h b/libavformat/flac_picture.h
> index 4374b6f4f6..61fd0c8806 100644
> --- a/libavformat/flac_picture.h
> +++ b/libavformat/flac_picture.h
> @@ -26,6 +26,6 @@
>
>  #define RETURN_ERROR(code) do { ret = (code); goto fail; } while (0)
>
> -int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size);
> +int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size, int truncate_workaround);
>
>  #endif /* AVFORMAT_FLAC_PICTURE_H */
> diff --git a/libavformat/flacdec.c b/libavformat/flacdec.c
> index cb516fb1f3..79c05f14bf 100644
> --- a/libavformat/flacdec.c
> +++ b/libavformat/flacdec.c
> @@ -146,7 +146,7 @@ static int flac_read_header(AVFormatContext *s)
>              }
>              av_freep(&buffer);
>          } else if (metadata_type == FLAC_METADATA_TYPE_PICTURE) {
> -            ret = ff_flac_parse_picture(s, buffer, metadata_size);
> +            ret = ff_flac_parse_picture(s, buffer, metadata_size, 1);
>              av_freep(&buffer);
>              if (ret < 0) {
>                  av_log(s, AV_LOG_ERROR, "Error parsing attached picture.\n");
> diff --git a/libavformat/oggparsevorbis.c b/libavformat/oggparsevorbis.c
> index 27d2c686b6..6f15204ada 100644
> --- a/libavformat/oggparsevorbis.c
> +++ b/libavformat/oggparsevorbis.c
> @@ -165,7 +165,7 @@ int ff_vorbis_comment(AVFormatContext *as, AVDictionary **m,
>                  av_freep(&tt);
>                  av_freep(&ct);
>                  if (ret > 0)
> -                    ret = ff_flac_parse_picture(as, pict, ret);
> +                    ret = ff_flac_parse_picture(as, pict, ret, 0);
>                  av_freep(&pict);
>                  if (ret < 0) {
>                      av_log(as, AV_LOG_WARNING, "Failed to parse cover art block.\n");
> --
> 2.18.0
>