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Versions: (draft-valin-codec-opus-update) 00 01 02 03 04 05 06 07 08 09 10 RFC 8251

Network Working Group                                          JM. Valin
Internet-Draft                                       Mozilla Corporation
Updates: 6716 (if approved)                                       K. Vos
Intended status: Standards Track                                 vocTone
Expires: February 25, 2018                               August 24, 2017


                    Updates to the Opus Audio Codec
                    draft-ietf-codec-opus-update-10

Abstract

   This document addresses minor issues that were found in the
   specification of the Opus audio codec in RFC 6716.  It updates the
   normative decoder implementation included in the appendix of RFC
   6716.  The changes fixes real and potential security-related issues,
   as well minor quality-related issues.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 25, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Stereo State Reset in SILK  . . . . . . . . . . . . . . . . .   3
   4.  Parsing of the Opus Packet Padding  . . . . . . . . . . . . .   3
   5.  Resampler buffer  . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Integer wrap-around in inverse gain computation . . . . . . .   6
   7.  Integer wrap-around in LSF decoding . . . . . . . . . . . . .   6
   8.  Cap on Band Energy  . . . . . . . . . . . . . . . . . . . . .   7
   9.  Hybrid Folding  . . . . . . . . . . . . . . . . . . . . . . .   7
   10. Downmix to Mono . . . . . . . . . . . . . . . . . . . . . . .   9
   11. New Test Vectors  . . . . . . . . . . . . . . . . . . . . . .   9
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This document addresses minor issues that were discovered in the
   reference implementation of the Opus codec.  Unlike most IETF
   specifications, Opus is defined in RFC 6716 [RFC6716] in terms of a
   normative reference decoder implementation rather than from the
   associated text description.  That RFC includes the reference decoder
   implementation as Appendix A.  That's why only issues affecting the
   decoder are listed here.  An up-to-date implementation of the Opus
   encoder can be found at <https://opus-codec.org/>.

   Some of the changes in this document update normative behaviour in a
   way that requires new test vectors.  The English text of the
   specification is unaffected, only the C implementation is.  The
   updated specification remains fully compatible with the original
   specification.

   Note: due to RFC formatting conventions, lines exceeding the column
   width in the patch are split using a backslash character.  The
   backslashes at the end of a line and the white space at the beginning
   of the following line are not part of the patch.  A properly
   formatted patch including all changes is available at
   <https://www.ietf.org/proceedings/98/slides/materials-98-codec-opus-
   update-00.patch> and has a SHA-1 hash of
   029e3aa88fc342c91e67a21e7bfbc9458661cd5f.




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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Stereo State Reset in SILK

   The reference implementation does not reinitialize the stereo state
   during a mode switch.  The old stereo memory can produce a brief
   impulse (i.e. single sample) in the decoded audio.  This can be fixed
   by changing silk/dec_API.c at line 72:

   <CODE BEGINS>
        for( n = 0; n < DECODER_NUM_CHANNELS; n++ ) {
            ret  = silk_init_decoder( &channel_state[ n ] );
        }
   +    silk_memset(&((silk_decoder *)decState)->sStereo, 0,
   +                sizeof(((silk_decoder *)decState)->sStereo));
   +    /* Not strictly needed, but it's cleaner that way */
   +    ((silk_decoder *)decState)->prev_decode_only_middle = 0;

        return ret;
    }
   <CODE ENDS>

   This change affects the normative output of the decoder, but the
   amount of change is within the tolerance and too small to make the
   testvector check fail.

4.  Parsing of the Opus Packet Padding

   It was discovered that some invalid packets of very large size could
   trigger an out-of-bounds read in the Opus packet parsing code
   responsible for padding.  This is due to an integer overflow if the
   signaled padding exceeds 2^31-1 bytes (the actual packet may be
   smaller).  The code can be fixed by decrementing the (signed) len
   value, instead of incrementing a separate padding counter.  This is
   done by applying the following changes at line 596 of src/
   opus_decoder.c:











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   <CODE BEGINS>
          /* Padding flag is bit 6 */
          if (ch&0x40)
          {
   -         int padding=0;
             int p;
             do {
                if (len<=0)
                   return OPUS_INVALID_PACKET;
                p = *data++;
                len--;
   -            padding += p==255 ? 254: p;
   +            len -= p==255 ? 254: p;
             } while (p==255);
   -         len -= padding;
          }
   <CODE ENDS>

   This packet parsing issue is limited to reading memory up to about 60
   kB beyond the compressed buffer.  This can only be triggered by a
   compressed packet more than about 16 MB long, so it's not a problem
   for RTP.  In theory, it could crash a file decoder (e.g.  Opus in
   Ogg) if the memory just after the incoming packet is out-of-range,
   but our attempts to trigger such a crash in a production application
   built using an affected version of the Opus decoder failed.

5.  Resampler buffer

   The SILK resampler had the following issues:

   1.  The calls to memcpy() were using sizeof(opus_int32), but the type
       of the local buffer was opus_int16.

   2.  Because the size was wrong, this potentially allowed the source
       and destination regions of the memcpy() to overlap on the copy
       from "buf" to "buf".  We believe that nSamplesIn (number of input
       samples) is at least fs_in_khZ (sampling rate in kHz), which is
       at least 8.  Since RESAMPLER_ORDER_FIR_12 is only 8, that should
       not be a problem once the type size is fixed.

   3.  The size of the buffer used RESAMPLER_MAX_BATCH_SIZE_IN, but the
       data stored in it was actually twice the input batch size
       (nSamplesIn<<1).

   The code can be fixed by applying the following changes to line 78 of
   silk/resampler_private_IIR_FIR.c:

   <CODE BEGINS>



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    )
    {
        silk_resampler_state_struct *S = \
   (silk_resampler_state_struct *)SS;
        opus_int32 nSamplesIn;
        opus_int32 max_index_Q16, index_increment_Q16;
   -    opus_int16 buf[ RESAMPLER_MAX_BATCH_SIZE_IN + \
   RESAMPLER_ORDER_FIR_12 ];
   +    opus_int16 buf[ 2*RESAMPLER_MAX_BATCH_SIZE_IN + \
   RESAMPLER_ORDER_FIR_12 ];

        /* Copy buffered samples to start of buffer */
   -    silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 \
   * sizeof( opus_int32 ) );
   +    silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 \
   * sizeof( opus_int16 ) );

        /* Iterate over blocks of frameSizeIn input samples */
        index_increment_Q16 = S->invRatio_Q16;
        while( 1 ) {
            nSamplesIn = silk_min( inLen, S->batchSize );

            /* Upsample 2x */
            silk_resampler_private_up2_HQ( S->sIIR, &buf[ \
   RESAMPLER_ORDER_FIR_12 ], in, nSamplesIn );

            max_index_Q16 = silk_LSHIFT32( nSamplesIn, 16 + 1 \
   );         /* + 1 because 2x upsampling */
            out = silk_resampler_private_IIR_FIR_INTERPOL( out, \
   buf, max_index_Q16, index_increment_Q16 );
            in += nSamplesIn;
            inLen -= nSamplesIn;

            if( inLen > 0 ) {
                /* More iterations to do; copy last part of \
   filtered signal to beginning of buffer */
   -            silk_memcpy( buf, &buf[ nSamplesIn << 1 ], \
   RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
   +            silk_memmove( buf, &buf[ nSamplesIn << 1 ], \
   RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
            } else {
                break;
            }
        }

        /* Copy last part of filtered signal to the state for \
   the next call */
   -    silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], \



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   RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
   +    silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], \
   RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
    }
   <CODE ENDS>

6.  Integer wrap-around in inverse gain computation

   It was discovered through decoder fuzzing that some bitstreams could
   produce integer values exceeding 32-bits in
   LPC_inverse_pred_gain_QA(), causing a wrap-around.  The C standard
   considers this behavior as undefined.  The following patch to line 87
   of silk/LPC_inv_pred_gain.c detects values that do not fit in a
   32-bit integer and considers the corresponding filters unstable:

  <CODE BEGINS>
           /* Update AR coefficient */
           for( n = 0; n < k; n++ ) {
  -            tmp_QA = Aold_QA[ n ] - MUL32_FRAC_Q( \
  Aold_QA[ k - n - 1 ], rc_Q31, 31 );
  -            Anew_QA[ n ] = MUL32_FRAC_Q( tmp_QA, rc_mult2 , mult2Q );
  +            opus_int64 tmp64;
  +            tmp_QA = silk_SUB_SAT32( Aold_QA[ n ], MUL32_FRAC_Q( \
  Aold_QA[ k - n - 1 ], rc_Q31, 31 ) );
  +            tmp64 = silk_RSHIFT_ROUND64( silk_SMULL( tmp_QA, \
  rc_mult2 ), mult2Q);
  +            if( tmp64 > silk_int32_MAX || tmp64 < silk_int32_MIN ) {
  +               return 0;
  +            }
  +            Anew_QA[ n ] = ( opus_int32 )tmp64;
           }
  <CODE ENDS>

7.  Integer wrap-around in LSF decoding

   It was discovered -- also from decoder fuzzing -- that an integer
   wrap-around could occur when decoding bitstreams with extremely large
   values for the high LSF parameters.  The end result of the wrap-
   around is an illegal read access on the stack, which the authors do
   not believe is exploitable but should nonetheless be fixed.  The
   following patch to line 137 of silk/NLSF_stabilize.c prevents the
   problem:









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   <CODE BEGINS>
              /* Keep delta_min distance between the NLSFs */
            for( i = 1; i < L; i++ )
   -            NLSF_Q15[i] = silk_max_int( NLSF_Q15[i], \
   NLSF_Q15[i-1] + NDeltaMin_Q15[i] );
   +            NLSF_Q15[i] = silk_max_int( NLSF_Q15[i], \
   silk_ADD_SAT16( NLSF_Q15[i-1], NDeltaMin_Q15[i] ) );

            /* Last NLSF should be no higher than 1 - NDeltaMin[L] */
   <CODE ENDS>

8.  Cap on Band Energy

   On extreme bit-streams, it is possible for log-domain band energy
   levels to exceed the maximum single-precision floating point value
   once converted to a linear scale.  This would later cause the decoded
   values to be NaN (not a number), possibly causing problems in the
   software using the PCM values.  This can be avoided with the
   following patch to line 552 of celt/quant_bands.c:

   <CODE BEGINS>
          {
             opus_val16 lg = ADD16(oldEBands[i+c*m->nbEBands],
                             SHL16((opus_val16)eMeans[i],6));
   +         lg = MIN32(QCONST32(32.f, 16), lg);
             eBands[i+c*m->nbEBands] = PSHR32(celt_exp2(lg),4);
          }
          for (;i<m->nbEBands;i++)
   <CODE ENDS>

9.  Hybrid Folding

   When encoding in hybrid mode at low bitrate, we sometimes only have
   enough bits to code a single CELT band (8 - 9.6 kHz).  When that
   happens, the second band (CELT band 18, from 9.6 to 12 kHz) cannot
   use folding because it is wider than the amount already coded, and
   falls back to white noise.  Because it can also happen on transients
   (e.g. stops), it can cause audible pre-echo.

   To address the issue, we change the folding behavior so that it is
   never forced to fall back to LCG due to the first band not containing
   enough coefficients to fold onto the second band.  This is achieved
   by simply repeating part of the first band in the folding of the
   second band.  This changes the code in celt/bands.c around line 1237:







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  <CODE BEGINS>
            b = 0;
         }

  -      if (resynth && M*eBands[i]-N >= M*eBands[start] && \
  (update_lowband || lowband_offset==0))
  +      if (resynth && (M*eBands[i]-N >= M*eBands[start] || \
  i==start+1) && (update_lowband || lowband_offset==0))
               lowband_offset = i;

  +      if (i == start+1)
  +      {
  +         int n1, n2;
  +         int offset;
  +         n1 = M*(eBands[start+1]-eBands[start]);
  +         n2 = M*(eBands[start+2]-eBands[start+1]);
  +         offset = M*eBands[start];
  +         /* Duplicate enough of the first band folding data to \
  be able to fold the second band.
  +            Copies no data for CELT-only mode. */
  +         OPUS_COPY(&norm[offset+n1], &norm[offset+2*n1 - n2], n2-n1);
  +         if (C==2)
  +            OPUS_COPY(&norm2[offset+n1], &norm2[offset+2*n1 - n2], \
  n2-n1);
  +      }
  +
         tf_change = tf_res[i];
         if (i>=m->effEBands)
         {
  <CODE ENDS>

   as well as line 1260:

   <CODE BEGINS>
             fold_start = lowband_offset;
             while(M*eBands[--fold_start] > effective_lowband);
             fold_end = lowband_offset-1;
   -         while(M*eBands[++fold_end] < effective_lowband+N);
   +         while(++fold_end < i && M*eBands[fold_end] < \
   effective_lowband+N);
             x_cm = y_cm = 0;
             fold_i = fold_start; do {
               x_cm |= collapse_masks[fold_i*C+0];

   <CODE ENDS>

   The fix does not impact compatibility, because the improvement does
   not depend on the encoder doing anything special.  There is also no



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   reasonable way for an encoder to use the original behavior to improve
   quality over the proposed change.

10.  Downmix to Mono

   The last issue is not strictly a bug, but it is an issue that has
   been reported when downmixing an Opus decoded stream to mono, whether
   this is done inside the decoder or as a post-processing step on the
   stereo decoder output.  Opus intensity stereo allows optionally
   coding the two channels 180-degrees out of phase on a per-band basis.
   This provides better stereo quality than forcing the two channels to
   be in phase, but when the output is downmixed to mono, the energy in
   the affected bands is cancelled sometimes resulting in audible
   artifacts.

   As a work-around for this issue, the decoder MAY choose not to apply
   the 180-degree phase shift.  This can be useful when downmixing to
   mono inside or outside of the decoder (e.g. user-controllable).

11.  New Test Vectors

   Changes in Section 9 and Section 10 have sufficient impact on the
   testvectors to make them fail.  For this reason, this document also
   updates the Opus test vectors.  The new test vectors now include two
   decoded outputs for the same bitstream.  The outputs with suffix 'm'
   do not apply the CELT 180-degree phase shift as allowed in
   Section 10, while the outputs without the suffix do.  An
   implementation is compliant as long as it passes either set of
   vectors.

   Any Opus implementation that passes either the original test vectors
   from RFC 6716 [RFC6716] or one of the new sets of test vectors is
   compliant with the Opus specification.  However, newer
   implementations SHOULD be based on the new test vectors rather than
   the old ones.

   The new test vectors are located at
   <https://www.ietf.org/proceedings/98/slides/materials-98-codec-opus-
   newvectors-00.tar.gz>.  The SHA-1 hashes of the test vectors are:












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   e49b2862ceec7324790ed8019eb9744596d5be01  testvector01.bit
   b809795ae1bcd606049d76de4ad24236257135e0  testvector02.bit
   e0c4ecaeab44d35a2f5b6575cd996848e5ee2acc  testvector03.bit
   a0f870cbe14ebb71fa9066ef3ee96e59c9a75187  testvector04.bit
   9b3d92b48b965dfe9edf7b8a85edd4309f8cf7c8  testvector05.bit
   28e66769ab17e17f72875283c14b19690cbc4e57  testvector06.bit
   bacf467be3215fc7ec288f29e2477de1192947a6  testvector07.bit
   ddbe08b688bbf934071f3893cd0030ce48dba12f  testvector08.bit
   3932d9d61944dab1201645b8eeaad595d5705ecb  testvector09.bit
   521eb2a1e0cc9c31b8b740673307c2d3b10c1900  testvector10.bit
   6bc8f3146fcb96450c901b16c3d464ccdf4d5d96  testvector11.bit
   338c3f1b4b97226bc60bc41038becbc6de06b28f  testvector12.bit
   f5ef93884da6a814d311027918e9afc6f2e5c2c8  testvector01.dec
   48ac1ff1995250a756e1e17bd32acefa8cd2b820  testvector02.dec
   d15567e919db2d0e818727092c0af8dd9df23c95  testvector03.dec
   1249dd28f5bd1e39a66fd6d99449dca7a8316342  testvector04.dec
   b85675d81deef84a112c466cdff3b7aaa1d2fc76  testvector05.dec
   55f0b191e90bfa6f98b50d01a64b44255cb4813e  testvector06.dec
   61e8b357ab090b1801eeb578a28a6ae935e25b7b  testvector07.dec
   a58539ee5321453b2ddf4c0f2500e856b3966862  testvector08.dec
   bb96aad2cde188555862b7bbb3af6133851ef8f4  testvector09.dec
   1b6cdf0413ac9965b16184b1bea129b5c0b2a37a  testvector10.dec
   b1fff72b74666e3027801b29dbc48b31f80dee0d  testvector11.dec
   98e09bbafed329e341c3b4052e9c4ba5fc83f9b1  testvector12.dec
   1e7d984ea3fbb16ba998aea761f4893fbdb30157  testvector01m.dec
   48ac1ff1995250a756e1e17bd32acefa8cd2b820  testvector02m.dec
   d15567e919db2d0e818727092c0af8dd9df23c95  testvector03m.dec
   1249dd28f5bd1e39a66fd6d99449dca7a8316342  testvector04m.dec
   d70b0bad431e7d463bc3da49bd2d49f1c6d0a530  testvector05m.dec
   6ac1648c3174c95fada565161a6c78bdbe59c77d  testvector06m.dec
   fc5e2f709693738324fb4c8bdc0dad6dda04e713  testvector07m.dec
   aad2ba397bf1b6a18e8e09b50e4b19627d479f00  testvector08m.dec
   6feb7a7b9d7cdc1383baf8d5739e2a514bd0ba08  testvector09m.dec
   1b6cdf0413ac9965b16184b1bea129b5c0b2a37a  testvector10m.dec
   fd3d3a7b0dfbdab98d37ed9aa04b659b9fefbd18  testvector11m.dec
   98e09bbafed329e341c3b4052e9c4ba5fc83f9b1  testvector12m.dec


   Note that the decoder input bitstream files (.bit) are unchanged.

12.  Security Considerations

   This document fixes two security issues reported on Opus and that
   affect the reference implementation in RFC 6716 [RFC6716]: CVE-
   2013-0899 <https://nvd.nist.gov/vuln/detail/CVE-2013-0899> and CVE-
   2017-0381 <https://nvd.nist.gov/vuln/detail/CVE-2017-0381>.  CVE-
   2013-0899 theoretically could have caused an information leak.  The
   leaked information would have gone through the decoder process before



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   being accessible to the attacker.  It is fixed by Section 4.  CVE-
   2017-0381 could have resulted in a 16-bit out-of-bounds read from a
   fixed location.  It is fixed in Section 7.  Beyond the two fixed
   CVEs, this document adds no new security considerations on top of RFC
   6716 [RFC6716].

13.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

14.  Acknowledgements

   We would like to thank Juri Aedla for reporting the issue with the
   parsing of the Opus padding.  Thanks to Felicia Lim for reporting the
   LSF integer overflow issue.  Also, thanks to Tina le Grand, Jonathan
   Lennox, and Mark Harris for their feedback on this document.

15.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
              September 2012, <https://www.rfc-editor.org/info/rfc6716>.

Authors' Addresses

   Jean-Marc Valin
   Mozilla Corporation
   331 E. Evelyn Avenue
   Mountain View, CA  94041
   USA

   Phone: +1 650 903-0800
   Email: jmvalin@jmvalin.ca


   Koen Vos
   vocTone

   Email: koenvos74@gmail.com




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