rfc:rfc8251
Internet Engineering Task Force (IETF) JM. Valin Request for Comments: 8251 Mozilla Corporation Updates: 6716 K. Vos Category: Standards Track vocTone ISSN: 2070-1721 October 2017
Updates to the Opus Audio Codec
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 Appendix A of RFC 6716. The changes fix real and potential security-related issues, as well as minor quality-related issues.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8251.
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 (https://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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Valin & Vos Standards Track [Page 1] RFC 8251 Opus Update October 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Stereo State Reset in SILK . . . . . . . . . . . . . . . . . 3 4. Parsing of the Opus Packet Padding . . . . . . . . . . . . . 4 5. Resampler Buffer . . . . . . . . . . . . . . . . . . . . . . 4 6. Integer Wrap-Around in Inverse Gain Computation . . . . . . . 6 7. Integer Wrap-Around in LSF Decoding . . . . . . . . . . . . . 7 8. Cap on Band Energy . . . . . . . . . . . . . . . . . . . . . 7 9. Hybrid Folding . . . . . . . . . . . . . . . . . . . . . . . 8 10. Downmix to Mono . . . . . . . . . . . . . . . . . . . . . . . 9 11. New Test Vectors . . . . . . . . . . . . . . . . . . . . . . 9 12. Security Considerations . . . . . . . . . . . . . . . . . . . 11 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 14. Normative References . . . . . . . . . . . . . . . . . . . . 11 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
This document addresses minor issues that were discovered in the reference implementation of the Opus codec. Unlike most IETF specifications, RFC 6716 [RFC6716] defines Opus in terms of a normative reference decoder implementation rather than from the associated text description. Appendix A of that RFC includes the reference decoder implementation, which is 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 behavior in a way that requires new test vectors. Only the C implementation is affected, not the English text of the specification. This specification remains fully compatible with RFC 6716 [RFC6716].
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. Referenced line numbers are approximations. 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.
Valin & Vos Standards Track [Page 2] RFC 8251 Opus Update October 2017
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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 around 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 is too small to make the test vector check fail.
Valin & Vos Standards Track [Page 3] RFC 8251 Opus Update October 2017
4. Parsing of the Opus Packet Padding
It was discovered that some invalid packets of a 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 around line 596 of src/opus_decoder.c:
<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.
Valin & Vos Standards Track [Page 4] RFC 8251 Opus Update October 2017
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 around line 78 of silk/resampler_private_IIR_FIR.c:
<CODE BEGINS>
)
{
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;
Valin & Vos Standards Track [Page 5] RFC 8251 Opus Update October 2017
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 ], \ 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 around 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>
Valin & Vos Standards Track [Page 6] RFC 8251 Opus Update October 2017
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 Line Spectral Frequency (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 around line 137 of silk/ NLSF_stabilize.c prevents the problem:
<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 bitstreams, 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 around 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>
Valin & Vos Standards Track [Page 7] RFC 8251 Opus Update October 2017
9. Hybrid Folding
When encoding in hybrid mode at low bitrate, we sometimes only have enough bits to code a single Constrained-Energy Lapped Transform (CELT) band (8 - 9.6 kHz). When that happens, the second band (CELT band 18, from 9.6 - 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 Linear Congruential Generator (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:
<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>
Valin & Vos Standards Track [Page 8] RFC 8251 Opus Update October 2017
as well as around 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 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 canceled, 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., requested explicitly from an API).
11. New Test Vectors
Changes in Sections 9 and 10 have sufficient impact on the test
vectors 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.
Valin & Vos Standards Track [Page 9] RFC 8251 Opus Update October 2017
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:
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.
Valin & Vos Standards Track [Page 10] RFC 8251 Opus Update October 2017
12. Security Considerations
This document fixes two security issues reported on Opus 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 being accessible to the attacker. The update in Section 4 fixes this. CVE-2017-0381 could have resulted in a 16-bit out-of-bounds read from a fixed location. The update in Section 7 fixes this. Beyond the two fixed Common Vulnerabilities and Exposures (CVEs), this document adds no new security considerations beyond those in RFC 6716 [RFC6716].
13. IANA Considerations
This document does not require any IANA actions.
14. 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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.
Valin & Vos Standards Track [Page 11] RFC 8251 Opus Update October 2017
Authors' Addresses
Jean-Marc Valin Mozilla Corporation 331 E. Evelyn Avenue Mountain View, CA 94041 United States of America
Phone: +1 650 903-0800 Email: jmvalin@jmvalin.ca
Koen Vos vocTone
Email: koenvos74@gmail.com
Valin & Vos Standards Track [Page 12]
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