Internet Engineering Task Force (IETF) JM. Valin

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.

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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.

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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.

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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;

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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>

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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>

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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>

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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.

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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.

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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: [email protected]

Koen Vos
vocTone

Email: [email protected]

Valin & Vos Standards Track [Page 12]