514 lines
20 KiB
XML
514 lines
20 KiB
XML
<?xml version="1.0" encoding="US-ASCII"?>
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
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<?rfc toc="yes"?>
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<?rfc tocompact="yes"?>
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<?rfc tocdepth="3"?>
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<?rfc tocindent="yes"?>
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<?rfc symrefs="yes"?>
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<?rfc sortrefs="yes"?>
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<?rfc comments="yes"?>
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<?rfc inline="yes"?>
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<?rfc compact="yes"?>
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<?rfc subcompact="no"?>
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<rfc category="std" docName="draft-ietf-codec-opus-update-10"
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ipr="trust200902" updates="6716">
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<front>
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<title abbrev="Opus Update">Updates to the Opus Audio Codec</title>
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<author initials="JM" surname="Valin" fullname="Jean-Marc Valin">
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<organization>Mozilla Corporation</organization>
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<address>
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<postal>
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<street>331 E. Evelyn Avenue</street>
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<city>Mountain View</city>
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<region>CA</region>
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<code>94041</code>
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<country>USA</country>
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</postal>
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<phone>+1 650 903-0800</phone>
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<email>jmvalin@jmvalin.ca</email>
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</address>
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</author>
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<author initials="K." surname="Vos" fullname="Koen Vos">
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<organization>vocTone</organization>
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<address>
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<postal>
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<street></street>
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<city></city>
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<region></region>
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<code></code>
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<country></country>
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</postal>
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<phone></phone>
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<email>koenvos74@gmail.com</email>
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</address>
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</author>
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<date day="24" month="August" year="2017" />
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<abstract>
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<t>This document addresses minor issues that were found in the specification
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of the Opus audio codec in RFC 6716. It updates the normative decoder implementation
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included in the appendix of RFC 6716. The changes fixes real and potential security-related
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issues, as well minor quality-related issues.</t>
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</abstract>
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</front>
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<middle>
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<section title="Introduction">
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<t>This document addresses minor issues that were discovered in the reference
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implementation of the Opus codec. Unlike most IETF specifications, Opus is defined
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in <xref target="RFC6716">RFC 6716</xref> in terms of a normative reference
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decoder implementation rather than from the associated text description.
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That RFC includes the reference decoder implementation as Appendix A.
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That's why only issues affecting the decoder are
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listed here. An up-to-date implementation of the Opus encoder can be found at
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<eref target="https://opus-codec.org/"/>.</t>
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<t>
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Some of the changes in this document update normative behaviour in a way that requires
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new test vectors. The English text of the specification is unaffected, only
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the C implementation is. The updated specification remains fully compatible with
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the original specification.
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</t>
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<t>
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Note: due to RFC formatting conventions, lines exceeding the column width
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in the patch are split using a backslash character. The backslashes
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at the end of a line and the white space at the beginning
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of the following line are not part of the patch. A properly formatted patch
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including all changes is available at
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<eref target="https://www.ietf.org/proceedings/98/slides/materials-98-codec-opus-update-00.patch"/>
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and has a SHA-1 hash of 029e3aa88fc342c91e67a21e7bfbc9458661cd5f.
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</t>
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</section>
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<section title="Terminology">
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<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in <xref
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target="RFC2119">RFC 2119</xref>.</t>
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</section>
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<section title="Stereo State Reset in SILK">
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<t>The reference implementation does not reinitialize the stereo state
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during a mode switch. The old stereo memory can produce a brief impulse
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(i.e. single sample) in the decoded audio. This can be fixed by changing
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silk/dec_API.c at line 72:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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for( n = 0; n < DECODER_NUM_CHANNELS; n++ ) {
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ret = silk_init_decoder( &channel_state[ n ] );
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}
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+ silk_memset(&((silk_decoder *)decState)->sStereo, 0,
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+ sizeof(((silk_decoder *)decState)->sStereo));
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+ /* Not strictly needed, but it's cleaner that way */
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+ ((silk_decoder *)decState)->prev_decode_only_middle = 0;
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return ret;
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}
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<CODE ENDS>
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]]></artwork>
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</figure>
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<t>
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This change affects the normative output of the decoder, but the
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amount of change is within the tolerance and too small to make the testvector check fail.
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</t>
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</section>
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<section anchor="padding" title="Parsing of the Opus Packet Padding">
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<t>It was discovered that some invalid packets of very large size could trigger
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an out-of-bounds read in the Opus packet parsing code responsible for padding.
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This is due to an integer overflow if the signaled padding exceeds 2^31-1 bytes
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(the actual packet may be smaller). The code can be fixed by decrementing the
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(signed) len value, instead of incrementing a separate padding counter.
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This is done by applying the following changes at line 596 of src/opus_decoder.c:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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/* Padding flag is bit 6 */
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if (ch&0x40)
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{
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- int padding=0;
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int p;
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do {
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if (len<=0)
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return OPUS_INVALID_PACKET;
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p = *data++;
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len--;
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- padding += p==255 ? 254: p;
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+ len -= p==255 ? 254: p;
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} while (p==255);
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- len -= padding;
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}
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<CODE ENDS>
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]]></artwork>
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</figure>
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<t>This packet parsing issue is limited to reading memory up
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to about 60 kB beyond the compressed buffer. This can only be triggered
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by a compressed packet more than about 16 MB long, so it's not a problem
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for RTP. In theory, it could crash a file
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decoder (e.g. Opus in Ogg) if the memory just after the incoming packet
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is out-of-range, but our attempts to trigger such a crash in a production
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application built using an affected version of the Opus decoder failed.</t>
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</section>
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<section anchor="resampler" title="Resampler buffer">
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<t>The SILK resampler had the following issues:
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<list style="numbers">
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<t>The calls to memcpy() were using sizeof(opus_int32), but the type of the
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local buffer was opus_int16.</t>
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<t>Because the size was wrong, this potentially allowed the source
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and destination regions of the memcpy() to overlap on the copy from "buf" to "buf".
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We believe that nSamplesIn (number of input samples) is at least fs_in_khZ (sampling rate in kHz),
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which is at least 8.
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Since RESAMPLER_ORDER_FIR_12 is only 8, that should not be a problem once
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the type size is fixed.</t>
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<t>The size of the buffer used RESAMPLER_MAX_BATCH_SIZE_IN, but the
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data stored in it was actually twice the input batch size
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(nSamplesIn<<1).</t>
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</list></t>
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<t>The code can be fixed by applying the following changes to line 78 of silk/resampler_private_IIR_FIR.c:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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)
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{
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silk_resampler_state_struct *S = \
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(silk_resampler_state_struct *)SS;
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opus_int32 nSamplesIn;
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opus_int32 max_index_Q16, index_increment_Q16;
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- opus_int16 buf[ RESAMPLER_MAX_BATCH_SIZE_IN + \
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RESAMPLER_ORDER_FIR_12 ];
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+ opus_int16 buf[ 2*RESAMPLER_MAX_BATCH_SIZE_IN + \
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RESAMPLER_ORDER_FIR_12 ];
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/* Copy buffered samples to start of buffer */
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- silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 \
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* sizeof( opus_int32 ) );
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+ silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 \
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* sizeof( opus_int16 ) );
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/* Iterate over blocks of frameSizeIn input samples */
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index_increment_Q16 = S->invRatio_Q16;
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while( 1 ) {
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nSamplesIn = silk_min( inLen, S->batchSize );
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/* Upsample 2x */
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silk_resampler_private_up2_HQ( S->sIIR, &buf[ \
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RESAMPLER_ORDER_FIR_12 ], in, nSamplesIn );
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max_index_Q16 = silk_LSHIFT32( nSamplesIn, 16 + 1 \
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); /* + 1 because 2x upsampling */
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out = silk_resampler_private_IIR_FIR_INTERPOL( out, \
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buf, max_index_Q16, index_increment_Q16 );
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in += nSamplesIn;
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inLen -= nSamplesIn;
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if( inLen > 0 ) {
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/* More iterations to do; copy last part of \
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filtered signal to beginning of buffer */
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- silk_memcpy( buf, &buf[ nSamplesIn << 1 ], \
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RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
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+ silk_memmove( buf, &buf[ nSamplesIn << 1 ], \
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RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
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} else {
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break;
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}
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}
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/* Copy last part of filtered signal to the state for \
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the next call */
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- silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], \
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RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
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+ silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], \
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RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
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}
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<CODE ENDS>
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]]></artwork>
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</figure>
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</section>
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<section title="Integer wrap-around in inverse gain computation">
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<t>
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It was discovered through decoder fuzzing that some bitstreams could produce
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integer values exceeding 32-bits in LPC_inverse_pred_gain_QA(), causing
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a wrap-around. The C standard considers
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this behavior as undefined. The following patch to line 87 of silk/LPC_inv_pred_gain.c
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detects values that do not fit in a 32-bit integer and considers the corresponding filters unstable:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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/* Update AR coefficient */
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for( n = 0; n < k; n++ ) {
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- tmp_QA = Aold_QA[ n ] - MUL32_FRAC_Q( \
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Aold_QA[ k - n - 1 ], rc_Q31, 31 );
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- Anew_QA[ n ] = MUL32_FRAC_Q( tmp_QA, rc_mult2 , mult2Q );
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+ opus_int64 tmp64;
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+ tmp_QA = silk_SUB_SAT32( Aold_QA[ n ], MUL32_FRAC_Q( \
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Aold_QA[ k - n - 1 ], rc_Q31, 31 ) );
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+ tmp64 = silk_RSHIFT_ROUND64( silk_SMULL( tmp_QA, \
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rc_mult2 ), mult2Q);
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+ if( tmp64 > silk_int32_MAX || tmp64 < silk_int32_MIN ) {
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+ return 0;
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+ }
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+ Anew_QA[ n ] = ( opus_int32 )tmp64;
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}
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<CODE ENDS>
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]]></artwork>
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</figure>
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</section>
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<section title="Integer wrap-around in LSF decoding" anchor="lsf_overflow">
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<t>
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It was discovered -- also from decoder fuzzing -- that an integer wrap-around could
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occur when decoding bitstreams with extremely large values for the high LSF parameters.
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The end result of the wrap-around is an illegal read access on the stack, which
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the authors do not believe is exploitable but should nonetheless be fixed. The following
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patch to line 137 of silk/NLSF_stabilize.c prevents the problem:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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/* Keep delta_min distance between the NLSFs */
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for( i = 1; i < L; i++ )
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- NLSF_Q15[i] = silk_max_int( NLSF_Q15[i], \
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NLSF_Q15[i-1] + NDeltaMin_Q15[i] );
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+ NLSF_Q15[i] = silk_max_int( NLSF_Q15[i], \
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silk_ADD_SAT16( NLSF_Q15[i-1], NDeltaMin_Q15[i] ) );
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/* Last NLSF should be no higher than 1 - NDeltaMin[L] */
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<CODE ENDS>
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]]></artwork>
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</figure>
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</section>
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<section title="Cap on Band Energy">
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<t>On extreme bit-streams, it is possible for log-domain band energy levels
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to exceed the maximum single-precision floating point value once converted
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to a linear scale. This would later cause the decoded values to be NaN (not a number),
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possibly causing problems in the software using the PCM values. This can be
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avoided with the following patch to line 552 of celt/quant_bands.c:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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{
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opus_val16 lg = ADD16(oldEBands[i+c*m->nbEBands],
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SHL16((opus_val16)eMeans[i],6));
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+ lg = MIN32(QCONST32(32.f, 16), lg);
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eBands[i+c*m->nbEBands] = PSHR32(celt_exp2(lg),4);
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}
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for (;i<m->nbEBands;i++)
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<CODE ENDS>
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]]></artwork>
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</figure>
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</section>
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<section title="Hybrid Folding" anchor="folding">
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<t>When encoding in hybrid mode at low bitrate, we sometimes only have
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enough bits to code a single CELT band (8 - 9.6 kHz). When that happens,
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the second band (CELT band 18, from 9.6 to 12 kHz) cannot use folding
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because it is wider than the amount already coded, and falls back to
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white noise. Because it can also happen on transients (e.g. stops), it
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can cause audible pre-echo.
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</t>
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<t>
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To address the issue, we change the folding behavior so that it is
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never forced to fall back to LCG due to the first band not containing
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enough coefficients to fold onto the second band. This
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is achieved by simply repeating part of the first band in the folding
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of the second band. This changes the code in celt/bands.c around line 1237:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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b = 0;
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}
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- if (resynth && M*eBands[i]-N >= M*eBands[start] && \
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(update_lowband || lowband_offset==0))
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+ if (resynth && (M*eBands[i]-N >= M*eBands[start] || \
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i==start+1) && (update_lowband || lowband_offset==0))
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lowband_offset = i;
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+ if (i == start+1)
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+ {
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+ int n1, n2;
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+ int offset;
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+ n1 = M*(eBands[start+1]-eBands[start]);
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+ n2 = M*(eBands[start+2]-eBands[start+1]);
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+ offset = M*eBands[start];
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+ /* Duplicate enough of the first band folding data to \
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be able to fold the second band.
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+ Copies no data for CELT-only mode. */
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+ OPUS_COPY(&norm[offset+n1], &norm[offset+2*n1 - n2], n2-n1);
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+ if (C==2)
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+ OPUS_COPY(&norm2[offset+n1], &norm2[offset+2*n1 - n2], \
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n2-n1);
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+ }
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+
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tf_change = tf_res[i];
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if (i>=m->effEBands)
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{
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<CODE ENDS>
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]]></artwork>
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</figure>
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<t>
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as well as line 1260:
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</t>
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<figure>
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<artwork><![CDATA[
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<CODE BEGINS>
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fold_start = lowband_offset;
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while(M*eBands[--fold_start] > effective_lowband);
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fold_end = lowband_offset-1;
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- while(M*eBands[++fold_end] < effective_lowband+N);
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+ while(++fold_end < i && M*eBands[fold_end] < \
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effective_lowband+N);
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x_cm = y_cm = 0;
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fold_i = fold_start; do {
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x_cm |= collapse_masks[fold_i*C+0];
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<CODE ENDS>
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]]></artwork>
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</figure>
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<t>
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The fix does not impact compatibility, because the improvement does
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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
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improve quality over the proposed change.
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</t>
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</section>
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<section title="Downmix to Mono" anchor="stereo">
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<t>The last issue is not strictly a bug, but it is an issue that has been reported
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when downmixing an Opus decoded stream to mono, whether this is done inside the decoder
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or as a post-processing step on the stereo decoder output. Opus intensity stereo allows
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optionally coding the two channels 180-degrees out of phase on a per-band basis.
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This provides better stereo quality than forcing the two channels to be in phase,
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but when the output is downmixed to mono, the energy in the affected bands is cancelled
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sometimes resulting in audible artifacts.
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</t>
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<t>As a work-around for this issue, the decoder MAY choose not to apply the 180-degree
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phase shift. This can be useful when downmixing to mono inside or
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outside of the decoder (e.g. user-controllable).
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</t>
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</section>
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<section title="New Test Vectors">
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<t>Changes in <xref target="folding"/> and <xref target="stereo"/> have
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sufficient impact on the testvectors to make them fail. For this reason,
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this document also updates the Opus test vectors. The new test vectors now
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include two decoded outputs for the same bitstream. The outputs with
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suffix 'm' do not apply the CELT 180-degree phase shift as allowed in
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<xref target="stereo"/>, while the outputs without the suffix do. An
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implementation is compliant as long as it passes either set of vectors.
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</t>
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<t>
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Any Opus implementation
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that passes either the original test vectors from <xref target="RFC6716">RFC 6716</xref>
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or one of the new sets of test vectors is compliant with the Opus specification. However, newer implementations
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SHOULD be based on the new test vectors rather than the old ones.
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</t>
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<t>The new test vectors are located at
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<eref target="https://www.ietf.org/proceedings/98/slides/materials-98-codec-opus-newvectors-00.tar.gz"/>.
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The SHA-1 hashes of the test vectors are:
|
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<figure>
|
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<artwork>
|
|
<![CDATA[
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e49b2862ceec7324790ed8019eb9744596d5be01 testvector01.bit
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b809795ae1bcd606049d76de4ad24236257135e0 testvector02.bit
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e0c4ecaeab44d35a2f5b6575cd996848e5ee2acc testvector03.bit
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a0f870cbe14ebb71fa9066ef3ee96e59c9a75187 testvector04.bit
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9b3d92b48b965dfe9edf7b8a85edd4309f8cf7c8 testvector05.bit
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28e66769ab17e17f72875283c14b19690cbc4e57 testvector06.bit
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bacf467be3215fc7ec288f29e2477de1192947a6 testvector07.bit
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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
|
|
]]>
|
|
</artwork>
|
|
</figure>
|
|
Note that the decoder input bitstream files (.bit) are unchanged.
|
|
</t>
|
|
</section>
|
|
|
|
<section anchor="security" title="Security Considerations">
|
|
<t>This document fixes two security issues reported on Opus and that affect the
|
|
reference implementation in <xref target="RFC6716">RFC 6716</xref>: CVE-2013-0899
|
|
<eref target="https://nvd.nist.gov/vuln/detail/CVE-2013-0899"/>
|
|
and CVE-2017-0381 <eref target="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. It is fixed by <xref target="padding"/>.
|
|
CVE-2017-0381 could have resulted in a 16-bit out-of-bounds read from a fixed
|
|
location. It is fixed in <xref target="lsf_overflow"/>.
|
|
Beyond the two fixed CVEs, this document adds no new security considerations on top of
|
|
<xref target="RFC6716">RFC 6716</xref>.
|
|
</t>
|
|
</section>
|
|
|
|
<section anchor="IANA" title="IANA Considerations">
|
|
<t>This document makes no request of IANA.</t>
|
|
|
|
<t>Note to RFC Editor: this section may be removed on publication as an
|
|
RFC.</t>
|
|
</section>
|
|
|
|
<section anchor="Acknowledgements" title="Acknowledgements">
|
|
<t>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.</t>
|
|
</section>
|
|
</middle>
|
|
|
|
<back>
|
|
<references title="Normative References">
|
|
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
|
|
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6716.xml"?>
|
|
|
|
|
|
</references>
|
|
</back>
|
|
</rfc>
|