Re: [FFmpeg-devel] [PATCH 1/5] avcodec/aac/aacdec_usac: Test ac in usac

[Lynne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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On 26/06/2024 01:57, Michael Niedermayer wrote:
> On Wed, Jun 26, 2024 at 01:35:18AM +0200, Lynne via ffmpeg-devel wrote:
>> On 24/06/2024 01:01, Michael Niedermayer wrote:
>>> ff_aac_usac_config_decode() needs AACDecContext to be set but some callers
>>> pass NULL.
>>>
>>> I have no real testcase to implement/test this, so failing in this case
>>> seems safest.
>>>
>>> Fixes: member access within null pointer of type 'AACDecContext' (aka 'struct AACDecContext')
>>> Fixes: 69435/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_LATM_fuzzer-5733527483121664
>>>
>>> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>>> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
>>> ---
>>>    libavcodec/aac/aacdec_usac.c | 3 +++
>>>    1 file changed, 3 insertions(+)
>>>
>>> diff --git a/libavcodec/aac/aacdec_usac.c b/libavcodec/aac/aacdec_usac.c
>>> index 132ffee9c20..4856c1786b7 100644
>>> --- a/libavcodec/aac/aacdec_usac.c
>>> +++ b/libavcodec/aac/aacdec_usac.c
>>> @@ -348,6 +348,9 @@ int ff_aac_usac_config_decode(AACDecContext *ac, AVCodecContext *avctx,
>>>        int map_pos_set = 0;
>>>        uint8_t layout_map[MAX_ELEM_ID*4][3] = { 0 };
>>> +    if (!ac)
>>> +        return AVERROR_PATCHWELCOME;
>>> +
>>>        memset(usac, 0, sizeof(*usac));
>>>        freq_idx = get_bits(gb, 5); /* usacSamplingFrequencyIndex */
>>
>> This doesn't seem possible at all.
>> There are 2 callers, parse_audio_preroll and
>> decode_audio_specific_config_gb. Both of these will crash way before the
>> function is called.
>>
>> Could you at least get a backtrace?
> 
> sure
> 
> libavcodec/aac/aacdec_usac.c:402:39: runtime error: member access within null pointer of type 'AACDecContext' (aka 'struct AACDecContext')
> SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior libavcodec/aac/aacdec_usac.c:402:39 in
> AddressSanitizer:DEADLYSIGNAL
> =================================================================
> ==215602==ERROR: AddressSanitizer: SEGV on unknown address 0x000000005b10 (pc 0x00000154771e bp 0x7ffcf5049e90 sp 0x7ffcf5049e70 T0)
> ==215602==The signal is caused by a READ memory access.
>      #0 0x154771d in av_channel_layout_uninit ffmpeg/libavutil/channel_layout.c:439:25
>      #1 0x57346e in ff_aac_usac_config_decode ffmpeg/libavcodec/aac/aacdec_usac.c:402:9
>      #2 0x500a0a in decode_audio_specific_config_gb ffmpeg/libavcodec/aac/aacdec.c:1050:20
>      #3 0x50a542 in latm_decode_audio_specific_config ffmpeg/libavcodec/aac/aacdec_latm.h:80:21
>      #4 0x4f8638 in read_stream_mux_config ffmpeg/libavcodec/aac/aacdec_latm.h:160:24
>      #5 0x4f8638 in read_audio_mux_element ffmpeg/libavcodec/aac/aacdec_latm.h:233
>      #6 0x4f8638 in latm_decode_frame ffmpeg/libavcodec/aac/aacdec_latm.h:275
>      #7 0x68f26f in decode_simple_internal ffmpeg/libavcodec/decode.c:429:20
>      #8 0x68f26f in decode_simple_receive_frame ffmpeg/libavcodec/decode.c:600
>      #9 0x68f26f in decode_receive_frame_internal ffmpeg/libavcodec/decode.c:631
>      #10 0x68dc9d in avcodec_send_packet ffmpeg/libavcodec/decode.c:721:15
>      #11 0x4d1e55 in LLVMFuzzerTestOneInput ffmpeg/tools/target_dec_fuzzer.c:534:25
> 
> 
> [...]
> 
> 
> 
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This patch actually is correct.
USAC in LATM is not supported AFAIK.

LGTM with a note like:
"Happens only when the LATM decoder is used, and USAC is not supported 
in LATM".

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