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 <mich...@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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