[FFmpeg-devel] [PATCH v2] avformat/flvdec: use avio operation instead of pb->buf_ptr use

[Steven Liu]

check ensure seekback 4 bytes before read 4 bytes from pb,
and seek back 4 byte from current position after read 4 bytes.

fix segfaults:
READ of size 1 at 0x6100000003b7 thread T0
    #0 0x7f928d in flv_same_video_codec ffmpeg/libavformat/flvdec.c:317:29
    #1 0x7f928d in flv_read_packet ffmpeg/libavformat/flvdec.c:1177
    #2 0x6ff32f in ff_read_packet ffmpeg/libavformat/demux.c:575:15
    #3 0x70a2fd in read_frame_internal ffmpeg/libavformat/demux.c:1263:15
    #4 0x71d158 in avformat_find_stream_info ffmpeg/libavformat/demux.c:2634:15
    #5 0x4c821b in LLVMFuzzerTestOneInput ffmpeg/tools/target_dem_fuzzer.c:206:11

Signed-off-by: Steven Liu <lq at chinaffmpeg.org>
---
 libavformat/flvdec.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
index 3fe21622f7..38f34567dd 100644
--- a/libavformat/flvdec.c
+++ b/libavformat/flvdec.c
@@ -35,6 +35,7 @@
 #include "libavutil/intreadwrite.h"
 #include "libavutil/mathematics.h"
 #include "avformat.h"
+#include "avio_internal.h"
 #include "demux.h"
 #include "internal.h"
 #include "flv.h"
@@ -313,8 +314,14 @@ static int flv_same_video_codec(AVFormatContext *s, AVCodecParameters *vpar, int
         return 1;
 
     if (flv->exheader) {
-        uint8_t *codec_id_str = (uint8_t *)s->pb->buf_ptr;
-        uint32_t codec_id = codec_id_str[3] | codec_id_str[2] << 8 | codec_id_str[1] << 16 | codec_id_str[0] << 24;
+        uint32_t codec_id = 0;
+        int ret = ffio_ensure_seekback(s->pb, 4);
+        if (ret < 0) {
+            av_log(s, AV_LOG_WARNING, "Could not ensure seekback, because %s", av_err2str(ret));
+            return 0;
+        }
+        codec_id = avio_rb32(s->pb);
+        avio_seek(s->pb, -4, SEEK_CUR);
         switch(codec_id) {
             case MKBETAG('h', 'v', 'c', '1'):
                 return vpar->codec_id == AV_CODEC_ID_HEVC;
-- 
2.40.0