[FFmpeg-cvslog] avformat/matroskadec: Reset state also on failure in matroska_reset_status()

[Michael Niedermayer]

ffmpeg | branch: release/4.2 | Michael Niedermayer <michael at niedermayer.cc> | Thu Jun 24 19:53:47 2021 +0200| [c0b6f4b38da9523621eb1f3956fa2b0a0f64331d] | committer: Michael Niedermayer

avformat/matroskadec: Reset state also on failure in matroska_reset_status()

The calling code does not handle failures and will fail with assertion failures later.
Seeking can always fail even when the position was previously read.

Fixes: Assertion failure
Fixes: 35253/clusterfuzz-testcase-minimized-ffmpeg_dem_MATROSKA_fuzzer-4693059982983168

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
(cherry picked from commit d115eec97929e23fd1b06df2d95f48cf5000eb87)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=c0b6f4b38da9523621eb1f3956fa2b0a0f64331d
---

 libavformat/matroskadec.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 4b189902a3..e316dbd0a2 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -757,20 +757,22 @@ static int matroska_read_close(AVFormatContext *s);
 static int matroska_reset_status(MatroskaDemuxContext *matroska,
                                  uint32_t id, int64_t position)
 {
+    int64_t err = 0;
     if (position >= 0) {
-        int64_t err = avio_seek(matroska->ctx->pb, position, SEEK_SET);
-        if (err < 0)
-            return err;
-    }
+        err = avio_seek(matroska->ctx->pb, position, SEEK_SET);
+        if (err > 0)
+            err = 0;
+    } else
+        position = avio_tell(matroska->ctx->pb);
 
     matroska->current_id    = id;
     matroska->num_levels    = 1;
     matroska->unknown_count = 0;
-    matroska->resync_pos = avio_tell(matroska->ctx->pb);
+    matroska->resync_pos    = position;
     if (id)
         matroska->resync_pos -= (av_log2(id) + 7) / 8;
 
-    return 0;
+    return err;
 }
 
 static int matroska_resync(MatroskaDemuxContext *matroska, int64_t last_pos)
@@ -1814,6 +1816,7 @@ static int matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
     uint32_t saved_id  = matroska->current_id;
     int64_t before_pos = avio_tell(matroska->ctx->pb);
     int ret = 0;
+    int ret2;
 
     /* seek */
     if (avio_seek(matroska->ctx->pb, pos, SEEK_SET) == pos) {
@@ -1838,7 +1841,9 @@ static int matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
     }
     /* Seek back - notice that in all instances where this is used
      * it is safe to set the level to 1. */
-    matroska_reset_status(matroska, saved_id, before_pos);
+    ret2 = matroska_reset_status(matroska, saved_id, before_pos);
+    if (ret >= 0)
+        ret = ret2;
 
     return ret;
 }