demux_mkv: improve robustness by explicitly checking for level 1 elements

Matroska makes it pretty hard to resync correctly on broken files:
random data returns "valid" EBML IDs with a high probability, and when
trying to skip them it's likely that you skip a random amount of data
(instead of considering the element length invalid).

Improve upon this by skipping known level 1 elements only. Consider
everything else invalid and call the resync code. This might result in
annoying behavior when Matroska adds new level 1 elements, although it
won't be particularly harmful. Matroska doesn't really allow us to do
better (even mkvtoolnix explicitly checks for known level 1 elements).

Since we now don't always want to combine EBML element skipping and
resyncing, remove ebml_read_skip_or_resync_cluster(), and make
ebml_read_skip() more tolerant against skipping broken elements.

Also, don't resync when reading sub-elements, and instead do resyncing
when reading them results in an error.

demux/demux_mkv.c

@@ -723,7 +723,7 @@ static int demux_mkv_read_cues(demuxer_t *demuxer)
     stream_t *s = demuxer->stream;
 
     if (opts->index_mode == 0 || opts->index_mode == 2) {
-        ebml_read_skip(s, NULL);
+        ebml_read_skip(demuxer->log, -1, s);
         return 0;
     }
 
@@ -1128,7 +1128,7 @@ static int read_header_element(struct demuxer *demuxer, uint32_t id,
         res = 2;
     }
     if (!at_filepos && id != EBML_ID_INVALID)
-        ebml_read_skip(s, NULL);
+        ebml_read_skip(demuxer->log, -1, s);
     return res;
 }
 
@@ -2454,11 +2454,12 @@ static int read_block_group(demuxer_t *demuxer, int64_t end,
                 block->keyframe = false;
             break;
 
+        case MATROSKA_ID_CLUSTER:
         case EBML_ID_INVALID:
             goto error;
 
         default:
-            if (ebml_read_skip_or_resync_cluster(demuxer->log, end, s) != 0)
+            if (ebml_read_skip(demuxer->log, end, s) != 0)
                 goto error;
             break;
         }
@@ -2519,8 +2520,7 @@ static int read_next_block(demuxer_t *demuxer, struct block_info *block)
                 goto find_next_cluster;
 
             default: ;
-                if (ebml_read_skip_or_resync_cluster
+                if (ebml_read_skip(demuxer->log, mkv_d->cluster_end, s) != 0)
-                        (demuxer->log, mkv_d->cluster_end, s) != 0)
                     goto find_next_cluster;
                 break;
             }
@@ -2535,7 +2535,13 @@ static int read_next_block(demuxer_t *demuxer, struct block_info *block)
                 break;
             if (s->eof)
                 return -1;
-            ebml_read_skip_or_resync_cluster(demuxer->log, -1, s);
+            // For the sake of robustness, consider even unknown level 1
+            // elements the same as unknown/broken IDs.
+            if (!ebml_is_mkv_level1_id(id) ||
+                ebml_read_skip(demuxer->log, -1, s) != 0)
+            {
+                ebml_resync_cluster(demuxer->log, s);
+            }
         }
     next_cluster:
         mkv_d->cluster_end = ebml_read_length(s, NULL);

demux/ebml.c

@@ -41,6 +41,26 @@
 #define SIZE_MAX ((size_t)-1)
 #endif
 
+// Whether the id is a known Matroska level 1 element (allowed as element on
+// global file level, after the level 0 MATROSKA_ID_SEGMENT).
+// This (intentionally) doesn't include "global" elements.
+bool ebml_is_mkv_level1_id(uint32_t id)
+{
+    switch (id) {
+    case MATROSKA_ID_SEEKHEAD:
+    case MATROSKA_ID_INFO:
+    case MATROSKA_ID_CLUSTER:
+    case MATROSKA_ID_TRACKS:
+    case MATROSKA_ID_CUES:
+    case MATROSKA_ID_ATTACHMENTS:
+    case MATROSKA_ID_CHAPTERS:
+    case MATROSKA_ID_TAGS:
+        return true;
+    default:
+        return false;
+    }
+}
+
 /*
  * Read: the element content data ID.
  * Return: the ID.
@@ -252,21 +272,32 @@ char *ebml_read_utf8(stream_t *s, uint64_t *length)
 
 /*
  * Skip the current element.
+ * end: the end of the parent element or -1 (for robust error handling)
  */
-int ebml_read_skip(stream_t *s, uint64_t *length)
+int ebml_read_skip(struct mp_log *log, int64_t end, stream_t *s)
 {
     uint64_t len;
     int l;
 
+    int64_t pos = stream_tell(s);
+
     len = ebml_read_length(s, &l);
     if (len == EBML_UINT_INVALID)
-        return 1;
+        goto invalid;
-    if (length)
-        *length = len + l;
 
-    stream_skip(s, len);
+    int64_t pos2 = stream_tell(s);
+    if (len >= INT64_MAX - pos2 || (end > 0 && pos2 + len > end))
+        goto invalid;
+
+    if (!stream_skip(s, len))
+        goto invalid;
 
     return 0;
+
+invalid:
+    mp_err(log, "Invalid EBML length at position %"PRId64"\n", pos);
+    stream_seek(s, pos);
+    return 1;
 }
 
 /*
@@ -291,38 +322,6 @@ int ebml_resync_cluster(struct mp_log *log, stream_t *s)
     return -1;
 }
 
-/*
- * Skip the current element, or on error, call ebml_resync_cluster().
- * end gives the maximum possible file pos (due to EBML parent element size).
- */
-int ebml_read_skip_or_resync_cluster(struct mp_log *log, int64_t end,
-                                     stream_t *s)
-{
-    uint64_t len;
-    int l;
-
-    len = ebml_read_length(s, &l);
-    if (len == EBML_UINT_INVALID)
-        goto resync;
-
-    int64_t pos = stream_tell(s);
-
-    if (end >= 0 && pos + len > end)
-        goto resync;
-
-    // When reading corrupted elements, len will often be a random high number,
-    // and stream_skip() will fail when skipping past EOF.
-    if (!stream_skip(s, len)) {
-        stream_seek(s, pos);
-        goto resync;
-    }
-
-    return 0;
-
-resync:
-    return ebml_resync_cluster(log, s) < 0 ? -1 : 1;
-}
-
 /*
  * Read the next element, but only the header. The contents
  * are supposed to be sub-elements which can be read separately.

demux/ebml.h

@@ -92,6 +92,7 @@ struct ebml_parse_ctx {
 #define EBML_FLOAT_INVALID  -1000000000.0
 
 
+bool ebml_is_mkv_level1_id(uint32_t id);
 uint32_t ebml_read_id (stream_t *s, int *length);
 uint64_t ebml_read_vlen_uint (bstr *buffer);
 int64_t ebml_read_vlen_int (bstr *buffer);
@@ -101,9 +102,7 @@ int64_t ebml_read_int (stream_t *s, uint64_t *length);
 double ebml_read_float (stream_t *s, uint64_t *length);
 char *ebml_read_ascii (stream_t *s, uint64_t *length);
 char *ebml_read_utf8 (stream_t *s, uint64_t *length);
-int ebml_read_skip (stream_t *s, uint64_t *length);
+int ebml_read_skip(struct mp_log *log, int64_t end, stream_t *s);
-int ebml_read_skip_or_resync_cluster(struct mp_log *log, int64_t end,
-                                     stream_t *s);
 int ebml_resync_cluster(struct mp_log *log, stream_t *s);
 uint32_t ebml_read_master (stream_t *s, uint64_t *length);