Add protection against out-of-bounds read in ttconv

This commit adds to counter to track how many tables are read from the table
directory and stops the search for further tables early if the total number of
tables has been reached.

Author: sauerburger

extern/ttconv/pprdrv_tt.cpp

@@ -757,33 +757,36 @@ void ttfont_sfnts(TTStreamWriter& stream, struct TTFONT *font)
     ** Find the tables we want and store there vital
     ** statistics in tables[].
     */
-    for (x=0; x < 9; x++ )
-    {
-        do
-        {
-            diff = strncmp( (char*)ptr, table_names[x], 4 );
-
-            if ( diff > 0 )             /* If we are past it. */
-            {
-                tables[x].length = 0;
-                diff = 0;
-            }
-            else if ( diff < 0 )        /* If we haven't hit it yet. */
-            {
-                ptr += 16;
-            }
-            else if ( diff == 0 )       /* Here it is! */
-            {
-                tables[x].newoffset = nextoffset;
-                tables[x].checksum = getULONG( ptr + 4 );
-                tables[x].oldoffset = getULONG( ptr + 8 );
-                tables[x].length = getULONG( ptr + 12 );
-                nextoffset += ( ((tables[x].length + 3) / 4) * 4 );
-                count++;
-                ptr += 16;
-            }
-        }
-        while (diff != 0);
+    ULONG num_tables_read = 0;  /* Number of tables read from the directory */
+    for (x = 0; x < 9; x++) {
+        do {
+          if (num_tables_read < font->numTables) {
+              /* There are still tables to read from ptr */
+              diff = strncmp((char*)ptr, table_names[x], 4);
+
+              if (diff > 0) {           /* If we are past it. */
+                  tables[x].length = 0;
+                  diff = 0;
+              } else if (diff < 0) {      /* If we haven't hit it yet. */
+                  ptr += 16;
+                  num_tables_read++;
+              } else if (diff == 0) {     /* Here it is! */
+                  tables[x].newoffset = nextoffset;
+                  tables[x].checksum = getULONG( ptr + 4 );
+                  tables[x].oldoffset = getULONG( ptr + 8 );
+                  tables[x].length = getULONG( ptr + 12 );
+                  nextoffset += ( ((tables[x].length + 3) / 4) * 4 );
+                  count++;
+                  ptr += 16;
+                  num_tables_read++;
+              }
+          } else {
+            /* We've read the whole table directory already */
+            /* Some tables couldn't be found */
+            tables[x].length = 0;
+            break;  /* Proceed to next tables[x] */
+          }
+        } while (diff != 0);
 
     } /* end of for loop which passes over the table directory */