JS: Make ReDoS check string-based regexes

javascript/ql/src/Performance/ReDoS.ql

@@ -84,16 +84,19 @@ import javascript
  * whose root node is not a disjunction.
  */
 class RegExpRoot extends @regexpterm {
+  RegExpParent parent;
+
   // RegExpTerm is abstract, so do not extend it.
   RegExpRoot() {
-    exists(RegExpLiteral literal, RegExpAlt alt | alt.getParent() = literal |
-      this = alt.getAChild()
+    exists(RegExpAlt alt |
+      alt.isRootTerm() and
+      this = alt.getAChild() and
+      parent = alt.getParent()
     )
     or
-    exists(RegExpLiteral literal |
-      not exists(RegExpAlt alt | alt.getParent() = literal) and
-      this.(RegExpTerm).getParent() = literal
-    )
+    this.(RegExpTerm).isRootTerm() and
+    not this instanceof RegExpAlt and
+    parent = this.(RegExpTerm).getParent()
   }
 
   /**
@@ -103,7 +106,13 @@ class RegExpRoot extends @regexpterm {
     // there is at least one repetition
     exists(RegExpRepetition rep | getRoot(rep) = this) and
     // there are no lookbehinds
-    not exists(RegExpLookbehind lbh | getRoot(lbh) = this)
+    not exists(RegExpLookbehind lbh | getRoot(lbh) = this) and
+    // is actually used as a RegExp
+    (
+      parent instanceof RegExpLiteral
+      or
+      parent.(StringLiteral).flow() instanceof RegExpPatternSource
+    )
   }
 
   string toString() { result = this.(RegExpTerm).toString() }
@@ -352,10 +361,10 @@ predicate delta(State q1, EdgeLabel lbl, State q2) {
     )
   )
   or
-  exists(RegExpDot dot, RegExpLiteral rel |
-    q1 = before(dot) and q2 = after(dot) and rel = dot.getLiteral()
+  exists(RegExpDot dot |
+    q1 = before(dot) and q2 = after(dot)
   |
-    if rel.isDotAll() then lbl = Any() else lbl = Dot()
+    if dot.getLiteral().isDotAll() then lbl = Any() else lbl = Dot()
   )
   or
   exists(RegExpCharacterClass cc |

javascript/ql/src/semmle/javascript/Regexp.qll

@@ -105,6 +105,13 @@ abstract class RegExpTerm extends Locatable, @regexpterm {
    * it has an enclosing lookbehind assertions.
    */
   predicate isInBackwardMatchingContext() { this = any(RegExpLookbehind lbh).getAChild+() }
+
+  /**
+   * Holds if this is the root term of a regular expression.
+   */
+  predicate isRootTerm() {
+    not getParent() instanceof RegExpTerm
+  }
 }
 
 /**

javascript/ql/test/query-tests/Performance/ReDoS/ReDoS.expected

@@ -49,3 +49,4 @@
 | tst.js:74:14:74:21 | (b\|a?b)* | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of 'b'. |
 | tst.js:77:14:77:21 | (a\|aa?)* | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of 'a'. |
 | tst.js:83:14:83:20 | (.\|\\n)* | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of '\\n'. |
+| tst.js:89:25:89:32 | (a\|aa?)* | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of 'a'. |

javascript/ql/test/query-tests/Performance/ReDoS/tst.js

@@ -84,3 +84,9 @@ var bad16 = /(.|\n)*!/s;
 
 // GOOD
 var good8 = /([\w.]+)*/;
+
+// NOT GOOD
+var bad17 = new RegExp('(a|aa?)*b');
+
+// GOOD - not used as regexp
+var good9 = '(a|aa?)*b';