CVE-2018-18854 Use TreeMap instead of HashMap for JsObject key/value pairs, fixes #277

The problem is that with String's hashCode implementation it is too simple to
create synthetic collisions. This allows an attacker to create an object with
keys that all collide which leads to a performance drop for the HashMap
just for creating the map in the first place. See https://github.com/scala/bug/issues/11203
for more information about the underlying HashMap issue.

For the time being, it seems safer to use a TreeMap which uses String ordering.
Benchmarks suggest that using a TreeMap is only ~6% slower for reasonably sized JSON objects
up to 100 keys.

Benchmark for non-colliding keys:

Benchmark                         (_size)  (parser)   Mode  Cnt        Score       Error  Units
ExtractFieldsBenchmark.readSpray        1   HashMap  thrpt    5  1195832.262 ± 64366.605  ops/s
ExtractFieldsBenchmark.readSpray        1   TreeMap  thrpt    5  1342009.641 ± 17307.555  ops/s
ExtractFieldsBenchmark.readSpray       10   HashMap  thrpt    5   237173.327 ± 70341.742  ops/s
ExtractFieldsBenchmark.readSpray       10   TreeMap  thrpt    5   233510.618 ± 69638.750  ops/s
ExtractFieldsBenchmark.readSpray      100   HashMap  thrpt    5    23202.016 ±  1514.763  ops/s
ExtractFieldsBenchmark.readSpray      100   TreeMap  thrpt    5    21899.072 ±   823.225  ops/s
ExtractFieldsBenchmark.readSpray     1000   HashMap  thrpt    5     2073.754 ±    66.093  ops/s
ExtractFieldsBenchmark.readSpray     1000   TreeMap  thrpt    5     1793.329 ±    43.603  ops/s
ExtractFieldsBenchmark.readSpray    10000   HashMap  thrpt    5      208.160 ±     7.466  ops/s
ExtractFieldsBenchmark.readSpray    10000   TreeMap  thrpt    5      160.349 ±     5.809  ops/s

1 files changed, 26 insertions, 0 deletions

diff --git a/src/test/scala/spray/json/HashCodeCollider.scala b/src/test/scala/spray/json/HashCodeCollider.scala
new file mode 100644
index 0000000..57388b9
--- /dev/null
+++ b/src/test/scala/spray/json/HashCodeCollider.scala
@@ -0,0 +1,26 @@
+package spray.json
+
+/**
+ * Helper that creates strings that all share the same hashCode == 0.
+ *
+ * Adapted from MIT-licensed code by Andriy Plokhotnyuk
+ * at https://github.com/plokhotnyuk/jsoniter-scala/blob/26b5ecdd4f8c2ab7e97bd8106cefdda4c1e701ce/jsoniter-scala-benchmark/src/main/scala/com/github/plokhotnyuk/jsoniter_scala/macros/HashCodeCollider.scala#L6.
+ */
+object HashCodeCollider {
+  val visibleChars = (33 until 127).filterNot(c => c == '\\' || c == '"')
+  def asciiChars: Iterator[Int] = visibleChars.toIterator
+  def asciiCharsAndHash(previousHash: Int): Iterator[(Int, Int)] = visibleChars.toIterator.map(c => c -> (previousHash + c) * 31)
+
+  /** Creates an iterator of Strings that all have hashCode == 0 */
+  def zeroHashCodeIterator(): Iterator[String] =
+    for {
+      (i0, h0) <- asciiCharsAndHash(0)
+      (i1, h1) <- asciiCharsAndHash(h0)
+      (i2, h2) <- asciiCharsAndHash(h1) if (((h2 + 32) * 923521) ^ ((h2 + 127) * 923521)) < 0
+      (i3, h3) <- asciiCharsAndHash(h2) if (((h3 + 32) * 29791) ^ ((h3 + 127) * 29791)) < 0
+      (i4, h4) <- asciiCharsAndHash(h3) if (((h4 + 32) * 961) ^ ((h4 + 127) * 961)) < 0
+      (i5, h5) <- asciiCharsAndHash(h4) if (((h5 + 32) * 31) ^ ((h5 + 127) * 31)) < 0
+      (i6, h6) <- asciiCharsAndHash(h5) if ((h6 + 32) ^ (h6 + 127)) < 0
+      (i7, h7) <- asciiCharsAndHash(h6) if h6 + i7 == 0
+    } yield new String(Array(i0, i1, i2, i3, i4, i5, i6, i7).map(_.toChar))
+}