DTLS fragments may not be split across two records.

See also upstream's 9dcab127e14467733523ff7626da8906e67eedd6. The root problem
is dtls1_read_bytes is wrong, but we can get the right behavior now and add a
regression test for it before cleaning it up.

Change-Id: I4e5c39ab254a872d9f64242c9b77b020bdded6e6
Reviewed-on: https://boringssl-review.googlesource.com/5123
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 207de0c..928c2b2 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -668,17 +668,10 @@
 	// handshake fragments in DTLS to have the wrong message length.
 	FragmentMessageLengthMismatch bool
 
-	// SplitFragmentHeader, if true, causes the handshake fragments in DTLS
-	// to be split across two records.
-	SplitFragmentHeader bool
-
-	// SplitFragmentBody, if true, causes the handshake bodies in DTLS to be
-	// split across two records.
-	//
-	// TODO(davidben): There's one final split to test: when the header and
-	// body are split across two records. But those are (incorrectly)
-	// accepted right now.
-	SplitFragmentBody bool
+	// SplitFragments, if non-zero, causes the handshake fragments in DTLS
+	// to be split across two records. The value of |SplitFragments| is the
+	// number of bytes in the first fragment.
+	SplitFragments int
 
 	// SendEmptyFragments, if true, causes handshakes to include empty
 	// fragments in DTLS.
diff --git a/ssl/test/runner/dtls.go b/ssl/test/runner/dtls.go
index 50f7786..538bf51 100644
--- a/ssl/test/runner/dtls.go
+++ b/ssl/test/runner/dtls.go
@@ -216,13 +216,10 @@
 	// Pack handshake fragments into records.
 	var records [][]byte
 	for _, fragment := range fragments {
-		if c.config.Bugs.SplitFragmentHeader {
-			records = append(records, fragment[:2])
-			records = append(records, fragment[2:])
-		} else if c.config.Bugs.SplitFragmentBody {
-			if len(fragment) > 12 {
-				records = append(records, fragment[:13])
-				records = append(records, fragment[13:])
+		if n := c.config.Bugs.SplitFragments; n > 0 {
+			if len(fragment) > n {
+				records = append(records, fragment[:n])
+				records = append(records, fragment[n:])
 			} else {
 				records = append(records, fragment)
 			}
diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 524b3d0..f02a221 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -913,10 +913,10 @@
 	},
 	{
 		protocol: dtls,
-		name:     "SplitFragmentHeader-DTLS",
+		name:     "SplitFragments-Header-DTLS",
 		config: Config{
 			Bugs: ProtocolBugs{
-				SplitFragmentHeader: true,
+				SplitFragments: 2,
 			},
 		},
 		shouldFail:    true,
@@ -924,14 +924,25 @@
 	},
 	{
 		protocol: dtls,
-		name:     "SplitFragmentBody-DTLS",
+		name:     "SplitFragments-Boundary-DTLS",
 		config: Config{
 			Bugs: ProtocolBugs{
-				SplitFragmentBody: true,
+				SplitFragments: dtlsRecordHeaderLen,
 			},
 		},
 		shouldFail:    true,
-		expectedError: ":UNEXPECTED_MESSAGE:",
+		expectedError: ":EXCESSIVE_MESSAGE_SIZE:",
+	},
+	{
+		protocol: dtls,
+		name:     "SplitFragments-Body-DTLS",
+		config: Config{
+			Bugs: ProtocolBugs{
+				SplitFragments: dtlsRecordHeaderLen + 1,
+			},
+		},
+		shouldFail:    true,
+		expectedError: ":EXCESSIVE_MESSAGE_SIZE:",
 	},
 	{
 		protocol: dtls,