crypto/cipher: avoid out of bounds error in CryptBlocks

Fixes #4699.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/7231065

src/pkg/crypto/cipher/cbc.go

@@ -42,6 +42,12 @@ func NewCBCEncrypter(b Block, iv []byte) BlockMode {
 func (x *cbcEncrypter) BlockSize() int { return x.blockSize }
 
 func (x *cbcEncrypter) CryptBlocks(dst, src []byte) {
+	if len(src)%x.blockSize != 0 {
+		panic("crypto/cipher: input not full blocks")
+	}
+	if len(dst) < len(src) {
+		panic("crypto/cipher: output smaller than input")
+	}
 	for len(src) > 0 {
 		for i := 0; i < x.blockSize; i++ {
 			x.iv[i] ^= src[i]
@@ -70,6 +76,12 @@ func NewCBCDecrypter(b Block, iv []byte) BlockMode {
 func (x *cbcDecrypter) BlockSize() int { return x.blockSize }
 
 func (x *cbcDecrypter) CryptBlocks(dst, src []byte) {
+	if len(src)%x.blockSize != 0 {
+		panic("crypto/cipher: input not full blocks")
+	}
+	if len(dst) < len(src) {
+		panic("crypto/cipher: output smaller than input")
+	}
 	for len(src) > 0 {
 		x.b.Decrypt(x.tmp, src[:x.blockSize])
 		for i := 0; i < x.blockSize; i++ {

src/pkg/crypto/cipher/cipher_test.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cipher_test
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"testing"
+)
+
+func TestCryptBlocks(t *testing.T) {
+	buf := make([]byte, 16)
+	block, _ := aes.NewCipher(buf)
+
+	mode := cipher.NewCBCDecrypter(block, buf)
+	mustPanic(t, "crypto/cipher: input not full blocks", func() { mode.CryptBlocks(buf, buf[:3]) })
+	mustPanic(t, "crypto/cipher: output smaller than input", func() { mode.CryptBlocks(buf[:3], buf) })
+
+	mode = cipher.NewCBCEncrypter(block, buf)
+	mustPanic(t, "crypto/cipher: input not full blocks", func() { mode.CryptBlocks(buf, buf[:3]) })
+	mustPanic(t, "crypto/cipher: output smaller than input", func() { mode.CryptBlocks(buf[:3], buf) })
+}
+
+func mustPanic(t *testing.T, msg string, f func()) {
+	defer func() {
+		err := recover()
+		if err == nil {
+			t.Errorf("function did not panic, wanted %q", msg)
+		} else if err != msg {
+			t.Errorf("got panic %v, wanted %q", err, msg)
+		}
+	}()
+	f()
+}