crypto: add secure random reader using getrandom

This commit adds in RandReader, a cryptographically secure io.Reader
that will fail when the os has insufficient randomness. This is done
using the getrandom() syscall in non-blocking mode.
  see: http://man7.org/linux/man-pages/man2/getrandom.2.html
Any kernel new enough to have filesystem encryption will also have this
syscall.

This RandReader is preferable to the one provided by the standard
library in crypto/rand. See the bugs:
	https://github.com/golang/go/issues/11833
	https://github.com/golang/go/issues/19274
This will be removed when go updates the crypto/rand implementation.

Change-Id: Icccaf07bc6011b95cd31a5c268e7486807dcffe2

1 files changed, 62 insertions, 0 deletions

diff --git a/crypto/rand.go b/crypto/rand.go
new file mode 100644
index 0000000..d9d4cff
--- /dev/null
+++ b/crypto/rand.go
@@ -0,0 +1,62 @@
+/*
+ * rand.go - Reader used to generate secure random data for fscrypt.
+ *
+ * Copyright 2017 Google Inc.
+ * Author: Joe Richey (joerichey@google.com)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package crypto
+
+import (
+	"io"
+
+	"golang.org/x/sys/unix"
+
+	"fscrypt/util"
+)
+
+/*
+RandReader uses the Linux Getrandom() syscall to read random bytes. If the
+operating system has insufficient randomness, the read will fail. This is an
+improvement over Go's built-in crypto/rand which will still return bytes if the
+system has insufficiency entropy (https://github.com/golang/go/issues/19274).
+
+While this syscall was only introduced in Kernel v3.17, it predates the
+introduction of filesystem encryption, so it introduces no additional
+compatibility issues.
+*/
+var RandReader io.Reader = randReader{}
+
+// As we just call into Getrandom, no internal data is needed.
+type randReader struct{}
+
+func (r randReader) Read(buffer []byte) (int, error) {
+	n, err := unix.Getrandom(buffer, unix.GRND_NONBLOCK)
+	switch err {
+	case nil:
+		return n, nil
+	case unix.EAGAIN:
+		return 0, util.SystemErrorF("entropy pool not yet initialized")
+	case unix.ENOSYS:
+		return 0, util.SystemErrorF("getrandom not implemented; kernel must be v3.17 or later")
+	default:
+		return 0, util.SystemErrorF("cannot get randomness: %v", err)
+	}
+}
+
+// NewRandomKey creates a random key (from RandReader) of the specified length.
+func NewRandomKey(length int) (*Key, error) {
+	return NewFixedLengthKeyFromReader(RandReader, length)
+}