crypto/rand: prevent Read argument from escaping to heap

Mateusz had this idea before me in CL 578516, but it got much easier
after the recent cleanup.

It's unfortunate we lose the test coverage of batched, but the package
is significantly simpler than when we introduced it, so it should be
easier to review that everything does what it's supposed to do.

Fixes #66779

Co-authored-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Change-Id: Id35f1172e678fec184efb0efae3631afac8121d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/602498
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/rand/rand.go

@@ -70,28 +70,20 @@ func fatal(string)
 // If [Reader] is set to a non-default value, Read calls [io.ReadFull] on
 // [Reader] and crashes the program irrecoverably if an error is returned.
 func Read(b []byte) (n int, err error) {
-	_, err = io.ReadFull(Reader, b)
+	// We don't want b to escape to the heap, but escape analysis can't see
+	// through a potentially overridden Reader, so we special-case the default
+	// case which we can keep non-escaping, and in the general case we read into
+	// a heap buffer and copy from it.
+	if r, ok := Reader.(*reader); ok {
+		_, err = r.Read(b)
+	} else {
+		bb := make([]byte, len(b))
+		_, err = io.ReadFull(Reader, bb)
+		copy(b, bb)
+	}
 	if err != nil {
 		fatal("crypto/rand: failed to read random data (see https://go.dev/issue/66821): " + err.Error())
 		panic("unreachable") // To be sure.
 	}
 	return len(b), nil
 }
-
-// batched returns a function that calls f to populate a []byte by chunking it
-// into subslices of, at most, readMax bytes.
-func batched(f func([]byte) error, readMax int) func([]byte) error {
-	return func(out []byte) error {
-		for len(out) > 0 {
-			read := len(out)
-			if read > readMax {
-				read = readMax
-			}
-			if err := f(out[:read]); err != nil {
-				return err
-			}
-			out = out[read:]
-		}
-		return nil
-	}
-}

src/crypto/rand/rand_batched_test.go

@@ -1,75 +0,0 @@
-// Copyright 2014 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.
-
-//go:build unix
-
-package rand
-
-import (
-	"bytes"
-	"errors"
-	prand "math/rand"
-	"testing"
-)
-
-func TestBatched(t *testing.T) {
-	fillBatched := batched(func(p []byte) error {
-		for i := range p {
-			p[i] = byte(i)
-		}
-		return nil
-	}, 5)
-
-	p := make([]byte, 13)
-	if err := fillBatched(p); err != nil {
-		t.Fatalf("batched function returned error: %s", err)
-	}
-	expected := []byte{0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2}
-	if !bytes.Equal(expected, p) {
-		t.Errorf("incorrect batch result: got %x, want %x", p, expected)
-	}
-}
-
-func TestBatchedBuffering(t *testing.T) {
-	backingStore := make([]byte, 1<<23)
-	prand.Read(backingStore)
-	backingMarker := backingStore[:]
-	output := make([]byte, len(backingStore))
-	outputMarker := output[:]
-
-	fillBatched := batched(func(p []byte) error {
-		n := copy(p, backingMarker)
-		backingMarker = backingMarker[n:]
-		return nil
-	}, 731)
-
-	for len(outputMarker) > 0 {
-		max := 9200
-		if max > len(outputMarker) {
-			max = len(outputMarker)
-		}
-		howMuch := prand.Intn(max + 1)
-		if err := fillBatched(outputMarker[:howMuch]); err != nil {
-			t.Fatalf("batched function returned error: %s", err)
-		}
-		outputMarker = outputMarker[howMuch:]
-	}
-	if !bytes.Equal(backingStore, output) {
-		t.Error("incorrect batch result")
-	}
-}
-
-func TestBatchedError(t *testing.T) {
-	b := batched(func(p []byte) error { return errors.New("failure") }, 5)
-	if b(make([]byte, 13)) == nil {
-		t.Fatal("batched function should have returned an error")
-	}
-}
-
-func TestBatchedEmpty(t *testing.T) {
-	b := batched(func(p []byte) error { return errors.New("failure") }, 5)
-	if b(make([]byte, 0)) != nil {
-		t.Fatal("empty slice should always return successful")
-	}
-}

src/crypto/rand/rand_getentropy.go

@@ -8,5 +8,17 @@ package rand
 
 import "internal/syscall/unix"
 
-// getentropy(2) returns a maximum of 256 bytes per call.
-var read = batched(unix.GetEntropy, 256)
+func read(b []byte) error {
+	for len(b) > 0 {
+		size := len(b)
+		if size > 256 {
+			size = 256
+		}
+		// getentropy(2) returns a maximum of 256 bytes per call.
+		if err := unix.GetEntropy(b[:size]); err != nil {
+			return err
+		}
+		b = b[size:]
+	}
+	return nil
+}

src/crypto/rand/rand_js.go

@@ -24,3 +24,21 @@ func getRandom(b []byte) error {
 	js.CopyBytesToGo(b, a)
 	return nil
 }
+
+// batched returns a function that calls f to populate a []byte by chunking it
+// into subslices of, at most, readMax bytes.
+func batched(f func([]byte) error, readMax int) func([]byte) error {
+	return func(out []byte) error {
+		for len(out) > 0 {
+			read := len(out)
+			if read > readMax {
+				read = readMax
+			}
+			if err := f(out[:read]); err != nil {
+				return err
+			}
+			out = out[read:]
+		}
+		return nil
+	}
+}

src/crypto/rand/rand_test.go

@@ -7,8 +7,10 @@ package rand_test
 import (
 	"bytes"
 	"compress/flate"
+	"crypto/internal/boring"
 	. "crypto/rand"
 	"io"
+	"runtime"
 	"sync"
 	"testing"
 )
@@ -121,6 +123,30 @@ func TestConcurrentRead(t *testing.T) {
 	wg.Wait()
 }
 
+var sink byte
+
+func TestAllocations(t *testing.T) {
+	if boring.Enabled {
+		// Might be fixable with https://go.dev/issue/56378.
+		t.Skip("boringcrypto allocates")
+	}
+	if runtime.GOOS == "aix" {
+		t.Skip("/dev/urandom read path allocates")
+	}
+	if runtime.GOOS == "js" {
+		t.Skip("syscall/js allocates")
+	}
+
+	n := int(testing.AllocsPerRun(10, func() {
+		buf := make([]byte, 32)
+		Read(buf)
+		sink ^= buf[0]
+	}))
+	if n > 0 {
+		t.Errorf("allocs = %d, want 0", n)
+	}
+}
+
 func BenchmarkRead(b *testing.B) {
 	b.Run("4", func(b *testing.B) {
 		benchmarkRead(b, 4)

src/internal/syscall/unix/getentropy_openbsd.go

@@ -14,4 +14,5 @@ func GetEntropy(p []byte) error {
 }
 
 //go:linkname getentropy syscall.getentropy
+//go:noescape
 func getentropy(p []byte) error