[release-branch.go1.18] [release-branch.go1.18 crypto/rand: properly handle large Read on windows

Use the batched reader to chunk large Read calls on windows to a max of
1 << 31 - 1 bytes. This prevents an infinite loop when trying to read
more than 1 << 32 -1 bytes, due to how RtlGenRandom works.

This change moves the batched function from rand_unix.go to rand.go,
since it is now needed for both windows and unix implementations.

Updates #52561
Fixes #52933
Fixes CVE-2022-30634

Change-Id: Id98fc4b1427e5cb2132762a445b2aed646a37473
Reviewed-on: https://go-review.googlesource.com/c/go/+/402257
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <valsorda@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit bb1f441618)
Reviewed-on: https://go-review.googlesource.com/c/go/+/406634
Reviewed-by: Damien Neil <dneil@google.com>

src/crypto/rand/rand.go

@@ -24,3 +24,21 @@ var Reader io.Reader
 func Read(b []byte) (n int, err error) {
 	return io.ReadFull(Reader, b)
 }
+
+// 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.go

@@ -7,6 +7,7 @@
 package rand
 
 import (
+	"errors"
 	"internal/syscall/unix"
 )
 
@@ -15,20 +16,6 @@ func init() {
 	altGetRandom = batched(getRandomBatch, maxGetRandomRead)
 }
 
-// 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) bool, readMax int) func([]byte) bool {
-	return func(buf []byte) bool {
-		for len(buf) > readMax {
-			if !f(buf[:readMax]) {
-				return false
-			}
-			buf = buf[readMax:]
-		}
-		return len(buf) == 0 || f(buf)
-	}
-}
-
 // If the kernel is too old to support the getrandom syscall(),
 // unix.GetRandom will immediately return ENOSYS and we will then fall back to
 // reading from /dev/urandom in rand_unix.go. unix.GetRandom caches the ENOSYS
@@ -36,7 +23,10 @@ func batched(f func([]byte) bool, readMax int) func([]byte) bool {
 // If the kernel supports the getrandom() syscall, unix.GetRandom will block
 // until the kernel has sufficient randomness (as we don't use GRND_NONBLOCK).
 // In this case, unix.GetRandom will not return an error.
-func getRandomBatch(p []byte) (ok bool) {
+func getRandomBatch(p []byte) (err error) {
 	n, err := unix.GetRandom(p, 0)
-	return n == len(p) && err == nil
+	if n != len(p) {
+		return errors.New("short read")
+	}
+	return err
 }

src/crypto/rand/rand_batched_test.go

@@ -8,20 +8,21 @@ package rand
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 )
 
 func TestBatched(t *testing.T) {
-	fillBatched := batched(func(p []byte) bool {
+	fillBatched := batched(func(p []byte) error {
 		for i := range p {
 			p[i] = byte(i)
 		}
-		return true
+		return nil
 	}, 5)
 
 	p := make([]byte, 13)
-	if !fillBatched(p) {
-		t.Fatal("batched function returned false")
+	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) {
@@ -30,15 +31,15 @@ func TestBatched(t *testing.T) {
 }
 
 func TestBatchedError(t *testing.T) {
-	b := batched(func(p []byte) bool { return false }, 5)
-	if b(make([]byte, 13)) {
-		t.Fatal("batched function should have returned false")
+	b := batched(func(p []byte) error { return errors.New("") }, 5)
+	if b(make([]byte, 13)) == nil {
+		t.Fatal("batched function should have returned error")
 	}
 }
 
 func TestBatchedEmpty(t *testing.T) {
-	b := batched(func(p []byte) bool { return false }, 5)
-	if !b(make([]byte, 0)) {
-		t.Fatal("empty slice should always return true")
+	b := batched(func(p []byte) error { return errors.New("") }, 5)
+	if err := b(make([]byte, 0)); err != nil {
+		t.Fatalf("empty slice should always return nil: %s", err)
 	}
 }

src/crypto/rand/rand_getentropy.go

@@ -14,7 +14,7 @@ func init() {
 	altGetRandom = getEntropy
 }
 
-func getEntropy(p []byte) (ok bool) {
+func getEntropy(p []byte) error {
 	// getentropy(2) returns a maximum of 256 bytes per call
 	for i := 0; i < len(p); i += 256 {
 		end := i + 256
@@ -23,8 +23,8 @@ func getEntropy(p []byte) (ok bool) {
 		}
 		err := unix.GetEntropy(p[i:end])
 		if err != nil {
-			return false
+			return err
 		}
 	}
-	return true
+	return nil
 }

src/crypto/rand/rand_unix.go

@@ -45,7 +45,7 @@ type devReader struct {
 
 // altGetRandom if non-nil specifies an OS-specific function to get
 // urandom-style randomness.
-var altGetRandom func([]byte) (ok bool)
+var altGetRandom func([]byte) (err error)
 
 func warnBlocked() {
 	println("crypto/rand: blocked for 60 seconds waiting to read random data from the kernel")
@@ -58,7 +58,7 @@ func (r *devReader) Read(b []byte) (n int, err error) {
 		t := time.AfterFunc(60*time.Second, warnBlocked)
 		defer t.Stop()
 	}
-	if altGetRandom != nil && r.name == urandomDevice && altGetRandom(b) {
+	if altGetRandom != nil && r.name == urandomDevice && altGetRandom(b) == nil {
 		return len(b), nil
 	}
 	r.mu.Lock()

src/crypto/rand/rand_windows.go

@@ -9,7 +9,6 @@ package rand
 
 import (
 	"internal/syscall/windows"
-	"os"
 )
 
 func init() { Reader = &rngReader{} }
@@ -17,16 +16,11 @@ func init() { Reader = &rngReader{} }
 type rngReader struct{}
 
 func (r *rngReader) Read(b []byte) (n int, err error) {
-	// RtlGenRandom only accepts 2**32-1 bytes at a time, so truncate.
-	inputLen := uint32(len(b))
-
-	if inputLen == 0 {
-		return 0, nil
+	// RtlGenRandom only returns 1<<32-1 bytes at a time. We only read at
+	// most 1<<31-1 bytes at a time so that  this works the same on 32-bit
+	// and 64-bit systems.
+	if err := batched(windows.RtlGenRandom, 1<<31-1)(b); err != nil {
+		return 0, err
 	}
-
-	err = windows.RtlGenRandom(b)
-	if err != nil {
-		return 0, os.NewSyscallError("RtlGenRandom", err)
-	}
-	return int(inputLen), nil
+	return len(b), nil
 }