internal/subtle: add Any/InexactOverlap (new package) and apply them across packages

AnyOverlap and InexactOverlap implement checks for the aliasing
requirements defined by the crypto/cipher interfaces. Apply them to all
implementations as the actual requirement could be architecture-dependent
and user code should not rely on undefined behavior.

Updates golang/go#21624

Change-Id: I465de02fb3fec4e0c6f1fdee1ef6ae7ed5abff10
Reviewed-on: https://go-review.googlesource.com/112236
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: FiloSottile

chacha20poly1305/chacha20poly1305_amd64.go

@@ -9,6 +9,7 @@ package chacha20poly1305
 import (
 	"encoding/binary"
 
+	"golang.org/x/crypto/internal/subtle"
 	"golang.org/x/sys/cpu"
 )
 
@@ -55,6 +56,9 @@ func (c *chacha20poly1305) seal(dst, nonce, plaintext, additionalData []byte) []
 	setupState(&state, &c.key, nonce)
 
 	ret, out := sliceForAppend(dst, len(plaintext)+16)
+	if subtle.InexactOverlap(out, plaintext) {
+		panic("chacha20poly1305: invalid buffer overlap")
+	}
 	chacha20Poly1305Seal(out[:], state[:], plaintext, additionalData)
 	return ret
 }
@@ -69,6 +73,9 @@ func (c *chacha20poly1305) open(dst, nonce, ciphertext, additionalData []byte) (
 
 	ciphertext = ciphertext[:len(ciphertext)-16]
 	ret, out := sliceForAppend(dst, len(ciphertext))
+	if subtle.InexactOverlap(out, ciphertext) {
+		panic("chacha20poly1305: invalid buffer overlap")
+	}
 	if !chacha20Poly1305Open(out, state[:], ciphertext, additionalData) {
 		for i := range out {
 			out[i] = 0

chacha20poly1305/chacha20poly1305_generic.go

@@ -8,6 +8,7 @@ import (
 	"encoding/binary"
 
 	"golang.org/x/crypto/internal/chacha20"
+	"golang.org/x/crypto/internal/subtle"
 	"golang.org/x/crypto/poly1305"
 )
 
@@ -17,6 +18,9 @@ func roundTo16(n int) int {
 
 func (c *chacha20poly1305) sealGeneric(dst, nonce, plaintext, additionalData []byte) []byte {
 	ret, out := sliceForAppend(dst, len(plaintext)+poly1305.TagSize)
+	if subtle.InexactOverlap(out, plaintext) {
+		panic("chacha20poly1305: invalid buffer overlap")
+	}
 
 	var polyKey [32]byte
 	s := chacha20.New(c.key, [3]uint32{
@@ -62,6 +66,9 @@ func (c *chacha20poly1305) openGeneric(dst, nonce, ciphertext, additionalData []
 	binary.LittleEndian.PutUint64(polyInput[len(polyInput)-8:], uint64(len(ciphertext)))
 
 	ret, out := sliceForAppend(dst, len(ciphertext))
+	if subtle.InexactOverlap(out, ciphertext) {
+		panic("chacha20poly1305: invalid buffer overlap")
+	}
 	if !poly1305.Verify(&tag, polyInput, &polyKey) {
 		for i := range out {
 			out[i] = 0

internal/chacha20/chacha_generic.go

@@ -9,6 +9,8 @@ package chacha20
 import (
 	"crypto/cipher"
 	"encoding/binary"
+
+	"golang.org/x/crypto/internal/subtle"
 )
 
 // assert that *Cipher implements cipher.Stream
@@ -41,6 +43,13 @@ func New(key [8]uint32, nonce [3]uint32) *Cipher {
 // the src buffers was passed in a single run. That is, Cipher
 // maintains state and does not reset at each XORKeyStream call.
 func (s *Cipher) XORKeyStream(dst, src []byte) {
+	if len(dst) < len(src) {
+		panic("chacha20: output smaller than input")
+	}
+	if subtle.InexactOverlap(dst[:len(src)], src) {
+		panic("chacha20: invalid buffer overlap")
+	}
+
 	// xor src with buffered keystream first
 	if s.len != 0 {
 		buf := s.buf[len(s.buf)-s.len:]

internal/subtle/aliasing.go

@@ -0,0 +1,30 @@
+// Copyright 2018 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 subtle implements functions that are often useful in cryptographic
+// code but require careful thought to use correctly.
+package subtle // import "golang.org/x/crypto/internal/subtle"
+
+import "unsafe"
+
+// AnyOverlap reports whether x and y share memory at any (not necessarily
+// corresponding) index. The memory beyond the slice length is ignored.
+func AnyOverlap(x, y []byte) bool {
+	return len(x) > 0 && len(y) > 0 &&
+		uintptr(unsafe.Pointer(&x[0])) <= uintptr(unsafe.Pointer(&y[len(y)-1])) &&
+		uintptr(unsafe.Pointer(&y[0])) <= uintptr(unsafe.Pointer(&x[len(x)-1]))
+}
+
+// InexactOverlap reports whether x and y share memory at any non-corresponding
+// index. The memory beyond the slice length is ignored. Note that x and y can
+// have different lengths and still not have any inexact overlap.
+//
+// InexactOverlap can be used to implement the requirements of the crypto/cipher
+// AEAD, Block, BlockMode and Stream interfaces.
+func InexactOverlap(x, y []byte) bool {
+	if len(x) == 0 || len(y) == 0 || &x[0] == &y[0] {
+		return false
+	}
+	return AnyOverlap(x, y)
+}

internal/subtle/aliasing_test.go

@@ -0,0 +1,48 @@
+// Copyright 2018 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 subtle_test
+
+import (
+	"testing"
+
+	"golang.org/x/crypto/internal/subtle"
+)
+
+var a, b [100]byte
+
+var aliasingTests = []struct {
+	x, y                       []byte
+	anyOverlap, inexactOverlap bool
+}{
+	{a[:], b[:], false, false},
+	{a[:], b[:0], false, false},
+	{a[:], b[:50], false, false},
+	{a[40:50], a[50:60], false, false},
+	{a[40:50], a[60:70], false, false},
+	{a[:51], a[50:], true, true},
+	{a[:], a[:], true, false},
+	{a[:50], a[:60], true, false},
+	{a[:], nil, false, false},
+	{nil, nil, false, false},
+	{a[:], a[:0], false, false},
+}
+
+func testAliasing(t *testing.T, i int, x, y []byte, anyOverlap, inexactOverlap bool) {
+	any := subtle.AnyOverlap(x, y)
+	if any != anyOverlap {
+		t.Errorf("%d: wrong AnyOverlap result, expected %v, got %v", i, anyOverlap, any)
+	}
+	inexact := subtle.InexactOverlap(x, y)
+	if inexact != inexactOverlap {
+		t.Errorf("%d: wrong InexactOverlap result, expected %v, got %v", i, inexactOverlap, any)
+	}
+}
+
+func TestAliasing(t *testing.T) {
+	for i, tt := range aliasingTests {
+		testAliasing(t, i, tt.x, tt.y, tt.anyOverlap, tt.inexactOverlap)
+		testAliasing(t, i, tt.y, tt.x, tt.anyOverlap, tt.inexactOverlap)
+	}
+}

nacl/secretbox/secretbox.go

@@ -35,6 +35,7 @@ This package is interoperable with NaCl: https://nacl.cr.yp.to/secretbox.html.
 package secretbox // import "golang.org/x/crypto/nacl/secretbox"
 
 import (
+	"golang.org/x/crypto/internal/subtle"
 	"golang.org/x/crypto/poly1305"
 	"golang.org/x/crypto/salsa20/salsa"
 )
@@ -87,6 +88,9 @@ func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte {
 	copy(poly1305Key[:], firstBlock[:])
 
 	ret, out := sliceForAppend(out, len(message)+poly1305.TagSize)
+	if subtle.AnyOverlap(out, message) {
+		panic("nacl: invalid buffer overlap")
+	}
 
 	// We XOR up to 32 bytes of message with the keystream generated from
 	// the first block.
@@ -118,7 +122,7 @@ func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte {
 // Open authenticates and decrypts a box produced by Seal and appends the
 // message to out, which must not overlap box. The output will be Overhead
 // bytes smaller than box.
-func Open(out []byte, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool) {
+func Open(out, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool) {
 	if len(box) < Overhead {
 		return nil, false
 	}
@@ -143,6 +147,9 @@ func Open(out []byte, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool)
 	}
 
 	ret, out := sliceForAppend(out, len(box)-Overhead)
+	if subtle.AnyOverlap(out, box) {
+		panic("nacl: invalid buffer overlap")
+	}
 
 	// We XOR up to 32 bytes of box with the keystream generated from
 	// the first block.

nacl/sign/sign.go

@@ -24,6 +24,7 @@ import (
 	"io"
 
 	"golang.org/x/crypto/ed25519"
+	"golang.org/x/crypto/internal/subtle"
 )
 
 // Overhead is the number of bytes of overhead when signing a message.
@@ -47,6 +48,9 @@ func GenerateKey(rand io.Reader) (publicKey *[32]byte, privateKey *[64]byte, err
 func Sign(out, message []byte, privateKey *[64]byte) []byte {
 	sig := ed25519.Sign(ed25519.PrivateKey((*privateKey)[:]), message)
 	ret, out := sliceForAppend(out, Overhead+len(message))
+	if subtle.AnyOverlap(out, message) {
+		panic("nacl: invalid buffer overlap")
+	}
 	copy(out, sig)
 	copy(out[Overhead:], message)
 	return ret
@@ -63,6 +67,9 @@ func Open(out, signedMessage []byte, publicKey *[32]byte) ([]byte, bool) {
 		return nil, false
 	}
 	ret, out := sliceForAppend(out, len(signedMessage)-Overhead)
+	if subtle.AnyOverlap(out, signedMessage) {
+		panic("nacl: invalid buffer overlap")
+	}
 	copy(out, signedMessage[Overhead:])
 	return ret, true
 }

salsa20/salsa20.go

@@ -24,6 +24,7 @@ package salsa20 // import "golang.org/x/crypto/salsa20"
 // TODO(agl): implement XORKeyStream12 and XORKeyStream8 - the reduced round variants of Salsa20.
 
 import (
+	"golang.org/x/crypto/internal/subtle"
 	"golang.org/x/crypto/salsa20/salsa"
 )
 
@@ -34,6 +35,9 @@ func XORKeyStream(out, in []byte, nonce []byte, key *[32]byte) {
 	if len(out) < len(in) {
 		panic("salsa20: output smaller than input")
 	}
+	if subtle.InexactOverlap(out[:len(in)], in) {
+		panic("salsa20: invalid buffer overlap")
+	}
 
 	var subNonce [16]byte
 

xts/xts.go

@@ -25,6 +25,8 @@ import (
 	"crypto/cipher"
 	"encoding/binary"
 	"errors"
+
+	"golang.org/x/crypto/internal/subtle"
 )
 
 // Cipher contains an expanded key structure. It doesn't contain mutable state
@@ -64,6 +66,9 @@ func (c *Cipher) Encrypt(ciphertext, plaintext []byte, sectorNum uint64) {
 	if len(plaintext)%blockSize != 0 {
 		panic("xts: plaintext is not a multiple of the block size")
 	}
+	if subtle.InexactOverlap(ciphertext[:len(plaintext)], plaintext) {
+		panic("xts: invalid buffer overlap")
+	}
 
 	var tweak [blockSize]byte
 	binary.LittleEndian.PutUint64(tweak[:8], sectorNum)
@@ -95,6 +100,9 @@ func (c *Cipher) Decrypt(plaintext, ciphertext []byte, sectorNum uint64) {
 	if len(ciphertext)%blockSize != 0 {
 		panic("xts: ciphertext is not a multiple of the block size")
 	}
+	if subtle.InexactOverlap(plaintext[:len(ciphertext)], ciphertext) {
+		panic("xts: invalid buffer overlap")
+	}
 
 	var tweak [blockSize]byte
 	binary.LittleEndian.PutUint64(tweak[:8], sectorNum)