crypto/aes: fix overrun in assembly encrypt/decrypt

Fixes #7928.

LGTM=bradfitz
R=golang-codereviews
CC=agl, bradfitz, golang-codereviews
https://golang.org/cl/91320043

src/pkg/crypto/aes/aes_test.go

@@ -354,6 +354,34 @@ func TestCipherDecrypt(t *testing.T) {
 	}
 }
 
+// Test short input/output.
+// Assembly used to not notice.
+// See issue 7928.
+func TestShortBlocks(t *testing.T) {
+	bytes := func(n int) []byte { return make([]byte, n) }
+
+	c, _ := NewCipher(bytes(16))
+
+	mustPanic(t, "crypto/aes: input not full block", func() { c.Encrypt(bytes(1), bytes(1)) })
+	mustPanic(t, "crypto/aes: input not full block", func() { c.Decrypt(bytes(1), bytes(1)) })
+	mustPanic(t, "crypto/aes: input not full block", func() { c.Encrypt(bytes(100), bytes(1)) })
+	mustPanic(t, "crypto/aes: input not full block", func() { c.Decrypt(bytes(100), bytes(1)) })
+	mustPanic(t, "crypto/aes: output not full block", func() { c.Encrypt(bytes(1), bytes(100)) })
+	mustPanic(t, "crypto/aes: output not full block", func() { c.Decrypt(bytes(1), bytes(100)) })
+}
+
+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()
+}
+
 func BenchmarkEncrypt(b *testing.B) {
 	tt := encryptTests[0]
 	c, err := NewCipher(tt.key)

src/pkg/crypto/aes/cipher.go

@@ -46,9 +46,21 @@ func NewCipher(key []byte) (cipher.Block, error) {
 func (c *aesCipher) BlockSize() int { return BlockSize }
 
 func (c *aesCipher) Encrypt(dst, src []byte) {
+	if len(src) < BlockSize {
+		panic("crypto/aes: input not full block")
+	}
+	if len(dst) < BlockSize {
+		panic("crypto/aes: output not full block")
+	}
 	encryptBlock(c.enc, dst, src)
 }
 
 func (c *aesCipher) Decrypt(dst, src []byte) {
+	if len(src) < BlockSize {
+		panic("crypto/aes: input not full block")
+	}
+	if len(dst) < BlockSize {
+		panic("crypto/aes: output not full block")
+	}
 	decryptBlock(c.dec, dst, src)
 }

src/pkg/crypto/aes/cipher_asm.go

@@ -21,6 +21,7 @@ func encryptBlock(xk []uint32, dst, src []byte) {
 		encryptBlockGo(xk, dst, src)
 	}
 }
+
 func decryptBlock(xk []uint32, dst, src []byte) {
 	if useAsm {
 		decryptBlockAsm(len(xk)/4-1, &xk[0], &dst[0], &src[0])
@@ -28,6 +29,7 @@ func decryptBlock(xk []uint32, dst, src []byte) {
 		decryptBlockGo(xk, dst, src)
 	}
 }
+
 func expandKey(key []byte, enc, dec []uint32) {
 	if useAsm {
 		rounds := 10