tpmutil: Fix integer casting on 32-bit platforms (#253)

We need to make sure we only cast to ints once we are sure the value is
within the appropriate range (otherwise the value will wrap causing
errors).

Fixes #252

Signed-off-by: Joe Richey <joerichey@google.com>

tpmutil/structures.go

@@ -26,7 +26,7 @@ import (
 // from resulting in a massive memory allocation, potentially causing
 // an OOM condition on the system.
 // We expect no buffer from a TPM to approach 1Mb in size.
-const maxBytesBufferSize = 1024 * 1024 // 1Mb.
+const maxBytesBufferSize uint32 = 1024 * 1024 // 1Mb.
 
 // RawBytes is for Pack and RunCommand arguments that are already encoded.
 // Compared to []byte, RawBytes will not be prepended with slice length during
@@ -38,16 +38,16 @@ type U16Bytes []byte
 
 // TPMMarshal packs U16Bytes
 func (b *U16Bytes) TPMMarshal(out io.Writer) error {
-	size := uint16(len([]byte(*b)))
-	if err := binary.Write(out, binary.BigEndian, size); err != nil {
+	size := len([]byte(*b))
+	if err := binary.Write(out, binary.BigEndian, uint16(size)); err != nil {
 		return err
 	}
 
 	n, err := out.Write(*b)
 	if err != nil {
 		return err
 	}
-	if n != int(size) {
+	if n != size {
 		return fmt.Errorf("unable to write all contents of U16Bytes")
 	}
 	return nil
@@ -82,16 +82,16 @@ type U32Bytes []byte
 
 // TPMMarshal packs U32Bytes
 func (b *U32Bytes) TPMMarshal(out io.Writer) error {
-	size := uint32(len([]byte(*b)))
-	if err := binary.Write(out, binary.BigEndian, size); err != nil {
+	size := len([]byte(*b))
+	if err := binary.Write(out, binary.BigEndian, uint32(size)); err != nil {
 		return err
 	}
 
 	n, err := out.Write(*b)
 	if err != nil {
 		return err
 	}
-	if n != int(size) {
+	if n != size {
 		return fmt.Errorf("unable to write all contents of U32Bytes")
 	}
 	return nil
@@ -103,11 +103,12 @@ func (b *U32Bytes) TPMUnmarshal(in io.Reader) error {
 	if err := binary.Read(in, binary.BigEndian, &tmpSize); err != nil {
 		return err
 	}
-	size := int(tmpSize)
 
-	if size > maxBytesBufferSize {
+	if tmpSize > maxBytesBufferSize {
 		return bytes.ErrTooLarge
 	}
+	// We can now safely cast to an int on 32-bit or 64-bit machines
+	size := int(tmpSize)
 
 	if len(*b) >= size {
 		*b = (*b)[:size]