fleet/pkg/file/xar.go

package file

//		Copyright 2023 SAS Software
//
//	 Licensed under the Apache License, Version 2.0 (the "License");
//	 you may not use this file except in compliance with the License.
//	 You may obtain a copy of the License at
//
//	     http://www.apache.org/licenses/LICENSE-2.0
//
//	 Unless required by applicable law or agreed to in writing, software
//	 distributed under the License is distributed on an "AS IS" BASIS,
//	 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//	 See the License for the specific language governing permissions and
//	 limitations under the License.
//
// xar contains utilities to parse xar files, most of the logic here is a
// simplified version extracted from the logic to sign xar files in
// https://github.com/sassoftware/relic

import (
	"bytes"
	"compress/zlib"
	"crypto"
	"encoding/binary"
	"encoding/xml"
	"errors"
	"fmt"
	"io"
)

// xarMagic is the [file signature][1] (or magic bytes) for xar
//
// [1]: https://en.wikipedia.org/wiki/List_of_file_signatures
const xarMagic = 0x78617221

const (
	hashNone uint32 = iota
	hashSHA1
	hashMD5
	hashSHA256
	hashSHA512
)

var (
	// ErrInvalidType is used to signal that the provided package can't be
	// parsed because is an invalid file type.
	ErrInvalidType = errors.New("invalid file type")
	// ErrNotSigned is used to signal that the provided package doesn't
	// contain a signature.
	ErrNotSigned = errors.New("file is not signed")
)

type xarHeader struct {
	Magic            uint32
	HeaderSize       uint16
	Version          uint16
	CompressedSize   int64
	UncompressedSize int64
	HashType         uint32
}

type tocXar struct {
	TOC toc `xml:"toc"`
}

type toc struct {
	Signature  *any `xml:"signature"`
	XSignature *any `xml:"x-signature"`
}

// CheckPKGSignature checks if the provided bytes correspond to a signed pkg
// (xar) file.
//
// - If the file is not xar, it returns a ErrInvalidType error
// - If the file is not signed, it returns a ErrNotSigned error
func CheckPKGSignature(pkg io.Reader) error {
	buff := bytes.NewBuffer(nil)
	if _, err := io.Copy(buff, pkg); err != nil {
		return err
	}
	r := bytes.NewReader(buff.Bytes())

	hdr, hashType, err := parseHeader(io.NewSectionReader(r, 0, 28))
	if err != nil {
		return err
	}

	base := int64(hdr.HeaderSize)
	toc, err := parseTOC(io.NewSectionReader(r, base, hdr.CompressedSize), hashType)
	if err != nil {
		return err
	}

	if toc.Signature == nil && toc.XSignature == nil {
		return ErrNotSigned
	}

	return nil
}

func decompress(r io.Reader) ([]byte, error) {
	zr, err := zlib.NewReader(r)
	if err != nil {
		return nil, err
	}
	defer zr.Close()
	return io.ReadAll(zr)
}

func parseTOC(r io.Reader, hashType crypto.Hash) (*toc, error) {
	tocHash := hashType.New()
	r = io.TeeReader(r, tocHash)
	decomp, err := decompress(r)
	if err != nil {
		return nil, fmt.Errorf("decompressing TOC: %w", err)
	}
	var toc tocXar
	if err := xml.Unmarshal(decomp, &toc); err != nil {
		return nil, fmt.Errorf("decoding TOC: %w", err)
	}
	return &toc.TOC, nil
}

func parseHeader(r io.Reader) (xarHeader, crypto.Hash, error) {
	var hdr xarHeader
	if err := binary.Read(r, binary.BigEndian, &hdr); err != nil {
		return xarHeader{}, 0, err
	}

	if hdr.Magic != xarMagic {
		return hdr, 0, ErrInvalidType
	}

	var hashType crypto.Hash
	switch hdr.HashType {
	case hashSHA1:
		hashType = crypto.SHA1
	case hashSHA256:
		hashType = crypto.SHA256
	case hashSHA512:
		hashType = crypto.SHA512
	default:
		return xarHeader{}, 0, fmt.Errorf("unknown hash algorithm %d", hdr.HashType)
	}

	return hdr, hashType, nil
}