github.com/ubuntu-core/snappy@v0.0.0-20210827154228-9e584df982bb/cmd/snap-preseed/preseed_linux.go

// -*- Mode: Go; indent-tabs-mode: t -*-

/*
 * Copyright (C) 2019-2020 Canonical Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package main

import (
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
	"syscall"

	"github.com/snapcore/snapd/dirs"
	"github.com/snapcore/snapd/osutil"
	"github.com/snapcore/snapd/osutil/squashfs"
	"github.com/snapcore/snapd/seed"
	"github.com/snapcore/snapd/snapdtool"
	"github.com/snapcore/snapd/strutil"
	"github.com/snapcore/snapd/timings"
)

var (
	// snapdMountPath is where target core/snapd is going to be mounted in the target chroot
	snapdMountPath = "/tmp/snapd-preseed"
	syscallChroot  = syscall.Chroot
)

// checkChroot does a basic sanity check of the target chroot environment, e.g. makes
// sure critical virtual filesystems (such as proc) are mounted. This is not meant to
// be exhaustive check, but one that prevents running the tool against a wrong directory
// by an accident, which would lead to hard to understand errors from snapd in preseed
// mode.
func checkChroot(preseedChroot string) error {
	exists, isDir, err := osutil.DirExists(preseedChroot)
	if err != nil {
		return fmt.Errorf("cannot verify %q: %v", preseedChroot, err)
	}
	if !exists || !isDir {
		return fmt.Errorf("cannot verify %q: is not a directory", preseedChroot)
	}

	if osutil.FileExists(filepath.Join(preseedChroot, dirs.SnapStateFile)) {
		return fmt.Errorf("the system at %q appears to be preseeded, pass --reset flag to clean it up", preseedChroot)
	}

	// sanity checks of the critical mountpoints inside chroot directory.
	required := map[string]bool{}
	// required mountpoints are relative to the preseed chroot
	for _, p := range []string{"/sys/kernel/security", "/proc", "/dev"} {
		required[filepath.Join(preseedChroot, p)] = true
	}
	entries, err := osutil.LoadMountInfo()
	if err != nil {
		return fmt.Errorf("cannot parse mount info: %v", err)
	}
	for _, ent := range entries {
		if _, ok := required[ent.MountDir]; ok {
			delete(required, ent.MountDir)
		}
	}
	// non empty required indicates missing mountpoint(s)
	if len(required) > 0 {
		var sorted []string
		for path := range required {
			sorted = append(sorted, path)
		}
		sort.Strings(sorted)
		parts := append([]string{""}, sorted...)
		return fmt.Errorf("cannot preseed without the following mountpoints:%s", strings.Join(parts, "\n - "))
	}

	path := filepath.Join(preseedChroot, "/sys/kernel/security/apparmor")
	if exists := osutil.FileExists(path); !exists {
		return fmt.Errorf("cannot preseed without access to %q", path)
	}

	return nil
}

var seedOpen = seed.Open

var systemSnapFromSeed = func(rootDir string) (string, error) {
	seedDir := filepath.Join(dirs.SnapSeedDirUnder(rootDir))
	seed, err := seedOpen(seedDir, "")
	if err != nil {
		return "", err
	}

	// load assertions into temporary database
	if err := seed.LoadAssertions(nil, nil); err != nil {
		return "", err
	}
	model := seed.Model()

	tm := timings.New(nil)
	if err := seed.LoadMeta(tm); err != nil {
		return "", err
	}

	// TODO: implement preseeding for core.
	if !model.Classic() {
		return "", fmt.Errorf("preseeding is only supported on classic systems")
	}

	var required string
	if seed.UsesSnapdSnap() {
		required = "snapd"
	} else {
		required = "core"
	}

	var snapPath string
	ess := seed.EssentialSnaps()
	if len(ess) > 0 {
		// core / snapd snap is the first essential snap.
		if ess[0].SnapName() == required {
			snapPath = ess[0].Path
		}
	}

	if snapPath == "" {
		return "", fmt.Errorf("%s snap not found", required)
	}

	return snapPath, nil
}

const snapdPreseedSupportVer = `2.43.3+`

type targetSnapdInfo struct {
	path    string
	version string
}

// chooseTargetSnapdVersion checks if the version of snapd under chroot env
// is good enough for preseeding. It checks both the snapd from the deb
// and from the seeded snap mounted under snapdMountPath and returns the
// information (path, version) about snapd to execute as part of preseeding
// (it picks the newer version of the two).
// The function must be called after syscall.Chroot(..).
func chooseTargetSnapdVersion() (*targetSnapdInfo, error) {
	// read snapd version from the mounted core/snapd snap
	infoPath := filepath.Join(snapdMountPath, dirs.CoreLibExecDir, "info")
	verFromSnap, err := snapdtool.SnapdVersionFromInfoFile(infoPath)
	if err != nil {
		return nil, err
	}

	// read snapd version from the main fs under chroot (snapd from the deb);
	// assumes running under chroot already.
	infoPath = filepath.Join(dirs.GlobalRootDir, dirs.CoreLibExecDir, "info")
	verFromDeb, err := snapdtool.SnapdVersionFromInfoFile(infoPath)
	if err != nil {
		return nil, err
	}

	res, err := strutil.VersionCompare(verFromSnap, verFromDeb)
	if err != nil {
		return nil, err
	}

	var whichVer, snapdPath string
	if res < 0 {
		// snapd from the deb under chroot is the candidate to run
		whichVer = verFromDeb
		snapdPath = filepath.Join(dirs.GlobalRootDir, dirs.CoreLibExecDir, "snapd")
	} else {
		// snapd from the mounted core/snapd snap is the candidate to run
		whichVer = verFromSnap
		snapdPath = filepath.Join(snapdMountPath, dirs.CoreLibExecDir, "snapd")
	}

	res, err = strutil.VersionCompare(whichVer, snapdPreseedSupportVer)
	if err != nil {
		return nil, err
	}
	if res < 0 {
		return nil, fmt.Errorf("snapd %s from the target system does not support preseeding, the minimum required version is %s",
			whichVer, snapdPreseedSupportVer)
	}

	return &targetSnapdInfo{path: snapdPath, version: whichVer}, nil
}

func prepareChroot(preseedChroot string) (*targetSnapdInfo, func(), error) {
	if err := syscallChroot(preseedChroot); err != nil {
		return nil, nil, fmt.Errorf("cannot chroot into %s: %v", preseedChroot, err)
	}

	if err := os.Chdir("/"); err != nil {
		return nil, nil, fmt.Errorf("cannot chdir to /: %v", err)
	}

	// GlobalRootDir is now relative to chroot env. We assume all paths
	// inside the chroot to be identical with the host.
	rootDir := dirs.GlobalRootDir
	if rootDir == "" {
		rootDir = "/"
	}

	coreSnapPath, err := systemSnapFromSeed(rootDir)
	if err != nil {
		return nil, nil, err
	}

	// create mountpoint for core/snapd
	where := filepath.Join(rootDir, snapdMountPath)
	if err := os.MkdirAll(where, 0755); err != nil {
		return nil, nil, err
	}

	removeMountpoint := func() {
		if err := os.Remove(where); err != nil {
			fmt.Fprintf(Stderr, "%v", err)
		}
	}

	fstype, fsopts := squashfs.FsType()
	mountArgs := []string{"-t", fstype, "-o", strings.Join(fsopts, ","), coreSnapPath, where}
	cmd := exec.Command("mount", mountArgs...)
	if out, err := cmd.CombinedOutput(); err != nil {
		removeMountpoint()
		return nil, nil, fmt.Errorf("cannot mount %s at %s in preseed mode: %v\n'mount %s' failed with: %s", coreSnapPath, where, err, strings.Join(mountArgs, " "), out)
	}

	unmount := func() {
		fmt.Fprintf(Stdout, "unmounting: %s\n", snapdMountPath)
		cmd := exec.Command("umount", snapdMountPath)
		if err := cmd.Run(); err != nil {
			fmt.Fprintf(Stderr, "%v", err)
		}
	}

	targetSnapd, err := chooseTargetSnapdVersion()
	if err != nil {
		unmount()
		removeMountpoint()
		return nil, nil, err
	}

	return targetSnapd, func() {
		unmount()
		removeMountpoint()
	}, nil
}

// runPreseedMode runs snapd in a preseed mode. It assumes running in a chroot.
// The chroot is expected to be set-up and ready to use (critical system directories mounted).
func runPreseedMode(preseedChroot string, targetSnapd *targetSnapdInfo) error {
	// run snapd in preseed mode
	cmd := exec.Command(targetSnapd.path)
	cmd.Env = os.Environ()
	cmd.Env = append(cmd.Env, "SNAPD_PRESEED=1")
	cmd.Stderr = Stderr
	cmd.Stdout = Stdout

	// note, snapdPath is relative to preseedChroot
	fmt.Fprintf(Stdout, "starting to preseed root: %s\nusing snapd binary: %s (%s)\n", preseedChroot, targetSnapd.path, targetSnapd.version)

	if err := cmd.Run(); err != nil {
		return fmt.Errorf("error running snapd in preseed mode: %v\n", err)
	}

	return nil
}