github.com/kubiko/snapd@v0.0.0-20201013125620-d4f3094d9ddf/usersession/userd/launcher.go

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

/*
 * Copyright (C) 2017 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 userd

import (
	"fmt"
	"net/url"
	"os"
	"os/exec"
	"syscall"
	"time"

	"github.com/godbus/dbus"

	"github.com/snapcore/snapd/i18n"
	"github.com/snapcore/snapd/osutil/sys"
	"github.com/snapcore/snapd/strutil"
	"github.com/snapcore/snapd/usersession/userd/ui"
)

const launcherIntrospectionXML = `
<interface name="org.freedesktop.DBus.Peer">
	<method name='Ping'>
	</method>
	<method name='GetMachineId'>
               <arg type='s' name='machine_uuid' direction='out'/>
	</method>
</interface>
<interface name='io.snapcraft.Launcher'>
	<method name='OpenURL'>
		<arg type='s' name='url' direction='in'/>
	</method>
	<method name="OpenFile">
		<arg type="s" name="parent_window" direction="in"/>
		<arg type="h" name="fd" direction="in"/>
	</method>
</interface>`

// allowedURLSchemes are those that can be passed to xdg-open so that it may
// launch the handler for the url scheme on behalf of the snap (and therefore
// outside of the calling snap's confinement). Historically we've been
// conservative about adding url schemes but the thinking was refined in
// https://github.com/snapcore/snapd/pull/7731#pullrequestreview-362900171
//
// The current criteria for adding url schemes is:
// * understanding and documenting the scheme in this file
// * the scheme itself does not cause xdg-open to open files (eg, file:// or
//   matching '^[[:alpha:]+\.\-]+:' (from xdg-open source))
// * verifying that the recipient of the url (ie, what xdg-open calls) won't
//   process file paths/etc that can be leveraged to break out of the sandbox
//   (but understanding how the url can drive the recipient application is
//   important)
//
// This code uses golang's net/url.Parse() which will help ensure the url is
// ok before passing to xdg-open. xdg-open itself properly quotes the url so
// shell metacharacters are blocked.
var (
	allowedURLSchemes = []string{
		// apt: the scheme allows specifying a package for xdg-open to pass to an
		//   apt-handling application, like gnome-software, apturl, etc which are all
		//   protected by policykit
		//   - scheme: apt:<name of package>
		//   - https://github.com/snapcore/snapd/pull/7731
		"apt",
		// help: the scheme allows for specifying a help URL. This code ensures that
		//   the url is parseable
		//   - scheme: help://topic
		//   - https://github.com/snapcore/snapd/pull/6493
		"help",
		// http/https: the scheme allows specifying a web URL. This code ensures that
		//   the url is parseable
		//   - scheme: http(s)://example.com
		"http",
		"https",
		// mailto: the scheme allows for specifying an email address
		//   - scheme: mailto:foo@example.com
		"mailto",
		// msteams: the scheme is a thin wrapper around https.
		//   - scheme: msteams:...
		//   - https://github.com/snapcore/snapd/pull/8761
		"msteams",
		// TODO: document slack URL scheme.
		"slack",
		// snap: the scheme allows specifying a package for xdg-open to pass to a
		//   snap-handling installer application, like snap-store, etc which are
		//   protected by policykit/snap login
		//   - https://github.com/snapcore/snapd/pull/5181
		"snap",
		// zoommtg: the scheme is a modified web url scheme
		//   - scheme: https://medium.com/zoom-developer-blog/zoom-url-schemes-748b95fd9205
		//     (eg, zoommtg://zoom.us/...)
		//   - https://github.com/snapcore/snapd/pull/8304
		"zoommtg",
		// zoomphonecall: another zoom URL scheme, for dialing phone numbers
		//   - https://github.com/snapcore/snapd/pull/8910
		"zoomphonecall",
		// zoomus: alternative name for zoommtg
		//   - https://github.com/snapcore/snapd/pull/8910
		"zoomus",
	}
)

// Launcher implements the 'io.snapcraft.Launcher' DBus interface.
type Launcher struct {
	conn *dbus.Conn
}

// Interface returns the name of the interface this object implements
func (s *Launcher) Interface() string {
	return "io.snapcraft.Launcher"
}

// ObjectPath returns the path that the object is exported as
func (s *Launcher) ObjectPath() dbus.ObjectPath {
	return "/io/snapcraft/Launcher"
}

// IntrospectionData gives the XML formatted introspection description
// of the DBus service.
func (s *Launcher) IntrospectionData() string {
	return launcherIntrospectionXML
}

func makeAccessDeniedError(err error) *dbus.Error {
	return &dbus.Error{
		Name: "org.freedesktop.DBus.Error.AccessDenied",
		Body: []interface{}{err.Error()},
	}
}

// OpenURL implements the 'OpenURL' method of the 'io.snapcraft.Launcher'
// DBus interface. Before the provided url is passed to xdg-open the scheme is
// validated against a list of allowed schemes. All other schemes are denied.
func (s *Launcher) OpenURL(addr string, sender dbus.Sender) *dbus.Error {
	u, err := url.Parse(addr)
	if err != nil {
		return &dbus.ErrMsgInvalidArg
	}

	if !strutil.ListContains(allowedURLSchemes, u.Scheme) {
		return makeAccessDeniedError(fmt.Errorf("Supplied URL scheme %q is not allowed", u.Scheme))
	}

	if err := exec.Command("xdg-open", addr).Run(); err != nil {
		return dbus.MakeFailedError(fmt.Errorf("cannot open supplied URL"))
	}

	return nil
}

// fdToFilename determines the path associated with an open file descriptor.
//
// The file descriptor cannot be opened using O_PATH and must refer to
// a regular file or to a directory. The symlink at /proc/self/fd/<fd>
// is read to determine the filename. The descriptor is also fstat'ed
// and the resulting device number and inode number are compared to
// stat on the path determined earlier. The numbers must match.
func fdToFilename(fd int) (string, error) {
	flags, err := sys.FcntlGetFl(fd)
	if err != nil {
		return "", err
	}
	// File descriptors opened with O_PATH do not imply access to
	// the file in question.
	if flags&sys.O_PATH != 0 {
		return "", fmt.Errorf("cannot use file descriptors opened using O_PATH")
	}

	// Determine the file name associated with the passed file descriptor.
	filename, err := os.Readlink(fmt.Sprintf("/proc/self/fd/%d", fd))
	if err != nil {
		return "", err
	}

	var fileStat, fdStat syscall.Stat_t
	if err := syscall.Stat(filename, &fileStat); err != nil {
		return "", err
	}
	if err := syscall.Fstat(fd, &fdStat); err != nil {
		return "", err
	}

	// Sanity check to ensure we've got the right file
	if fdStat.Dev != fileStat.Dev || fdStat.Ino != fileStat.Ino {
		return "", fmt.Errorf("cannot determine file name")
	}

	fileType := fileStat.Mode & syscall.S_IFMT
	if fileType != syscall.S_IFREG && fileType != syscall.S_IFDIR {
		return "", fmt.Errorf("cannot open anything other than regular files or directories")
	}

	return filename, nil
}

func (s *Launcher) OpenFile(parentWindow string, clientFd dbus.UnixFD, sender dbus.Sender) *dbus.Error {
	// godbus transfers ownership of this file descriptor to us
	fd := int(clientFd)
	defer syscall.Close(fd)

	filename, err := fdToFilename(fd)
	if err != nil {
		return dbus.MakeFailedError(err)
	}

	snap, err := snapFromSender(s.conn, sender)
	if err != nil {
		return dbus.MakeFailedError(err)
	}
	dialog, err := ui.New()
	if err != nil {
		return dbus.MakeFailedError(err)
	}
	answeredYes := dialog.YesNo(
		i18n.G("Allow opening file?"),
		fmt.Sprintf(i18n.G("Allow snap %q to open file %q?"), snap, filename),
		&ui.DialogOptions{
			Timeout: 5 * 60 * time.Second,
			Footer:  i18n.G("This dialog will close automatically after 5 minutes of inactivity."),
		},
	)
	if !answeredYes {
		return dbus.MakeFailedError(fmt.Errorf("permission denied"))
	}

	if err = exec.Command("xdg-open", filename).Run(); err != nil {
		return dbus.MakeFailedError(fmt.Errorf("cannot open supplied URL"))
	}

	return nil
}