/
backend.go
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/
backend.go
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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2016 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 dbus implements interaction between snappy and dbus.
//
// Snappy creates dbus configuration files that describe how various
// services on the system bus can communicate with other peers.
//
// Each configuration is an XML file containing <busconfig>...</busconfig>.
// Particular security snippets define whole <policy>...</policy> entires.
// This is explained in detail in https://dbus.freedesktop.org/doc/dbus-daemon.1.html
package dbus
import (
"bytes"
"fmt"
"os"
"path/filepath"
"github.com/snapcore/snapd/dirs"
"github.com/snapcore/snapd/interfaces"
"github.com/snapcore/snapd/logger"
"github.com/snapcore/snapd/osutil"
"github.com/snapcore/snapd/release"
"github.com/snapcore/snapd/snap"
"github.com/snapcore/snapd/snapdtool"
"github.com/snapcore/snapd/timings"
"github.com/snapcore/snapd/wrappers"
)
// Backend is responsible for maintaining DBus policy files.
type Backend struct{}
// Initialize does nothing.
func (b *Backend) Initialize(*interfaces.SecurityBackendOptions) error {
return nil
}
// Name returns the name of the backend.
func (b *Backend) Name() interfaces.SecuritySystem {
return "dbus"
}
func shouldCopyConfigFiles(snapInfo *snap.Info) bool {
// Only copy config files on classic distros
if !release.OnClassic {
return false
}
// Only copy config files if we have been reexecuted
if reexecd, _ := snapdtool.IsReexecd(); !reexecd {
return false
}
switch snapInfo.Type() {
case snap.TypeOS:
// XXX: ugly but we need to make sure that the content
// of the "snapd" snap wins
//
// TODO: this is also racy but the content of the
// files in core and snapd is identical. Cleanup
// after link-snap and setup-profiles are unified
return !osutil.FileExists(filepath.Join(snapInfo.MountDir(), "../..", "snapd/current"))
case snap.TypeSnapd:
return true
default:
return false
}
}
// setupDbusServiceForUserd will setup the service file for the new
// `snap userd` instance on re-exec
func setupDbusServiceForUserd(snapInfo *snap.Info) error {
coreOrSnapdRoot := snapInfo.MountDir()
for _, srv := range []string{
"io.snapcraft.Launcher.service",
"io.snapcraft.Settings.service",
} {
dst := filepath.Join("/usr/share/dbus-1/services/", srv)
src := filepath.Join(coreOrSnapdRoot, dst)
// we only need the GlobalRootDir for testing
dst = filepath.Join(dirs.GlobalRootDir, dst)
if !osutil.FilesAreEqual(src, dst) {
if err := osutil.CopyFile(src, dst, osutil.CopyFlagPreserveAll); err != nil {
return err
}
}
}
return nil
}
func setupHostDBusConf(snapInfo *snap.Info) error {
sessionContent, systemContent, err := wrappers.DeriveSnapdDBusConfig(snapInfo)
if err != nil {
return err
}
// We don't use `dirs.SnapDBusSessionPolicyDir because we want
// to match the path the package on the host system uses.
dest := filepath.Join(dirs.GlobalRootDir, "/usr/share/dbus-1/session.d")
if err = os.MkdirAll(dest, 0755); err != nil {
return err
}
_, _, err = osutil.EnsureDirState(dest, "snapd.*.conf", sessionContent)
if err != nil {
return err
}
dest = filepath.Join(dirs.GlobalRootDir, "/usr/share/dbus-1/system.d")
if err = os.MkdirAll(dest, 0755); err != nil {
return err
}
_, _, err = osutil.EnsureDirState(dest, "snapd.*.conf", systemContent)
if err != nil {
return err
}
return nil
}
// Setup creates dbus configuration files specific to a given snap.
//
// DBus has no concept of a complain mode so confinment type is ignored.
func (b *Backend) Setup(snapInfo *snap.Info, opts interfaces.ConfinementOptions, repo *interfaces.Repository, tm timings.Measurer) error {
snapName := snapInfo.InstanceName()
// Get the snippets that apply to this snap
spec, err := repo.SnapSpecification(b.Name(), snapName)
if err != nil {
return fmt.Errorf("cannot obtain dbus specification for snap %q: %s", snapName, err)
}
// copy some config files when installing core/snapd if we reexec
if shouldCopyConfigFiles(snapInfo) {
if err := setupDbusServiceForUserd(snapInfo); err != nil {
logger.Noticef("cannot create host `snap userd` dbus service file: %s", err)
}
// TODO: Make this conditional on the dbus-activation
// feature flag.
if err := setupHostDBusConf(snapInfo); err != nil {
logger.Noticef("cannot create host dbus config: %s", err)
}
}
// Get the files that this snap should have
content, err := b.deriveContent(spec.(*Specification), snapInfo)
if err != nil {
return fmt.Errorf("cannot obtain expected DBus configuration files for snap %q: %s", snapName, err)
}
glob := fmt.Sprintf("%s.conf", interfaces.SecurityTagGlob(snapName))
dir := dirs.SnapDBusSystemPolicyDir
if err := os.MkdirAll(dir, 0755); err != nil {
return fmt.Errorf("cannot create directory for DBus configuration files %q: %s", dir, err)
}
_, _, err = osutil.EnsureDirState(dir, glob, content)
if err != nil {
return fmt.Errorf("cannot synchronize DBus configuration files for snap %q: %s", snapName, err)
}
return nil
}
// Remove removes dbus configuration files of a given snap.
//
// This method should be called after removing a snap.
func (b *Backend) Remove(snapName string) error {
glob := fmt.Sprintf("%s.conf", interfaces.SecurityTagGlob(snapName))
_, _, err := osutil.EnsureDirState(dirs.SnapDBusSystemPolicyDir, glob, nil)
if err != nil {
return fmt.Errorf("cannot synchronize DBus configuration files for snap %q: %s", snapName, err)
}
return nil
}
// deriveContent combines security snippets collected from all the interfaces
// affecting a given snap into a content map applicable to EnsureDirState.
func (b *Backend) deriveContent(spec *Specification, snapInfo *snap.Info) (content map[string]osutil.FileState, err error) {
for _, appInfo := range snapInfo.Apps {
securityTag := appInfo.SecurityTag()
appSnippets := spec.SnippetForTag(securityTag)
if appSnippets == "" {
continue
}
if content == nil {
content = make(map[string]osutil.FileState)
}
addContent(securityTag, appSnippets, content)
}
for _, hookInfo := range snapInfo.Hooks {
securityTag := hookInfo.SecurityTag()
hookSnippets := spec.SnippetForTag(securityTag)
if hookSnippets == "" {
continue
}
if content == nil {
content = make(map[string]osutil.FileState)
}
addContent(securityTag, hookSnippets, content)
}
return content, nil
}
func addContent(securityTag string, snippet string, content map[string]osutil.FileState) {
var buffer bytes.Buffer
buffer.Write(xmlHeader)
buffer.WriteString(snippet)
buffer.Write(xmlFooter)
content[fmt.Sprintf("%s.conf", securityTag)] = &osutil.MemoryFileState{
Content: buffer.Bytes(),
Mode: 0644,
}
}
func (b *Backend) NewSpecification() interfaces.Specification {
return &Specification{}
}
// SandboxFeatures returns list of features supported by snapd for dbus communication.
func (b *Backend) SandboxFeatures() []string {
return []string{"mediated-bus-access"}
}