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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2017-2019 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"
"path/filepath"
"syscall"
"github.com/snapcore/snapd/dirs"
"github.com/snapcore/snapd/osutil"
)
// UserProfileUpdateContext contains information about update to per-user mount namespace.
type UserProfileUpdateContext struct {
CommonProfileUpdateContext
// uid is the numeric user identifier associated with the user for which
// the update operation is occurring. It may be the current UID but doesn't
// need to be.
uid int
// home contains the user's real home directory
home string
homeError error
}
// isPlausibleHome returns an error if the path is empty, not clean, not absolute or cannot
// be opened for reading.
//
// See bootstrap.c function switch_to_privileged_user(). When snap-update-ns is invoked for
// user mounts, the effective uid and gid is changed to the calling user and supplementary
// groups dropped, while retaining capability SYS_ADMIN. Having the effective uid and gid
// changed to the calling user is a prerequisite for isPlausibleHome to function as intended.
func isPlausibleHome(path string) error {
if path == "" {
return fmt.Errorf("cannot allow empty path")
}
if path != filepath.Clean(path) {
return fmt.Errorf("cannot allow unclean path")
}
if !filepath.IsAbs(path) {
return fmt.Errorf("cannot allow relative path")
}
const openFlags = syscall.O_NOFOLLOW | syscall.O_CLOEXEC | syscall.O_DIRECTORY
fd, err := sysOpen(path, openFlags, 0)
if err != nil {
return err
}
sysClose(fd)
return nil
}
// NewUserProfileUpdateContext returns encapsulated information for performing a per-user mount namespace update.
func NewUserProfileUpdateContext(instanceName string, fromSnapConfine bool, uid int) (*UserProfileUpdateContext, error) {
realHome := os.Getenv("SNAP_REAL_HOME")
var realHomeError error
if realHome == "" {
realHomeError = fmt.Errorf("cannot retrieve home directory")
}
if err := isPlausibleHome(realHome); err != nil {
realHomeError = fmt.Errorf("cannot use invalid home directory %q: %v", realHome, err)
}
return &UserProfileUpdateContext{
CommonProfileUpdateContext: CommonProfileUpdateContext{
instanceName: instanceName,
fromSnapConfine: fromSnapConfine,
currentProfilePath: currentUserProfilePath(instanceName, uid),
desiredProfilePath: desiredUserProfilePath(instanceName),
},
uid: uid,
home: realHome,
homeError: realHomeError,
}, nil
}
// Lock acquires locks / freezes needed to synchronize mount namespace changes.
func (upCtx *UserProfileUpdateContext) Lock() (unlock func(), err error) {
// TODO: when persistent user mount namespaces are enabled, grab a lock
// protecting the snap and freeze snap processes here.
return func() {}, nil
}
// Assumptions returns information about file system mutability rules.
func (upCtx *UserProfileUpdateContext) Assumptions() *Assumptions {
// TODO: configure the secure helper and inform it about directories that
// can be created without trespassing.
as := &Assumptions{}
if upCtx.homeError == nil {
as.AddUnrestrictedPaths(upCtx.home)
}
// TODO: Handle /home/*/snap/* when we do per-user mount namespaces and
// allow defining layout items that refer to SNAP_USER_DATA and
// SNAP_USER_COMMON.
return as
}
// LoadDesiredProfile loads the desired, per-user mount profile, expanding user-specific variables.
func (upCtx *UserProfileUpdateContext) LoadDesiredProfile() (*osutil.MountProfile, error) {
profile, err := upCtx.CommonProfileUpdateContext.LoadDesiredProfile()
if err != nil {
return nil, err
}
home := func() (path string, err error) {
return upCtx.home, upCtx.homeError
}
if err := expandHomeDir(profile, home); err != nil {
return nil, err
}
// TODO: when SNAP_USER_DATA, SNAP_USER_COMMON or other variables relating
// to the user name and their home directory need to be expanded then
// handle them here.
expandXdgRuntimeDir(profile, upCtx.uid)
return profile, nil
}
// SaveCurrentProfile does nothing at all.
//
// Per-user mount profiles are not persisted yet.
func (upCtx *UserProfileUpdateContext) SaveCurrentProfile(profile *osutil.MountProfile) error {
// TODO: when persistent user mount namespaces are enabled save the
// current, per-user mount profile here.
return nil
}
// LoadCurrentProfile returns the empty profile.
//
// Per-user mount profiles are not persisted yet.
func (upCtx *UserProfileUpdateContext) LoadCurrentProfile() (*osutil.MountProfile, error) {
// TODO: when persistent user mount namespaces are enabled load the
// current, per-user mount profile here.
return &osutil.MountProfile{}, nil
}
// desiredUserProfilePath returns the path of the fstab-like file with the desired, user-specific mount profile for a snap.
func desiredUserProfilePath(snapName string) string {
return fmt.Sprintf("%s/snap.%s.user-fstab", dirs.SnapMountPolicyDir, snapName)
}
// currentUserProfilePath returns the path of the fstab-like file with the applied, user-specific mount profile for a snap.
func currentUserProfilePath(snapName string, uid int) string {
return fmt.Sprintf("%s/snap.%s.%d.user-fstab", dirs.SnapRunNsDir, snapName, uid)
}
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