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tailscale/ssh/tailssh/user.go

235 lines
6.3 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
//go:build linux || (darwin && !ios) || freebsd || openbsd
package tailssh
import (
"context"
"errors"
"io"
"log"
"os"
"os/exec"
"os/user"
"path/filepath"
"runtime"
"strconv"
"strings"
"time"
"unicode/utf8"
"go4.org/mem"
"tailscale.com/envknob"
"tailscale.com/hostinfo"
"tailscale.com/util/lineread"
"tailscale.com/version/distro"
)
// userMeta is a wrapper around *user.User with extra fields.
type userMeta struct {
user.User
// loginShellCached is the user's login shell, if known
// at the time of userLookup.
loginShellCached string
}
// GroupIds returns the list of group IDs that the user is a member of.
func (u *userMeta) GroupIds() ([]string, error) {
if runtime.GOOS == "linux" && distro.Get() == distro.Gokrazy {
// Gokrazy is a single-user appliance with ~no userspace.
// There aren't users to look up (no /etc/passwd, etc)
// so rather than fail below, just hardcode root.
// TODO(bradfitz): fix os/user upstream instead?
return []string{"0"}, nil
}
return u.User.GroupIds()
}
// userLookup is like os/user.Lookup but it returns a *userMeta wrapper
// around a *user.User with extra fields.
func userLookup(username string) (*userMeta, error) {
if runtime.GOOS != "linux" {
return userLookupStd(username)
}
// No getent on Gokrazy. So hard-code the login shell.
if distro.Get() == distro.Gokrazy {
um, err := userLookupStd(username)
if err != nil {
um.User = user.User{
Uid: "0",
Gid: "0",
Username: "root",
Name: "Gokrazy",
HomeDir: "/",
}
}
um.loginShellCached = "/tmp/serial-busybox/ash"
return um, err
}
// On Linux, default to using "getent" to look up users so that
// even with static tailscaled binaries without cgo (as we distribute),
// we can still look up PAM/NSS users which the standard library's
// os/user without cgo won't get (because of no libc hooks).
// But if "getent" fails, userLookupGetent falls back to the standard
// library anyway.
return userLookupGetent(username)
}
func validUsername(uid string) bool {
maxUid := 32
if runtime.GOOS == "linux" {
maxUid = 256
}
if len(uid) > maxUid || len(uid) == 0 {
return false
}
for _, r := range uid {
if r < ' ' || r == 0x7f || r == utf8.RuneError { // TODO(bradfitz): more?
return false
}
}
return true
}
func userLookupGetent(username string) (*userMeta, error) {
// Do some basic validation before passing this string to "getent", even though
// getent should do its own validation.
if !validUsername(username) {
return nil, errors.New("invalid username")
}
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
out, err := exec.CommandContext(ctx, "getent", "passwd", username).Output()
if err != nil {
log.Printf("error calling getent for user %q: %v", username, err)
return userLookupStd(username)
}
// output is "alice:x:1001:1001:Alice Smith,,,:/home/alice:/bin/bash"
f := strings.SplitN(strings.TrimSpace(string(out)), ":", 10)
for len(f) < 7 {
f = append(f, "")
}
um := &userMeta{
User: user.User{
Username: f[0],
Uid: f[2],
Gid: f[3],
Name: f[4],
HomeDir: f[5],
},
loginShellCached: f[6],
}
return um, nil
}
func userLookupStd(username string) (*userMeta, error) {
u, err := user.Lookup(username)
if err != nil {
return nil, err
}
return &userMeta{User: *u}, nil
}
func (u *userMeta) LoginShell() string {
if u.loginShellCached != "" {
// This field should be populated on Linux, at least, because
// func userLookup on Linux uses "getent" to look up the user
// and that populates it.
return u.loginShellCached
}
switch runtime.GOOS {
case "darwin":
// Note: /Users/username is key, and not the same as u.HomeDir.
out, _ := exec.Command("dscl", ".", "-read", filepath.Join("/Users", u.Username), "UserShell").Output()
// out is "UserShell: /bin/bash"
s, ok := strings.CutPrefix(string(out), "UserShell: ")
if ok {
return strings.TrimSpace(s)
}
}
if e := os.Getenv("SHELL"); e != "" {
return e
}
return "/bin/sh"
}
// defaultPathTmpl specifies the default PATH template to use for new sessions.
//
// If empty, a default value is used based on the OS & distro to match OpenSSH's
// usually-hardcoded behavior. (see
// https://github.com/tailscale/tailscale/issues/5285 for background).
//
// The template may contain @{HOME} or @{PAM_USER} which expand to the user's
// home directory and username, respectively. (PAM is not used, despite the
// name)
var defaultPathTmpl = envknob.RegisterString("TAILSCALE_SSH_DEFAULT_PATH")
func defaultPathForUser(u *user.User) string {
if s := defaultPathTmpl(); s != "" {
return expandDefaultPathTmpl(s, u)
}
isRoot := u.Uid == "0"
switch distro.Get() {
case distro.Debian:
hi := hostinfo.New()
if hi.Distro == "ubuntu" {
// distro.Get's Debian includes Ubuntu. But see if it's actually Ubuntu.
// Ubuntu doesn't empirically seem to distinguish between root and non-root for the default.
// And it includes /snap/bin.
return "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin"
}
if isRoot {
return "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
}
return "/usr/local/bin:/usr/bin:/bin:/usr/bn/games"
case distro.NixOS:
return defaultPathForUserOnNixOS(u)
}
if isRoot {
return "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
}
return "/usr/local/bin:/usr/bin:/bin"
}
func defaultPathForUserOnNixOS(u *user.User) string {
var path string
lineread.File("/etc/pam/environment", func(lineb []byte) error {
if v := pathFromPAMEnvLine(lineb, u); v != "" {
path = v
return io.EOF // stop iteration
}
return nil
})
return path
}
func pathFromPAMEnvLine(line []byte, u *user.User) (path string) {
if !mem.HasPrefix(mem.B(line), mem.S("PATH")) {
return ""
}
rest := strings.TrimSpace(strings.TrimPrefix(string(line), "PATH"))
if quoted, ok := strings.CutPrefix(rest, "DEFAULT="); ok {
if path, err := strconv.Unquote(quoted); err == nil {
return expandDefaultPathTmpl(path, u)
}
}
return ""
}
func expandDefaultPathTmpl(t string, u *user.User) string {
p := strings.NewReplacer(
"@{HOME}", u.HomeDir,
"@{PAM_USER}", u.Username,
).Replace(t)
if strings.Contains(p, "@{") {
// If there are unknown expansions, conservatively fail closed.
return ""
}
return p
}