mirror of https://github.com/tailscale/tailscale/
ssh/tailssh,util: extract new osuser package from ssh code (#10170)
This package is a wrapper for os/user that handles non-cgo builds, gokrazy and user shells. Updates tailscale/corp#15405 Signed-off-by: Andrew Lytvynov <awly@tailscale.com>pull/10163/head
Andrew Lytvynov
1 year ago
committed by
GitHub
parent
09de240934
commit
1fc1077052
@ -0,0 +1,139 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// Package osuser implements OS user lookup. It's a wrapper around os/user that
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// works on non-cgo builds.
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package osuser
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import (
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"context"
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"errors"
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"log"
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"os/exec"
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"os/user"
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"runtime"
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"strings"
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"time"
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"unicode/utf8"
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"tailscale.com/version/distro"
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)
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// LookupByUIDWithShell is like os/user.LookupId but handles a few edge cases
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// like gokrazy and non-cgo lookups, and returns the user shell. The user shell
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// lookup is best-effort and may be empty.
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func LookupByUIDWithShell(uid string) (u *user.User, shell string, err error) {
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return lookup(uid, user.LookupId, true)
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}
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// LookupByUsernameWithShell is like os/user.Lookup but handles a few edge
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// cases like gokrazy and non-cgo lookups, and returns the user shell. The user
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// shell lookup is best-effort and may be empty.
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func LookupByUsernameWithShell(username string) (u *user.User, shell string, err error) {
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return lookup(username, user.Lookup, true)
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}
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// LookupByUID is like os/user.LookupId but handles a few edge cases like
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// gokrazy and non-cgo lookups.
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func LookupByUID(uid string) (*user.User, error) {
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u, _, err := lookup(uid, user.LookupId, false)
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return u, err
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}
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// LookupByUsername is like os/user.Lookup but handles a few edge cases like
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// gokrazy and non-cgo lookups.
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func LookupByUsername(username string) (*user.User, error) {
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u, _, err := lookup(username, user.Lookup, false)
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return u, err
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}
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// lookupStd is either user.Lookup or user.LookupId.
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type lookupStd func(string) (*user.User, error)
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func lookup(usernameOrUID string, std lookupStd, wantShell bool) (*user.User, string, error) {
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// TODO(awly): we should use genet on more platforms, like FreeBSD.
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if runtime.GOOS != "linux" {
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u, err := std(usernameOrUID)
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return u, "", err
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}
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// No getent on Gokrazy. So hard-code the login shell.
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if distro.Get() == distro.Gokrazy {
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var shell string
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if wantShell {
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shell = "/tmp/serial-busybox/ash"
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}
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u, err := std(usernameOrUID)
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if err != nil {
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return &user.User{
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Uid: "0",
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Gid: "0",
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Username: "root",
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Name: "Gokrazy",
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HomeDir: "/",
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}, shell, nil
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}
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return u, shell, nil
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}
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// Start with getent if caller wants to get the user shell.
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if wantShell {
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return userLookupGetent(usernameOrUID, std)
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}
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// If shell is not required, try os/user.Lookup* first and only use getent
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// if that fails. This avoids spawning a child process when os/user lookup
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// succeeds.
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if u, err := std(usernameOrUID); err == nil {
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return u, "", nil
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}
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return userLookupGetent(usernameOrUID, std)
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}
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func checkGetentInput(usernameOrUID string) bool {
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maxUid := 32
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if runtime.GOOS == "linux" {
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maxUid = 256
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}
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if len(usernameOrUID) > maxUid || len(usernameOrUID) == 0 {
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return false
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}
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for _, r := range usernameOrUID {
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if r < ' ' || r == 0x7f || r == utf8.RuneError { // TODO(bradfitz): more?
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return false
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}
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}
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return true
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}
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// userLookupGetent uses "getent" to look up users so that even with static
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// tailscaled binaries without cgo (as we distribute), we can still look up
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// PAM/NSS users which the standard library's os/user without cgo won't get
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// (because of no libc hooks). If "getent" fails, userLookupGetent falls back
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// to the standard library.
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func userLookupGetent(usernameOrUID string, std lookupStd) (*user.User, string, error) {
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// Do some basic validation before passing this string to "getent", even though
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// getent should do its own validation.
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if !checkGetentInput(usernameOrUID) {
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return nil, "", errors.New("invalid username or UID")
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}
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ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
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defer cancel()
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out, err := exec.CommandContext(ctx, "getent", "passwd", usernameOrUID).Output()
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if err != nil {
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log.Printf("error calling getent for user %q: %v", usernameOrUID, err)
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u, err := std(usernameOrUID)
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return u, "", err
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}
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// output is "alice:x:1001:1001:Alice Smith,,,:/home/alice:/bin/bash"
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f := strings.SplitN(strings.TrimSpace(string(out)), ":", 10)
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for len(f) < 7 {
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f = append(f, "")
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}
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return &user.User{
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Username: f[0],
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Uid: f[2],
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Gid: f[3],
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Name: f[4],
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HomeDir: f[5],
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}, f[6], nil
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}
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