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>main
Andrew Lytvynov
2 years ago
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@ -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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