tailscale/ipn/ipnlocal/ssh.go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

//go:build linux || (darwin && !ios) || freebsd || openbsd

package ipnlocal

import (
	"bytes"
	"crypto/ecdsa"
	"crypto/ed25519"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"slices"
	"strings"
	"sync"

	"github.com/tailscale/golang-x-crypto/ssh"
	"go4.org/mem"
	"tailscale.com/tailcfg"
	"tailscale.com/util/lineread"
	"tailscale.com/util/mak"
)

// keyTypes are the SSH key types that we either try to read from the
// system's OpenSSH keys or try to generate for ourselves when not
// running as root.
var keyTypes = []string{"rsa", "ecdsa", "ed25519"}

// getSSHUsernames discovers and returns the list of usernames that are
// potential Tailscale SSH user targets.
//
// Invariant: must not be called with b.mu held.
func (b *LocalBackend) getSSHUsernames(req *tailcfg.C2NSSHUsernamesRequest) (*tailcfg.C2NSSHUsernamesResponse, error) {
	res := new(tailcfg.C2NSSHUsernamesResponse)
	if !b.tailscaleSSHEnabled() {
		return res, nil
	}

	max := 10
	if req != nil && req.Max != 0 {
		max = req.Max
	}

	add := func(u string) {
		if req != nil && req.Exclude[u] {
			return
		}
		switch u {
		case "nobody", "daemon", "sync":
			return
		}
		if slices.Contains(res.Usernames, u) {
			return
		}
		if len(res.Usernames) > max {
			// Enough for a hint.
			return
		}
		res.Usernames = append(res.Usernames, u)
	}

	if opUser := b.operatorUserName(); opUser != "" {
		add(opUser)
	}

	// Check popular usernames and see if they exist with a real shell.
	switch runtime.GOOS {
	case "darwin":
		out, err := exec.Command("dscl", ".", "list", "/Users").Output()
		if err != nil {
			return nil, err
		}
		lineread.Reader(bytes.NewReader(out), func(line []byte) error {
			line = bytes.TrimSpace(line)
			if len(line) == 0 || line[0] == '_' {
				return nil
			}
			add(string(line))
			return nil
		})
	default:
		lineread.File("/etc/passwd", func(line []byte) error {
			line = bytes.TrimSpace(line)
			if len(line) == 0 || line[0] == '#' || line[0] == '_' {
				return nil
			}
			if mem.HasSuffix(mem.B(line), mem.S("/nologin")) ||
				mem.HasSuffix(mem.B(line), mem.S("/false")) {
				return nil
			}
			colon := bytes.IndexByte(line, ':')
			if colon != -1 {
				add(string(line[:colon]))
			}
			return nil
		})
	}
	return res, nil
}

func (b *LocalBackend) GetSSH_HostKeys() (keys []ssh.Signer, err error) {
	var existing map[string]ssh.Signer
	if os.Geteuid() == 0 {
		existing = b.getSystemSSH_HostKeys()
	}
	return b.getTailscaleSSH_HostKeys(existing)
}

// getTailscaleSSH_HostKeys returns the three (rsa, ecdsa, ed25519) SSH host
// keys, reusing the provided ones in existing if present in the map.
func (b *LocalBackend) getTailscaleSSH_HostKeys(existing map[string]ssh.Signer) (keys []ssh.Signer, err error) {
	var keyDir string // lazily initialized $TAILSCALE_VAR/ssh dir.
	for _, typ := range keyTypes {
		if s, ok := existing[typ]; ok {
			keys = append(keys, s)
			continue
		}
		if keyDir == "" {
			root := b.TailscaleVarRoot()
			if root == "" {
				return nil, errors.New("no var root for ssh keys")
			}
			keyDir = filepath.Join(root, "ssh")
			if err := os.MkdirAll(keyDir, 0700); err != nil {
				return nil, err
			}
		}
		hostKey, err := b.hostKeyFileOrCreate(keyDir, typ)
		if err != nil {
			return nil, fmt.Errorf("error creating SSH host key type %q in %q: %w", typ, keyDir, err)
		}
		signer, err := ssh.ParsePrivateKey(hostKey)
		if err != nil {
			return nil, fmt.Errorf("error parsing SSH host key type %q from %q: %w", typ, keyDir, err)
		}
		keys = append(keys, signer)
	}
	return keys, nil
}

var keyGenMu sync.Mutex

func (b *LocalBackend) hostKeyFileOrCreate(keyDir, typ string) ([]byte, error) {
	keyGenMu.Lock()
	defer keyGenMu.Unlock()

	path := filepath.Join(keyDir, "ssh_host_"+typ+"_key")
	v, err := os.ReadFile(path)
	if err == nil {
		return v, nil
	}
	if !os.IsNotExist(err) {
		return nil, err
	}
	var priv any
	switch typ {
	default:
		return nil, fmt.Errorf("unsupported key type %q", typ)
	case "ed25519":
		_, priv, err = ed25519.GenerateKey(rand.Reader)
	case "ecdsa":
		// curve is arbitrary. We pick whatever will at
		// least pacify clients as the actual encryption
		// doesn't matter: it's all over WireGuard anyway.
		curve := elliptic.P256()
		priv, err = ecdsa.GenerateKey(curve, rand.Reader)
	case "rsa":
		// keySize is arbitrary. We pick whatever will at
		// least pacify clients as the actual encryption
		// doesn't matter: it's all over WireGuard anyway.
		const keySize = 2048
		priv, err = rsa.GenerateKey(rand.Reader, keySize)
	}
	if err != nil {
		return nil, err
	}
	mk, err := x509.MarshalPKCS8PrivateKey(priv)
	if err != nil {
		return nil, err
	}
	pemGen := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: mk})
	err = os.WriteFile(path, pemGen, 0700)
	return pemGen, err
}

func (b *LocalBackend) getSystemSSH_HostKeys() (ret map[string]ssh.Signer) {
	for _, typ := range keyTypes {
		filename := "/etc/ssh/ssh_host_" + typ + "_key"
		hostKey, err := os.ReadFile(filename)
		if err != nil || len(bytes.TrimSpace(hostKey)) == 0 {
			continue
		}
		signer, err := ssh.ParsePrivateKey(hostKey)
		if err != nil {
			b.logf("warning: error reading host key %s: %v (generating one instead)", filename, err)
			continue
		}
		mak.Set(&ret, typ, signer)
	}
	return ret
}

func (b *LocalBackend) getSSHHostKeyPublicStrings() ([]string, error) {
	signers, err := b.GetSSH_HostKeys()
	if err != nil {
		return nil, err
	}
	var keyStrings []string
	for _, signer := range signers {
		keyStrings = append(keyStrings, strings.TrimSpace(string(ssh.MarshalAuthorizedKey(signer.PublicKey()))))
	}
	return keyStrings, nil
}

// tailscaleSSHEnabled reports whether Tailscale SSH is currently enabled based
// on prefs. It returns false if there are no prefs set.
func (b *LocalBackend) tailscaleSSHEnabled() bool {
	b.mu.Lock()
	defer b.mu.Unlock()
	p := b.pm.CurrentPrefs()
	return p.Valid() && p.RunSSH()
}