ipn/ipnlocal: generate tailscaled-owned SSH keys as needed

Updates #3802

Change-Id: Ie1bc9ae3f3639603b88b4e19b7eb12bea528ff77
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/ssh.go

@@ -8,10 +8,20 @@
 package ipnlocal
 
 import (
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
 	"errors"
+	"fmt"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"strings"
+	"sync"
 
 	"golang.org/x/crypto/ssh"
 	"tailscale.com/envknob"
@@ -19,17 +29,94 @@
 
 var useHostKeys = envknob.Bool("TS_USE_SYSTEM_SSH_HOST_KEYS")
 
-func (b *LocalBackend) GetSSH_HostKeys() ([]ssh.Signer, error) {
-	// TODO(bradfitz): generate host keys, at least as needed if
-	// an existing SSH server didn't put them on disk. But also
-	// because people may want tailscale-specific ones. For now be
-	// lazy and reuse the host ones.
-	return b.getSystemSSH_HostKeys()
+// 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"}
+
+func (b *LocalBackend) GetSSH_HostKeys() (keys []ssh.Signer, err error) {
+	if os.Geteuid() == 0 {
+		keys, err = b.getSystemSSH_HostKeys()
+		if err != nil || len(keys) > 0 {
+			return keys, err
+		}
+		// Otherwise, perhaps they don't have OpenSSH etc installed.
+		// Generate our own keys...
+	}
+	return b.getTailscaleSSH_HostKeys()
+}
+
+func (b *LocalBackend) getTailscaleSSH_HostKeys() (keys []ssh.Signer, err error) {
+	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
+	}
+	for _, typ := range keyTypes {
+		hostKey, err := b.hostKeyFileOrCreate(keyDir, typ)
+		if err != nil {
+			return nil, err
+		}
+		signer, err := ssh.ParsePrivateKey(hostKey)
+		if err != nil {
+			return nil, 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 := ioutil.ReadFile(path)
+	if err == nil {
+		return v, nil
+	}
+	if !os.IsNotExist(err) {
+		return nil, err
+	}
+	var priv interface{}
+	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 []ssh.Signer, err error) {
 	// TODO(bradfitz): cache this?
-	for _, typ := range []string{"rsa", "ecdsa", "ed25519"} {
+	for _, typ := range keyTypes {
 		hostKey, err := ioutil.ReadFile("/etc/ssh/ssh_host_" + typ + "_key")
 		if os.IsNotExist(err) {
 			continue
@@ -43,9 +130,6 @@ func (b *LocalBackend) getSystemSSH_HostKeys() (ret []ssh.Signer, err error) {
 		}
 		ret = append(ret, signer)
 	}
-	if len(ret) == 0 {
-		return nil, errors.New("no system SSH host keys found")
-	}
 	return ret, nil
 }
 

ipn/ipnlocal/ssh_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux
+// +build linux
+
+package ipnlocal
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestSSHKeyGen(t *testing.T) {
+	dir := t.TempDir()
+	lb := &LocalBackend{varRoot: dir}
+	keys, err := lb.getTailscaleSSH_HostKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := map[string]bool{}
+	for _, k := range keys {
+		got[k.PublicKey().Type()] = true
+	}
+	want := map[string]bool{
+		"ssh-rsa":             true,
+		"ecdsa-sha2-nistp256": true,
+		"ssh-ed25519":         true,
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("keys = %v; want %v", got, want)
+	}
+
+	keys2, err := lb.getTailscaleSSH_HostKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(keys, keys2) {
+		t.Errorf("got different keys on second call")
+	}
+}