Stuff

Implement certificate authentication, certificate requires :gokrazy: principal
Read first line of /etc/passwd for home and shell
Shell uses `-l` to make it a login shell which will run .profile

breakglass.go

@@ -10,6 +10,7 @@ import (
 	"crypto/x509"
 	"encoding/json"
 	"encoding/pem"
+	"errors"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -17,6 +18,7 @@ import (
 	"net"
 	"net/http"
 	"os"
+	"path"
 	"strings"
 	"syscall"
 
@@ -30,6 +32,10 @@ var (
 		"/perm/breakglass.authorized_keys",
 		"path to an OpenSSH authorized_keys file; if the value is 'ec2', fetch the SSH key(s) from the AWS IMDSv2 metadata")
 
+	authorizedUserCAPath = flag.String("authorized_ca",
+		"/perm/breakglass.authorized_user_ca",
+		"path to an OpenSSH TrustedUserCAKeys file; note the certificate must list ':gokrazy:' as a valid principal")
+
 	hostKeyPath = flag.String("host_key",
 		"/perm/breakglass.host_key",
 		"path to a PEM-encoded RSA, DSA or ECDSA private key (create using e.g. ssh-keygen -f /perm/breakglass.host_key -N '' -t rsa)")
@@ -45,6 +51,9 @@ var (
 	forwarding = flag.String("forward",
 		"",
 		"allow port forwarding. Use `loopback` for loopback interfaces and `private-network` for private networks")
+
+	home  = "/perm/home"
+	shell = ""
 )
 
 func loadAuthorizedKeys(path string) (map[string]bool, error) {
@@ -80,6 +89,19 @@ func loadAuthorizedKeys(path string) (map[string]bool, error) {
 	return result, nil
 }
 
+func loadPasswd(passwd string) {
+	b, err := os.ReadFile(passwd)
+	if err != nil {
+		return
+	}
+	fields := bytes.SplitN(bytes.SplitN(b, []byte("\n"), 2)[0], []byte(":"), 7)
+	if len(fields) != 7 {
+		return
+	}
+	home = path.Clean(string(fields[5]))
+	shell = path.Clean(string(fields[6]))
+}
+
 func loadHostKey(path string) (ssh.Signer, error) {
 	b, err := ioutil.ReadFile(path)
 	if err != nil {
@@ -197,6 +219,8 @@ func main() {
 
 	gokrazy.DontStartOnBoot()
 
+	loadPasswd("/etc/passwd")
+
 	authorizedKeys, err := loadAuthorizedKeys(*authorizedKeysPath)
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -205,19 +229,59 @@ func main() {
 		log.Fatal(err)
 	}
 
+	authorizedUserCertificateCA, err := loadAuthorizedKeys(strings.TrimPrefix(*authorizedUserCAPath, "ec2"))
+	if err != nil {
+		if os.IsNotExist(err) {
+			log.Printf("TrustedUserCAKeys not loaded")
+		}
+	}
+
 	if err := initMOTD(); err != nil {
 		log.Print(err)
 	}
-
+	certChecker := ssh.CertChecker{
-	config := &ssh.ServerConfig{
+		IsUserAuthority: func(auth ssh.PublicKey) bool {
-		PublicKeyCallback: func(conn ssh.ConnMetadata, pubKey ssh.PublicKey) (*ssh.Permissions, error) {
+			return authorizedUserCertificateCA[string(auth.Marshal())]
+		},
+		UserKeyFallback: func(conn ssh.ConnMetadata, pubKey ssh.PublicKey) (*ssh.Permissions, error) {
 			if authorizedKeys[string(pubKey.Marshal())] {
 				log.Printf("user %q successfully authorized from remote addr %s", conn.User(), conn.RemoteAddr())
-				return nil, nil
+				return &ssh.Permissions{map[string]string{}, map[string]string{}}, nil
 			}
 			return nil, fmt.Errorf("public key not found in %s", *authorizedKeysPath)
 		},
 	}
+	config := &ssh.ServerConfig{
+		PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+			cert, ok := key.(*ssh.Certificate)
+			if !ok {
+				if certChecker.UserKeyFallback != nil {
+					return certChecker.UserKeyFallback(conn, key)
+				}
+				return nil, errors.New("ssh: normal key pairs not accepted")
+			}
+
+			if cert.CertType != ssh.UserCert {
+				return nil, fmt.Errorf("ssh: cert has type %d", cert.CertType)
+			}
+			if !certChecker.IsUserAuthority(cert.SignatureKey) {
+				return nil, fmt.Errorf("ssh: certificate signed by unrecognized authority")
+			}
+
+			if err := certChecker.CheckCert(":gokrazy:", cert); err != nil {
+				return nil, err
+			}
+			if cert.Permissions.CriticalOptions == nil {
+				cert.Permissions.CriticalOptions = map[string]string{}
+			}
+			if cert.Permissions.Extensions == nil {
+				cert.Permissions.Extensions = map[string]string{}
+			}
+
+			return &cert.Permissions, nil
+
+		},
+	}
 
 	signer, err := loadHostKey(*hostKeyPath)
 	if err != nil {
@@ -267,7 +331,7 @@ func main() {
 			}
 
 			go func(conn net.Conn) {
-				_, chans, reqs, err := ssh.NewServerConn(conn, config)
+				c, chans, reqs, err := ssh.NewServerConn(conn, config)
 				if err != nil {
 					log.Printf("handshake: %v", err)
 					return
@@ -277,7 +341,7 @@ func main() {
 				go ssh.DiscardRequests(reqs)
 
 				for newChannel := range chans {
-					handleChannel(newChannel)
+					handleChannel(newChannel, c)
 				}
 			}(conn)
 		}

ssh.go

@@ -2,12 +2,14 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"log"
 	"net"
 	"os"
 	"os/exec"
+	"path"
 	"strconv"
 	"strings"
 	"sync"
@@ -21,11 +23,15 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
-func handleChannel(newChan ssh.NewChannel) {
+func handleChannel(newChan ssh.NewChannel, conn *ssh.ServerConn) {
 	switch t := newChan.ChannelType(); t {
 	case "session":
-		handleSession(newChan)
+		handleSession(newChan, conn)
 	case "direct-tcpip":
+		if _, portForwardDenied := conn.Permissions.Extensions["no-port-forwarding"]; portForwardDenied {
+			newChan.Reject(ssh.Prohibited, "port forwarding is disabled. For you in particular :-P")
+			return
+		}
 		handleTCPIP(newChan)
 	default:
 		newChan.Reject(ssh.UnknownChannelType, fmt.Sprintf("unknown channel type: %q", t))
@@ -112,7 +118,7 @@ func handleTCPIP(newChan ssh.NewChannel) {
 	}()
 }
 
-func handleSession(newChannel ssh.NewChannel) {
+func handleSession(newChannel ssh.NewChannel, conn *ssh.ServerConn) {
 	channel, requests, err := newChannel.Accept()
 	if err != nil {
 		log.Printf("Could not accept channel (%s)", err)
@@ -120,12 +126,12 @@ func handleSession(newChannel ssh.NewChannel) {
 	}
 
 	// Sessions have out-of-band requests such as "shell", "pty-req" and "env"
-	go func(channel ssh.Channel, requests <-chan *ssh.Request) {
+	go func(channel ssh.Channel, requests <-chan *ssh.Request, conn *ssh.ServerConn) {
 		ctx, canc := context.WithCancel(context.Background())
 		defer canc()
 		s := session{channel: channel}
 		for req := range requests {
-			if err := s.request(ctx, req); err != nil {
+			if err := s.request(ctx, req, conn); err != nil {
 				log.Printf("request(%q): %v", req.Type, err)
 				errmsg := []byte(err.Error())
 				// Append a trailing newline; the error message is
@@ -139,7 +145,7 @@ func handleSession(newChannel ssh.NewChannel) {
 			}
 		}
 		log.Printf("requests exhausted")
-	}(channel, requests)
+	}(channel, requests, conn)
 }
 
 func expandPath(env []string) []string {
@@ -220,24 +226,31 @@ func findShell() string {
 		// in standard locations (makes Emacs TRAMP work, for example).
 		if err := installBusybox(); err != nil {
 			log.Printf("installing busybox failed: %v", err)
-			// fallthrough
+			// fallthrough, we don't return /bin/sh as we read /etc/passwd
-		} else {
-			return "/bin/sh" // available after installation
 		}
 	}
+	if _, err := exec.LookPath(shell); path.IsAbs(shell) && err == nil {
+		return shell
+	}
+	if path, err := exec.LookPath("bash"); err == nil {
+		return path
+	}
 	if path, err := exec.LookPath("sh"); err == nil {
 		return path
 	}
 	const wellKnownSerialShell = "/tmp/serial-busybox/ash"
-	if _, err := os.Stat(wellKnownSerialShell); err == nil {
+	if _, err := exec.LookPath(wellKnownSerialShell); err == nil {
 		return wellKnownSerialShell
 	}
 	return ""
 }
 
-func (s *session) request(ctx context.Context, req *ssh.Request) error {
+func (s *session) request(ctx context.Context, req *ssh.Request, conn *ssh.ServerConn) error {
 	switch req.Type {
 	case "pty-req":
+		if _, portForwardDenied := conn.Permissions.Extensions["no-pty"]; portForwardDenied {
+			return errors.New("Pseudo-Terminal is disabled. For you in particular :-P")
+		}
 		var r ptyreq
 		if err := ssh.Unmarshal(req.Payload, &r); err != nil {
 			return err
@@ -355,21 +368,25 @@ func (s *session) request(ctx context.Context, req *ssh.Request) error {
 
 		// Ensure the $HOME directory exists so that shell history works without
 		// any extra steps.
-		if err := os.MkdirAll("/perm/home", 0755); err != nil {
+		if err := os.MkdirAll(home, 0755); err != nil {
 			// TODO: Suppress -EROFS
 			log.Print(err)
 		}
 
 		var cmd *exec.Cmd
 		if shell := findShell(); shell != "" {
+			if r.Command == "sh" {
+				cmd = exec.CommandContext(ctx, shell, "-l")
+			} else {
 				cmd = exec.CommandContext(ctx, shell, "-c", r.Command)
+			}
 		} else {
 			cmd = exec.CommandContext(ctx, cmdline[0], cmdline[1:]...)
 		}
 		log.Printf("Starting cmd %q", cmd.Args)
 		env := expandPath(s.env)
 		env = append(env,
-			"HOME=/perm/home",
+			"HOME="+home,
 			"TMPDIR=/tmp")
 		cmd.Env = env
 		cmd.SysProcAttr = &syscall.SysProcAttr{}