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
2025-02-16 17:53:26 -08:00
7 changed files with 195 additions and 82 deletions
--- a/.github/workflows/push.yml
+++ b/.github/workflows/push.yml
@@ -9,18 +9,21 @@ jobs:
    name: CI
    runs-on: ubuntu-latest
    steps:
-    - uses: actions/checkout@v4

-    - name: Set up latest stable Go
-      uses: actions/setup-go@v5
+    - name: Set up Go 1.x
+      uses: actions/setup-go@v2
      with:
-        go-version: 'stable'
+        # Run on the latest minor release of Go 1.19:
+        go-version: ^1.19
+      id: go
+
+    - name: Check out code into the Go module directory
+      uses: actions/checkout@v2

    - name: Ensure all files were formatted as per gofmt
      run: |
        [ "$(gofmt -l $(find . -name '*.go') 2>&1)" = "" ]

-    - name: Build and test
+    - name: Build packages
      run: |
-        go install ./...
-        go test -v ./...
+        go install -v ./...
--- a/README.md
+++ b/README.md
@@ -97,7 +97,7 @@ busybox: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically li
 for GNU/Linux 3.7.0, BuildID[sha1]=c9e20e9849ed0ca3c2bd058427ac31a27c008efe, stripped
 $ ln -s busybox sh
 $ tar cf breakglass.tar --dereference sh
-$ breakglass -debug_tarball_pattern=breakglass.tar gokrazy
+$ breakglass -debug_tarball_pattern=debug.tar gokrazy
 /tmp/breakglass564067692 # df -h
 Filesystem                Size      Used Available Use% Mounted on
 /dev/root                60.5M     60.5M         0 100% /
--- a/breakglass.go
+++ b/breakglass.go
@@ -5,22 +5,23 @@ package main
 import (
 	"bufio"
 	"bytes"
-	"context"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/x509"
+	"encoding/json"
 	"encoding/pem"
+	"errors"
 	"flag"
 	"fmt"
 	"io/ioutil"
 	"log"
 	"net"
+	"net/http"
 	"os"
+	"path"
 	"strings"
 	"syscall"

-	"github.com/gokrazy/gokapi"
-	"github.com/gokrazy/gokapi/ondeviceapi"
 	"github.com/gokrazy/gokrazy"

 	"golang.org/x/crypto/ssh"
@@ -31,6 +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)")
@@ -46,6 +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) {
@@ -81,6 +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 {
@@ -116,16 +137,40 @@ func createHostKey(path string) (ssh.Signer, error) {
 }

 func buildTimestamp() (string, error) {
-	cfg, err := gokapi.ConnectOnDevice()
+	var statusReply struct {
+		BuildTimestamp string `json:"BuildTimestamp"`
+	}
+	pw, err := os.ReadFile("/etc/gokr-pw.txt")
 	if err != nil {
 		return "", err
 	}
-	cl := ondeviceapi.NewAPIClient(cfg)
-	res, _, err := cl.SuperviseApi.Index(context.Background())
+	port, err := os.ReadFile("/etc/http-port.txt")
 	if err != nil {
 		return "", err
 	}
-	return res.BuildTimestamp, nil
+	req, err := http.NewRequest("GET", "http://gokrazy:"+strings.TrimSpace(string(pw))+"@localhost:"+strings.TrimSpace(string(port))+"/", nil)
+	if err != nil {
+		return "", err
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+	if got, want := resp.StatusCode, http.StatusOK; got != want {
+		b, _ := ioutil.ReadAll(resp.Body)
+		return "", fmt.Errorf("unexpected HTTP status code: got %v, want %v (body: %s)", resp.Status, want, strings.TrimSpace(string(b)))
+	}
+	b, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return "", err
+	}
+	if err := json.Unmarshal(b, &statusReply); err != nil {
+		return "", err
+	}
+	return statusReply.BuildTimestamp, nil
 }

 var motd string
@@ -154,7 +199,7 @@ func initMOTD() error {
 		return err
 	}

-	motd = fmt.Sprintf(`              __                           
+	motd = fmt.Sprintf(`              __
 .-----.-----|  |--.----.---.-.-----.--.--.
 |  _  |  _  |    <|   _|  _  |-- __|  |  |
 |___  |_____|__|__|__| |___._|_____|___  |
@@ -174,6 +219,8 @@ func main() {

 	gokrazy.DontStartOnBoot()

+	loadPasswd("/etc/passwd")
+
 	authorizedKeys, err := loadAuthorizedKeys(*authorizedKeysPath)
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -182,19 +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)
 	}
-
-	config := &ssh.ServerConfig{
-		PublicKeyCallback: func(conn ssh.ConnMetadata, pubKey ssh.PublicKey) (*ssh.Permissions, error) {
+	certChecker := ssh.CertChecker{
+		IsUserAuthority: func(auth ssh.PublicKey) bool {
+			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 {
@@ -244,7 +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
@@ -254,7 +341,7 @@ func main() {
 				go ssh.DiscardRequests(reqs)

 				for newChannel := range chans {
-					handleChannel(newChannel)
+					handleChannel(newChannel, c)
 				}
 			}(conn)
 		}
--- a/cmd/breakglass/breakglass.go
+++ b/cmd/breakglass/breakglass.go
@@ -26,6 +26,7 @@ import (
 	"github.com/gokrazy/internal/config"
 	"github.com/gokrazy/internal/httpclient"
 	"github.com/gokrazy/internal/instanceflag"
+	"github.com/gokrazy/internal/tlsflag"
 	"github.com/gokrazy/internal/updateflag"
 )

@@ -34,11 +35,9 @@ type bg struct {
 	cfg          *config.Struct
 	forceRestart bool
 	sshConfig    string
-	insecure     bool

 	// state
 	GOARCH string
-	update updateflag.Value
 }

 func (bg *bg) startBreakglass() error {
@@ -47,26 +46,35 @@ func (bg *bg) startBreakglass() error {
 		return err
 	}

-	updateHttpClient, _, updateBaseURL, err := httpclient.For(bg.update, bg.cfg)
+	updateHttpClient, foundMatchingCertificate, updateBaseURL, err := httpclient.For(bg.cfg)
 	if err != nil {
 		return err
 	}
 	updateHttpClient.Jar = jar

+	remoteScheme, err := httpclient.GetRemoteScheme(updateBaseURL)
+	if remoteScheme == "https" && !tlsflag.Insecure() {
+		updateBaseURL.Scheme = "https"
+		updateflag.SetUpdate(updateBaseURL.String())
+	}
+
+	if updateBaseURL.Scheme != "https" && foundMatchingCertificate {
+		fmt.Printf("\n")
+		fmt.Printf("!!!WARNING!!! Possible SSL-Stripping detected!\n")
+		fmt.Printf("Found certificate for hostname in your client configuration but the host does not offer https!\n")
+		fmt.Printf("\n")
+		if !tlsflag.Insecure() {
+			log.Fatalf("update canceled: TLS certificate found, but negotiating a TLS connection with the target failed")
+		}
+		fmt.Printf("Proceeding anyway as requested (-insecure).\n")
+	}
+
+	if err != nil {
+		return err
+	}
+
 	form, err := updateHttpClient.Get(updateBaseURL.String() + "status?path=/user/breakglass")
 	if err != nil {
-		if updateBaseURL.Scheme == "https" && bg.insecure {
-			// Try falling back to HTTP
-			bg.cfg.Update.UseTLS = "off"
-			updateHttpClient, _, updateBaseURL, err = httpclient.For(bg.update, bg.cfg)
-			if err != nil {
-				return err
-			}
-			form, err = updateHttpClient.Get(updateBaseURL.String() + "status?path=/user/breakglass")
-			if err != nil {
-				return err
-			}
-		}
 		return err
 	}
 	if form.StatusCode == http.StatusNotFound {
@@ -190,11 +198,6 @@ func breakglass() error {
 			false,
 			"prepare the SSH connection only, but do not execute SSH (useful for using breakglass within an SSH ProxyCommand)")

-		insecure = flag.Bool(
-			"insecure",
-			false,
-			"Fall back to HTTP if HTTPS is configured, but does not work.")
-
 		proxy = flag.Bool(
 			"proxy",
 			false,
@@ -206,6 +209,13 @@ func breakglass() error {
 			"an alternative per-user configuration file for ssh and scp")
 	)

+	// TODO: remove the -tls and -gokrazy_url flags after 2023-June (half a year
+	// after the introduction of instance centric config), so that we can then
+	// merge these flags into tools/internal/oldpacker and remove their global
+	// state.
+	tlsflag.RegisterFlags(flag.CommandLine)
+	updateflag.RegisterFlags(flag.CommandLine, "gokrazy_url")
+
 	flag.Usage = func() {
 		fmt.Fprintf(os.Stderr, "Usage of %s:\n\n", os.Args[0])

@@ -221,6 +231,13 @@ func breakglass() error {
 		log.Fatalf("syntax: breakglass <hostname> [command]")
 	}

+	// If the user did not explicitly specify -update=yes, we default to it.
+	// This differs from the gokr-packer, but breakglass is only useful for
+	// gokrazy instances that already exist.
+	if updateflag.NewInstallation() {
+		updateflag.SetUpdate("yes")
+	}
+
 	instance := flag.Arg(0)
 	instanceflag.SetInstance(instance)

@@ -238,8 +255,6 @@ func breakglass() error {
 		cfg:          cfg,
 		forceRestart: *forceRestart,
 		sshConfig:    *sshConfig,
-		insecure:     *insecure,
-		update:       updateflag.Value{Update: "yes"},
 	}
 	if cfg.Update.Hostname == "" {
 		cfg.Update.Hostname = cfg.Hostname
--- a/go.mod
+++ b/go.mod
@@ -1,16 +1,15 @@
 module github.com/gokrazy/breakglass

-go 1.24.0
+go 1.21

 require (
-	github.com/gokrazy/gokapi v0.0.0-20250222071133-506fdb322775
-	github.com/gokrazy/gokrazy v0.0.0-20250222061409-bd0bb5f1d0b5
-	github.com/gokrazy/internal v0.0.0-20251208203110-3c1aa9087c82
+	github.com/gokrazy/gokrazy v0.0.0-20230812092215-346db1998f83
+	github.com/gokrazy/internal v0.0.0-20250126213949-423a5b587b57
 	github.com/google/renameio/v2 v2.0.0
 	github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510
 	github.com/kr/pty v1.1.8
 	github.com/pkg/sftp v1.13.5
-	golang.org/x/crypto v0.45.0
+	golang.org/x/crypto v0.31.0
 )

 require (
@@ -19,6 +18,5 @@ require (
 	github.com/kr/fs v0.1.0 // indirect
 	github.com/mdlayher/watchdog v0.0.0-20221003142519-49be0df7b3b5 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
-	golang.org/x/oauth2 v0.27.0 // indirect
-	golang.org/x/sys v0.38.0 // indirect
+	golang.org/x/sys v0.28.0 // indirect
 )
--- a/go.sum
+++ b/go.sum
@@ -3,12 +3,10 @@ github.com/creack/pty v1.1.18 h1:n56/Zwd5o6whRC5PMGretI4IdRLlmBXYNjScPaBgsbY=
 github.com/creack/pty v1.1.18/go.mod h1:MOBLtS5ELjhRRrroQr9kyvTxUAFNvYEK993ew/Vr4O4=
 github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/gokrazy/gokapi v0.0.0-20250222071133-506fdb322775 h1:f5+2UMRRbr3+e/gdWCBNn48chS/KMMljfbmlSSHfRBA=
-github.com/gokrazy/gokapi v0.0.0-20250222071133-506fdb322775/go.mod h1:q9mIV8al0wqmqFXJhKiO3SOHkL9/7Q4kIMynqUQWhgU=
-github.com/gokrazy/gokrazy v0.0.0-20250222061409-bd0bb5f1d0b5 h1:VQhDGxRliP4ZTQ8+33v4VKtOpX4VzN8pA4zBMZQSSxs=
-github.com/gokrazy/gokrazy v0.0.0-20250222061409-bd0bb5f1d0b5/go.mod h1:6fAh0J7aH6o5HWSiwN6uxNlm6Rjx1BxeNMWyNBQZ6sI=
-github.com/gokrazy/internal v0.0.0-20251208203110-3c1aa9087c82 h1:4ghNfD9NaZLpFrqQiBF6mPVFeMYXJSky38ubVA4ic2E=
-github.com/gokrazy/internal v0.0.0-20251208203110-3c1aa9087c82/go.mod h1:dQY4EMkD4L5ZjYJ0SPtpgYbV7MIUMCxNIXiOfnZ6jP4=
+github.com/gokrazy/gokrazy v0.0.0-20230812092215-346db1998f83 h1:Y4sADvUYd/c0eqnqebipHHl0GMpAxOQeTzPnwI4ievM=
+github.com/gokrazy/gokrazy v0.0.0-20230812092215-346db1998f83/go.mod h1:9q5Tg+q+YvRjC3VG0gfMFut46dhbhtAnvUEp4lPjc6c=
+github.com/gokrazy/internal v0.0.0-20250126213949-423a5b587b57 h1:f5bEvO4we3fbfiBkECrrUgWQ8OH6J3SdB2Dwxid/Yx4=
+github.com/gokrazy/internal v0.0.0-20250126213949-423a5b587b57/go.mod h1:SJG1KwuJQXFEoBgryaNCkMbdISyovDgZd0xmXJRZmiw=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
 github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/google/renameio/v2 v2.0.0 h1:UifI23ZTGY8Tt29JbYFiuyIU3eX+RNFtUwefq9qAhxg=
@@ -33,20 +31,18 @@ github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+
 github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 golang.org/x/crypto v0.0.0-20211215153901-e495a2d5b3d3/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
-golang.org/x/crypto v0.45.0 h1:jMBrvKuj23MTlT0bQEOBcAE0mjg8mK9RXFhRH6nyF3Q=
-golang.org/x/crypto v0.45.0/go.mod h1:XTGrrkGJve7CYK7J8PEww4aY7gM3qMCElcJQ8n8JdX4=
+golang.org/x/crypto v0.31.0 h1:ihbySMvVjLAeSH1IbfcRTkD/iNscyz8rGzjF/E5hV6U=
+golang.org/x/crypto v0.31.0/go.mod h1:kDsLvtWBEx7MV9tJOj9bnXsPbxwJQ6csT/x4KIN4Ssk=
 golang.org/x/net v0.0.0-20211112202133-69e39bad7dc2/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
-golang.org/x/oauth2 v0.27.0 h1:da9Vo7/tDv5RH/7nZDz1eMGS/q1Vv1N/7FCrBhI9I3M=
-golang.org/x/oauth2 v0.27.0/go.mod h1:onh5ek6nERTohokkhCD/y2cV4Do3fxFHFuAejCkRWT8=
 golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20211216021012-1d35b9e2eb4e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.38.0 h1:3yZWxaJjBmCWXqhN1qh02AkOnCQ1poK6oF+a7xWL6Gc=
-golang.org/x/sys v0.38.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/sys v0.28.0 h1:Fksou7UEQUWlKvIdsqzJmUmCX3cZuD2+P3XyyzwMhlA=
+golang.org/x/sys v0.28.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
-golang.org/x/term v0.37.0 h1:8EGAD0qCmHYZg6J17DvsMy9/wJ7/D/4pV/wfnld5lTU=
-golang.org/x/term v0.37.0/go.mod h1:5pB4lxRNYYVZuTLmy8oR2BH8dflOR+IbTYFD8fi3254=
+golang.org/x/term v0.27.0 h1:WP60Sv1nlK1T6SupCHbXzSaN0b9wUmsPoRS9b61A23Q=
+golang.org/x/term v0.27.0/go.mod h1:iMsnZpn0cago0GOrHO2+Y7u7JPn5AylBrcoWkElMTSM=
 golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
--- a/ssh.go
+++ b/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 {
@@ -220,31 +226,35 @@ 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
-		} else {
-			return "/bin/sh" // available after installation
+			// fallthrough, we don't return /bin/sh as we read /etc/passwd
 		}
 	}
+	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
 		}
-		if r.TERM != "" {
-			s.env = append(s.env, fmt.Sprintf("TERM=%s", r.TERM))
-		}

 		var err error
 		s.ptyf, s.ttyf, err = pty.Open()
@@ -358,21 +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 != "" {
-			cmd = exec.CommandContext(ctx, shell, "-c", r.Command)
+			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{}