Add ability to inject critical options into certs

You may now specify CriticalOptions in sign_certd's config on a
per-environment basis. This allows you to write a policy that says all
certs against this environment will have exactly these critical options.
You can ensure that certs always launch users into restricted shells or
from a defined range of source IPs as supported by sshd.

README.rst

@@ -315,6 +315,15 @@ Effectively the format is::
   indicate which users are allowed to sign requests.
 - ``AuthorizedUsers``: Same as ``AuthorizedSigners`` except that these are
   fingerprints of people allowed to submit requests.
+- ``CriticalOptions``: A hash of critical options to be added to all
+  certificate requests. By specifying these in your configuration file
+  all cert requests to this environment will have these options embedded
+  in them. You can use this option, for example, to restrict the IP
+  addresses that are allowed to use a certificate or to force a user
+  to only be able to run a single command. Those are the only two
+  options supported by sshd right now. This document describes them in
+  the section ``Critical options``:
+  http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
 
 The same users and fingerprints may appear in both ``AuthorizedSigners`` and
 ``AuthorizedUsers``.

sign_certd.go

@@ -30,6 +30,30 @@ import (
 	"time"
 )
 
+// Yanked from PROTOCOL.certkeys
+var supportedCriticalOptions = []string{
+	"force-command",
+	"source-address",
+}
+
+func isSupportedOption(x string) bool {
+	for optionIdx := range supportedCriticalOptions {
+		if supportedCriticalOptions[optionIdx] == x {
+			return true
+		}
+	}
+	return false
+}
+
+func areCriticalOptionsValid(criticalOptions map[string]string) error {
+	for optionName, _ := range criticalOptions {
+		if !isSupportedOption(optionName) {
+			return fmt.Errorf("Invalid critical option name: '%s'", optionName)
+		}
+	}
+	return nil
+}
+
 type certRequest struct {
 	// This struct tracks state for certificate requests. Imagine this one day
 	// being stored in a persistent data store.
@@ -192,13 +216,24 @@ func (h *certRequestHandler) createSigningRequest(rw http.ResponseWriter, req *h
 		http.Error(rw, "Unknown environment.", http.StatusBadRequest)
 		return
 	}
+
 	err = h.validateCert(cert, config.AuthorizedUsers)
 	if err != nil {
 		log.Printf("Invalid certificate signing request received from %s, ignoring", req.RemoteAddr)
 		http.Error(rw, fmt.Sprintf("%v", err), http.StatusBadRequest)
 		return
 	}
 
+	// Ideally we put the critical options into the cert and let validateCert
+	// do the validation. However, this also checks the signature on the cert
+	// which would fail if we modified it prior to validation. So we validate
+	// by hand.
+	if len(config.CriticalOptions) > 0 {
+		for optionName, optionVal := range config.CriticalOptions {
+			cert.CriticalOptions[optionName] = optionVal
+		}
+	}
+
 	requestID := make([]byte, 10)
 	rand.Reader.Read(requestID)
 	requestIDStr := base32.StdEncoding.EncodeToString(requestID)
@@ -324,6 +359,8 @@ func (h *certRequestHandler) validateCert(cert *ssh.Certificate, authorizedSigne
 		_, ok := authorizedSigners[fingerprint]
 		return ok
 	}
+	certChecker.SupportedCriticalOptions = supportedCriticalOptions
+
 	err := certChecker.CheckCert(cert.ValidPrincipals[0], cert)
 	if err != nil {
 		err := fmt.Errorf("Cert not valid: %v", err)
@@ -584,6 +621,12 @@ func signCertd(c *cli.Context) error {
 	if err != nil {
 		return cli.NewExitError(fmt.Sprintf("Load Config failed: %s", err), 1)
 	}
+	for envName, configObj := range config {
+		err = areCriticalOptionsValid(configObj.CriticalOptions)
+		if err != nil {
+			return cli.NewExitError(fmt.Sprintf("Error validation config for env '%s': %s", envName, err), 1)
+		}
+	}
 	err = runSignCertd(config)
 	return err
 }

sign_certd_test.go

@@ -255,6 +255,28 @@ func TestSaveRequestInvalidCert(t *testing.T) {
 	}
 }
 
+func TestSaveRequestInvalidCriticalOptions(t *testing.T) {
+	allConfig := SetupSignerdConfig(1, 0)
+	environment := "testing"
+	envConfig := allConfig[environment]
+	envConfig.CriticalOptions = make(map[string]string)
+	envConfig.CriticalOptions["non-existent-critical"] = "yes"
+	if areCriticalOptionsValid(envConfig.CriticalOptions) == nil {
+		t.Fatalf("Should have found invalid critical option and didn't")
+	}
+}
+
+func TestSaveRequestValidCriticalOptions(t *testing.T) {
+	allConfig := SetupSignerdConfig(1, 0)
+	environment := "testing"
+	envConfig := allConfig[environment]
+	envConfig.CriticalOptions = make(map[string]string)
+	envConfig.CriticalOptions["force-command"] = "/bin/ls"
+	if areCriticalOptionsValid(envConfig.CriticalOptions) != nil {
+		t.Fatalf("Critical option is valid. But our test failed.")
+	}
+}
+
 func getTwoBoringCerts(t *testing.T) (*ssh.Certificate, *ssh.Certificate) {
 	pubKeyOne, _, _, _, err := ssh.ParseAuthorizedKey([]byte(boringUserCertString))
 	if err != nil {

util/config.go

@@ -22,6 +22,7 @@ type SignerdConfig struct {
 	MaxCertLifetime       int
 	PrivateKeyFile        string
 	KmsRegion             string
+	CriticalOptions       map[string]string
 }
 
 type SignerConfig struct {