Merge pull request #2637 from Gibheer/ssh-publickeys

allow native and ssh-keygen public key check

4 files changed, 230 insertions, 12 deletions

diff --git a/conf/app.ini b/conf/app.ini
index 9ce2c6f9d1..4ec1b9b5ec 100644
--- a/conf/app.ini
+++ b/conf/app.ini
@@ -71,6 +71,13 @@ SSH_PORT = 22
 SSH_LISTEN_PORT = %(SSH_PORT)s
 ; Root path of SSH directory, default is '~/.ssh', but you have to use '/home/git/.ssh'.
 SSH_ROOT_PATH = 
+; override engine choice to check public keys (default: 'ssh-keygen' when
+; DISABLE_SSH is set to false else 'native')
+SSH_PUBLICKEY_CHECK =
+; directory to create temporary files when using ssh-keygen (default: /tmp)
+SSH_WORK_PATH =
+; path to ssh-keygen (default: result of `which ssh-keygen`)
+SSH_KEYGEN_PATH =
 ; Disable CDN even in "prod" mode
 OFFLINE_MODE = false
 DISABLE_ROUTER_LOG = false
@@ -132,6 +139,15 @@ ENABLE_REVERSE_PROXY_AUTHENTICATION = false
 ENABLE_REVERSE_PROXY_AUTO_REGISTRATION = false
 ; Enable captcha validation for registration
 ENABLE_CAPTCHA = true
+; Do not check minimum key size with corresponding type
+ENABLE_MINIMUM_KEY_SIZE_CHECK = false
+
+; define allowed algorithms and their minimum key length (use -1 to disable a type)
+[service.minimum_key_sizes]
+ED25519 = 256
+ECDSA   = 256
+RSA     = 2048
+DSA     = 1024
 
 [webhook]
 ; Hook task queue length
diff --git a/models/ssh_key.go b/models/ssh_key.go
index 325a40a481..c22b931c88 100644
--- a/models/ssh_key.go
+++ b/models/ssh_key.go
@@ -12,9 +12,11 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/big"
 	"os"
 	"path"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -33,7 +35,10 @@ const (
 	_TPL_PUBLICK_KEY = `command="%s serv key-%d --config='%s'",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty %s` + "\n"
 )
 
-var sshOpLocker = sync.Mutex{}
+var (
+	sshOpLocker       = sync.Mutex{}
+	SSHUnknownKeyType = fmt.Errorf("unknown key type")
+)
 
 type KeyType int
 
@@ -153,7 +158,110 @@ func parseKeyString(content string) (string, error) {
 	return keyType + " " + keyContent + " " + keyComment, nil
 }
 
+// extract key type and length using ssh-keygen
+func SSHKeyGenParsePublicKey(key string) (string, int, error) {
+	// The ssh-keygen in Windows does not print key type, so no need go further.
+	if setting.IsWindows {
+		return "", 0, nil
+	}
+
+	tmpFile, err := ioutil.TempFile(setting.SSHWorkPath, "gogs_keytest")
+	if err != nil {
+		return "", 0, err
+	}
+	tmpName := tmpFile.Name()
+	defer os.Remove(tmpName)
+
+	if ln, err := tmpFile.WriteString(key); err != nil {
+		tmpFile.Close()
+		return "", 0, err
+	} else if ln != len(key) {
+		tmpFile.Close()
+		return "", 0, fmt.Errorf("could not write complete public key (written: %d, should be: %d): %s", ln, len(key), key)
+	}
+	tmpFile.Close()
+
+	stdout, stderr, err := process.Exec("CheckPublicKeyString", setting.SSHKeyGenPath, "-lf", tmpName)
+	if err != nil {
+		return "", 0, fmt.Errorf("public key check failed with error '%s': %s", err, stderr)
+	}
+	if strings.HasSuffix(stdout, "is not a public key file.") {
+		return "", 0, SSHUnknownKeyType
+	}
+	fields := strings.Split(stdout, " ")
+	if len(fields) < 4 {
+		return "", 0, fmt.Errorf("invalid public key line: %s", stdout)
+	}
+
+	length, err := strconv.Atoi(fields[0])
+	if err != nil {
+		return "", 0, err
+	}
+	keyType := strings.Trim(fields[len(fields)-1], "()\r\n")
+	return strings.ToLower(keyType), length, nil
+}
+
+// extract the key type and length using the golang ssh library
+func SSHNativeParsePublicKey(keyLine string) (string, int, error) {
+	fields := strings.Fields(keyLine)
+	if len(fields) < 2 {
+		return "", 0, fmt.Errorf("not enough fields in public key line: %s", string(keyLine))
+	}
+
+	raw, err := base64.StdEncoding.DecodeString(fields[1])
+	if err != nil {
+		return "", 0, err
+	}
+
+	pkey, err := ssh.ParsePublicKey(raw)
+	if err != nil {
+		if strings.HasPrefix(err.Error(), "ssh: unknown key algorithm") {
+			return "", 0, SSHUnknownKeyType
+		}
+		return "", 0, err
+	}
+
+	// The ssh library can parse the key, so next we find out what key exactly we
+	// have.
+	switch pkey.Type() {
+	case ssh.KeyAlgoDSA:
+		rawPub := struct {
+			Name       string
+			P, Q, G, Y *big.Int
+		}{}
+		if err := ssh.Unmarshal(pkey.Marshal(), &rawPub); err != nil {
+			return "", 0, err
+		}
+		// as per https://bugzilla.mindrot.org/show_bug.cgi?id=1647 we should never
+		// see dsa keys != 1024 bit, but as it seems to work, we will not check here
+		return "dsa", rawPub.P.BitLen(), nil // use P as per crypto/dsa/dsa.go (is L)
+	case ssh.KeyAlgoRSA:
+		rawPub := struct {
+			Name string
+			E    *big.Int
+			N    *big.Int
+		}{}
+		if err := ssh.Unmarshal(pkey.Marshal(), &rawPub); err != nil {
+			return "", 0, err
+		}
+		return "rsa", rawPub.N.BitLen(), nil // use N as per crypto/rsa/rsa.go (is bits)
+	case ssh.KeyAlgoECDSA256:
+		return "ecdsa", 256, nil
+	case ssh.KeyAlgoECDSA384:
+		return "ecdsa", 384, nil
+	case ssh.KeyAlgoECDSA521:
+		return "ecdsa", 521, nil
+	case "ssh-ed25519": // TODO replace with ssh constant when available
+		return "ed25519", 256, nil
+	default:
+		return "", 0, fmt.Errorf("no support for key length detection for type %s", pkey.Type())
+	}
+	return "", 0, fmt.Errorf("SSHNativeParsePublicKey failed horribly, please investigate why")
+}
+
 // CheckPublicKeyString checks if the given public key string is recognized by SSH.
+//
+// The function returns the actual public key line on success.
 func CheckPublicKeyString(content string) (_ string, err error) {
 	content, err = parseKeyString(content)
 	if err != nil {
@@ -168,22 +276,34 @@ func CheckPublicKeyString(content string) (_ string, err error) {
 	// remove any unnecessary whitespace now
 	content = strings.TrimSpace(content)
 
-	fields := strings.Fields(content)
-	if len(fields) < 2 {
-		return "", errors.New("too less fields")
+	var (
+		keyType string
+		length  int
+	)
+	if setting.SSHPublicKeyCheck == setting.SSH_PUBLICKEY_CHECK_NATIVE {
+		keyType, length, err = SSHNativeParsePublicKey(content)
+	} else if setting.SSHPublicKeyCheck == setting.SSH_PUBLICKEY_CHECK_KEYGEN {
+		keyType, length, err = SSHKeyGenParsePublicKey(content)
+	} else {
+		log.Error(4, "invalid public key check type: %s", setting.SSHPublicKeyCheck)
+		return "", fmt.Errorf("invalid public key check type")
 	}
 
-	key, err := base64.StdEncoding.DecodeString(fields[1])
-	if err != nil {
-		return "", fmt.Errorf("StdEncoding.DecodeString: %v", err)
-	}
-	pkey, err := ssh.ParsePublicKey([]byte(key))
 	if err != nil {
+		log.Trace("invalid public key of type '%s' with length %d: %s", keyType, length, err)
 		return "", fmt.Errorf("ParsePublicKey: %v", err)
 	}
-	log.Trace("Key type: %s", pkey.Type())
+	log.Trace("Key type: %s", keyType)
 
-	return content, nil
+	if !setting.Service.EnableMinimumKeySizeCheck {
+		return content, nil
+	}
+	if minLen, found := setting.Service.MinimumKeySizes[keyType]; found && length >= minLen {
+		return content, nil
+	} else if found && length < minLen {
+		return "", fmt.Errorf("key not large enough - got %d, needs %d", length, minLen)
+	}
+	return "", fmt.Errorf("key type '%s' is not allowed", keyType)
 }
 
 // saveAuthorizedKeyFile writes SSH key content to authorized_keys file.
@@ -247,7 +367,7 @@ func addKey(e Engine, key *PublicKey) (err error) {
 	}
 	stdout, stderr, err := process.Exec("AddPublicKey", "ssh-keygen", "-lf", tmpPath)
 	if err != nil {
-		return errors.New("ssh-keygen -lf: " + stderr)
+		return fmt.Errorf("'ssh-keygen -lf %s' failed with error '%s': %s", tmpPath, err, stderr)
 	} else if len(stdout) < 2 {
 		return errors.New("not enough output for calculating fingerprint: " + stdout)
 	}
@@ -267,6 +387,7 @@ func addKey(e Engine, key *PublicKey) (err error) {
 
 // AddPublicKey adds new public key to database and authorized_keys file.
 func AddPublicKey(ownerID int64, name, content string) (*PublicKey, error) {
+	log.Trace(content)
 	if err := checkKeyContent(content); err != nil {
 		return nil, err
 	}
diff --git a/models/ssh_key_test.go b/models/ssh_key_test.go
new file mode 100644
index 0000000000..cfb8554ae2
--- /dev/null
+++ b/models/ssh_key_test.go
@@ -0,0 +1,39 @@
+package models
+
+import (
+	"github.com/gogits/gogs/modules/setting"
+	"testing"
+)
+
+func TestSSHKeyVerification(t *testing.T) {
+	setting.SSHWorkPath = "/tmp"
+	setting.SSHKeyGenPath = "/usr/bin/ssh-keygen"
+
+	keys := map[string]string{
+		"dsa-1024":  string("ssh-dss 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 nocomment"),
+		"rsa-1024":  string("ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDAu7tvIvX6ZHrRXuZNfkR3XLHSsuCK9Zn3X58lxBcQzuo5xZgB6vRwwm/QtJuF+zZPtY5hsQILBLmF+BZ5WpKZp1jBeSjH2G7lxet9kbcH+kIVj0tPFEoyKI9wvWqIwC4prx/WVk2wLTJjzBAhyNxfEq7C9CeiX9pQEbEqJfkKCQ== nocomment\n"),
+		"rsa-2048":  string("ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDMZXh+1OBUwSH9D45wTaxErQIN9IoC9xl7MKJkqvTvv6O5RR9YW/IK9FbfjXgXsppYGhsCZo1hFOOsXHMnfOORqu/xMDx4yPuyvKpw4LePEcg4TDipaDFuxbWOqc/BUZRZcXu41QAWfDLrInwsltWZHSeG7hjhpacl4FrVv9V1pS6Oc5Q1NxxEzTzuNLS/8diZrTm/YAQQ/+B+mzWI3zEtF4miZjjAljWd1LTBPvU23d29DcBmmFahcZ441XZsTeAwGxG/Q6j8NgNXj9WxMeWwxXV2jeAX/EBSpZrCVlCQ1yJswT6xCp8TuBnTiGWYMBNTbOZvPC4e0WI2/yZW/s5F nocomment"),
+		"ecdsa-256": string("ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBFQacN3PrOll7PXmN5B/ZNVahiUIqI05nbBlZk1KXsO3d06ktAWqbNflv2vEmA38bTFTfJ2sbn2B5ksT52cDDbA= nocomment"),
+		"ecdsa-384": string("ecdsa-sha2-nistp384 AAAAE2VjZHNhLXNoYTItbmlzdHAzODQAAAAIbmlzdHAzODQAAABhBINmioV+XRX1Fm9Qk2ehHXJ2tfVxW30ypUWZw670Zyq5GQfBAH6xjygRsJ5wWsHXBsGYgFUXIHvMKVAG1tpw7s6ax9oA+dJOJ7tj+vhn8joFqT+sg3LYHgZkHrfqryRasQ== nocomment"),
+		"ecdsa-512": string("ecdsa-sha2-nistp521 AAAAE2VjZHNhLXNoYTItbmlzdHA1MjEAAAAIbmlzdHA1MjEAAACFBACGt3UG3EzRwNOI17QR84l6PgiAcvCE7v6aXPj/SC6UWKg4EL8vW9ZBcdYL9wzs4FZXh4MOV8jAzu3KRWNTwb4k2wFNUpGOt7l28MztFFEtH5BDDrtAJSPENPy8pvPLMfnPg5NhvWycqIBzNcHipem5wSJFN5PdpNOC2xMrPWKNqj+ZjQ== nocomment"),
+	}
+
+	for name, pubkey := range keys {
+		keyTypeN, lengthN, errN := SSHNativeParsePublicKey(pubkey)
+		if errN != nil {
+			if errN != SSHUnknownKeyType {
+				t.Errorf("error parsing public key '%s': %s", name, errN)
+				continue
+			}
+		}
+		keyTypeK, lengthK, errK := SSHKeyGenParsePublicKey(pubkey)
+		if errK != nil {
+			t.Errorf("error parsing public key '%s': %s", name, errK)
+			continue
+		}
+		// we know that ed25519 is currently not supported by native and returns SSHUnknownKeyType
+		if (keyTypeN != keyTypeK || lengthN != lengthK) && errN != SSHUnknownKeyType {
+			t.Errorf("key mismatch for '%s': native: %s(%d), ssh-keygen: %s(%d)", name, keyTypeN, lengthN, keyTypeK, lengthK)
+		}
+	}
+}
diff --git a/modules/setting/setting.go b/modules/setting/setting.go
index c74416613d..2b1b553126 100644
--- a/modules/setting/setting.go
+++ b/modules/setting/setting.go
@@ -27,6 +27,11 @@ import (
 	"github.com/gogits/gogs/modules/user"
 )
 
+const (
+	SSH_PUBLICKEY_CHECK_NATIVE = "native"
+	SSH_PUBLICKEY_CHECK_KEYGEN = "ssh-keygen"
+)
+
 type Scheme string
 
 const (
@@ -67,6 +72,9 @@ var (
 	SSHPort            int
 	SSHListenPort      int
 	SSHRootPath        string
+	SSHPublicKeyCheck  string
+	SSHWorkPath        string
+	SSHKeyGenPath      string
 	OfflineMode        bool
 	DisableRouterLog   bool
 	CertFile, KeyFile  string
@@ -330,6 +338,29 @@ func NewContext() {
 	if err := os.MkdirAll(SSHRootPath, 0700); err != nil {
 		log.Fatal(4, "Fail to create '%s': %v", SSHRootPath, err)
 	}
+	checkDefault := SSH_PUBLICKEY_CHECK_KEYGEN
+	if StartSSHServer {
+		checkDefault = SSH_PUBLICKEY_CHECK_NATIVE
+	}
+	SSHPublicKeyCheck = sec.Key("SSH_PUBLICKEY_CHECK").MustString(checkDefault)
+	if SSHPublicKeyCheck != SSH_PUBLICKEY_CHECK_NATIVE &&
+		SSHPublicKeyCheck != SSH_PUBLICKEY_CHECK_KEYGEN {
+		log.Fatal(4, "SSH_PUBLICKEY_CHECK must be ssh-keygen or native")
+	}
+	SSHWorkPath = sec.Key("SSH_WORK_PATH").MustString(os.TempDir())
+	if !DisableSSH && (!StartSSHServer || SSHPublicKeyCheck == SSH_PUBLICKEY_CHECK_KEYGEN) {
+		if tmpDirStat, err := os.Stat(SSHWorkPath); err != nil || !tmpDirStat.IsDir() {
+			log.Fatal(4, "directory '%s' set in SSHWorkPath is not a directory: %s", SSHWorkPath, err)
+		}
+	}
+	SSHKeyGenPath = sec.Key("SSH_KEYGEN_PATH").MustString("")
+	if !DisableSSH && !StartSSHServer &&
+		SSHKeyGenPath == "" && SSHPublicKeyCheck == SSH_PUBLICKEY_CHECK_KEYGEN {
+		SSHKeyGenPath, err = exec.LookPath("ssh-keygen")
+		if err != nil {
+			log.Fatal(4, "could not find ssh-keygen, maybe set DISABLE_SSH to use the internal ssh server")
+		}
+	}
 	OfflineMode = sec.Key("OFFLINE_MODE").MustBool()
 	DisableRouterLog = sec.Key("DISABLE_ROUTER_LOG").MustBool()
 	StaticRootPath = sec.Key("STATIC_ROOT_PATH").MustString(workDir)
@@ -461,6 +492,8 @@ var Service struct {
 	EnableReverseProxyAuth         bool
 	EnableReverseProxyAutoRegister bool
 	EnableCaptcha                  bool
+	EnableMinimumKeySizeCheck      bool
+	MinimumKeySizes                map[string]int
 }
 
 func newService() {
@@ -473,6 +506,15 @@ func newService() {
 	Service.EnableReverseProxyAuth = sec.Key("ENABLE_REVERSE_PROXY_AUTHENTICATION").MustBool()
 	Service.EnableReverseProxyAutoRegister = sec.Key("ENABLE_REVERSE_PROXY_AUTO_REGISTRATION").MustBool()
 	Service.EnableCaptcha = sec.Key("ENABLE_CAPTCHA").MustBool()
+	Service.EnableMinimumKeySizeCheck = sec.Key("ENABLE_MINIMUM_KEY_SIZE_CHECK").MustBool()
+	Service.MinimumKeySizes = map[string]int{}
+
+	minimumKeySizes := Cfg.Section("service.minimum_key_sizes").Keys()
+	for _, key := range minimumKeySizes {
+		if key.MustInt() != -1 {
+			Service.MinimumKeySizes[strings.ToLower(key.Name())] = key.MustInt()
+		}
+	}
 }
 
 var logLevels = map[string]string{