sftp: add support for ssh keys

remote/types.go

@@ -70,6 +70,7 @@ type SftpAuthRequest struct {
 	IP            string `json:"ip"`
 	SessionID     []byte `json:"session_id"`
 	ClientVersion []byte `json:"client_version"`
+	Type          string `json:"type"`
 }
 
 // SftpAuthResponse is returned by the Panel when a pair of SFTP credentials
@@ -77,8 +78,8 @@ type SftpAuthRequest struct {
 // matched as well as the permissions that are assigned to the authenticated
 // user for the SFTP subsystem.
 type SftpAuthResponse struct {
+	SSHKeys     []string `json:"ssh_keys"`
 	Server      string   `json:"server"`
-	Token       string   `json:"token"`
 	Permissions []string `json:"permissions"`
 }
 

sftp/handler.go

@@ -118,7 +118,7 @@ func (h *Handler) Filewrite(request *sftp.Request) (io.WriterAt, error) {
 	return f, nil
 }
 
-// Filecmd hander for basic SFTP system calls related to files, but not anything to do with reading
+// Filecmd handler for basic SFTP system calls related to files, but not anything to do with reading
 // or writing to those files.
 func (h *Handler) Filecmd(request *sftp.Request) error {
 	if h.ro {

sftp/server.go

@@ -1,7 +1,11 @@
 package sftp
 
 import (
+	"bytes"
 	"context"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/x509"
@@ -50,18 +54,7 @@ func New(m *server.Manager) *SFTPServer {
 
 // Starts the SFTP server and add a persistent listener to handle inbound SFTP connections.
 func (c *SFTPServer) Run() error {
-	if _, err := os.Stat(path.Join(c.BasePath, ".sftp/id_rsa")); os.IsNotExist(err) {
+	keys, err := c.loadPrivateKeys()
-		if err := c.generatePrivateKey(); err != nil {
-			return err
-		}
-	} else if err != nil {
-		return errors.Wrap(err, "sftp/server: could not stat private key file")
-	}
-	pb, err := ioutil.ReadFile(path.Join(c.BasePath, ".sftp/id_rsa"))
-	if err != nil {
-		return errors.Wrap(err, "sftp/server: could not read private key file")
-	}
-	private, err := ssh.ParsePrivateKey(pb)
 	if err != nil {
 		return err
 	}
@@ -70,8 +63,12 @@ func (c *SFTPServer) Run() error {
 		NoClientAuth:      false,
 		MaxAuthTries:      6,
 		PasswordCallback:  c.passwordCallback,
+		PublicKeyCallback: c.publicKeyCallback,
+	}
+
+	for _, k := range keys {
+		conf.AddHostKey(k)
 	}
-	conf.AddHostKey(private)
 
 	listener, err := net.Listen("tcp", c.Listen)
 	if err != nil {
@@ -104,7 +101,7 @@ func (c *SFTPServer) AcceptInbound(conn net.Conn, config *ssh.ServerConfig) {
 		// If its not a session channel we just move on because its not something we
 		// know how to handle at this point.
 		if ch.ChannelType() != "session" {
-			ch.Reject(ssh.UnknownChannelType, "unknown channel type")
+			_ = ch.Reject(ssh.UnknownChannelType, "unknown channel type")
 			continue
 		}
 
@@ -118,7 +115,7 @@ func (c *SFTPServer) AcceptInbound(conn net.Conn, config *ssh.ServerConfig) {
 				// Channels have a type that is dependent on the protocol. For SFTP
 				// this is "subsystem" with a payload that (should) be "sftp". Discard
 				// anything else we receive ("pty", "shell", etc)
-				req.Reply(req.Type == "subsystem" && string(req.Payload[4:]) == "sftp", nil)
+				_ = req.Reply(req.Type == "subsystem" && string(req.Payload[4:]) == "sftp", nil)
 			}
 		}(requests)
 
@@ -136,6 +133,7 @@ func (c *SFTPServer) AcceptInbound(conn net.Conn, config *ssh.ServerConfig) {
 			return s.ID() == uuid
 		})
 		if srv == nil {
+			_ = conn.Close()
 			continue
 		}
 
@@ -143,31 +141,153 @@ func (c *SFTPServer) AcceptInbound(conn net.Conn, config *ssh.ServerConfig) {
 		// them access to the underlying filesystem.
 		handler := sftp.NewRequestServer(channel, NewHandler(sconn, srv.Filesystem()).Handlers())
 		if err := handler.Serve(); err == io.EOF {
-			handler.Close()
+			_ = handler.Close()
 		}
 	}
 }
 
-// Generates a private key that will be used by the SFTP server.
+func (c *SFTPServer) loadPrivateKeys() ([]ssh.Signer, error) {
-func (c *SFTPServer) generatePrivateKey() error {
+	if _, err := os.Stat(path.Join(c.BasePath, ".sftp/id_rsa")); err != nil {
-	key, err := rsa.GenerateKey(rand.Reader, 2048)
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+
+		if err := c.generateRSAPrivateKey(); err != nil {
+			return nil, err
+		}
+	}
+	rsaBytes, err := ioutil.ReadFile(path.Join(c.BasePath, ".sftp/id_rsa"))
 	if err != nil {
-		return errors.WithStack(err)
+		return nil, errors.Wrap(err, "sftp/server: could not read private key file")
+	}
+	rsaPrivateKey, err := ssh.ParsePrivateKey(rsaBytes)
+	if err != nil {
+		return nil, err
+	}
+
+	if _, err := os.Stat(path.Join(c.BasePath, ".sftp/id_ecdsa")); err != nil {
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+
+		if err := c.generateECDSAPrivateKey(); err != nil {
+			return nil, err
+		}
+	}
+	ecdsaBytes, err := ioutil.ReadFile(path.Join(c.BasePath, ".sftp/id_ecdsa"))
+	if err != nil {
+		return nil, errors.Wrap(err, "sftp/server: could not read private key file")
+	}
+	ecdsaPrivateKey, err := ssh.ParsePrivateKey(ecdsaBytes)
+	if err != nil {
+		return nil, err
+	}
+
+	if _, err := os.Stat(path.Join(c.BasePath, ".sftp/id_ed25519")); err != nil {
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+
+		if err := c.generateEd25519PrivateKey(); err != nil {
+			return nil, err
+		}
+	}
+	ed25519Bytes, err := ioutil.ReadFile(path.Join(c.BasePath, ".sftp/id_ed25519"))
+	if err != nil {
+		return nil, errors.Wrap(err, "sftp/server: could not read private key file")
+	}
+	ed25519PrivateKey, err := ssh.ParsePrivateKey(ed25519Bytes)
+	if err != nil {
+		return nil, err
+	}
+
+	return []ssh.Signer{
+		rsaPrivateKey,
+		ecdsaPrivateKey,
+		ed25519PrivateKey,
+	}, nil
+}
+
+// generateRSAPrivateKey generates a RSA-4096 private key that will be used by the SFTP server.
+func (c *SFTPServer) generateRSAPrivateKey() error {
+	key, err := rsa.GenerateKey(rand.Reader, 4096)
+	if err != nil {
+		return err
 	}
 	if err := os.MkdirAll(path.Join(c.BasePath, ".sftp"), 0755); err != nil {
 		return errors.Wrap(err, "sftp/server: could not create .sftp directory")
 	}
 	o, err := os.OpenFile(path.Join(c.BasePath, ".sftp/id_rsa"), os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
 	if err != nil {
-		return errors.WithStack(err)
+		return err
 	}
 	defer o.Close()
 
-	err = pem.Encode(o, &pem.Block{
+	if err := pem.Encode(o, &pem.Block{
 		Type:  "RSA PRIVATE KEY",
 		Bytes: x509.MarshalPKCS1PrivateKey(key),
-	})
+	}); err != nil {
-	return errors.WithStack(err)
+		return err
+	}
+	return nil
+}
+
+// generateECDSAPrivateKey generates a ECDSA-P256 private key that will be used by the SFTP server.
+func (c *SFTPServer) generateECDSAPrivateKey() error {
+	key, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(path.Join(c.BasePath, ".sftp"), 0755); err != nil {
+		return errors.Wrap(err, "sftp/server: could not create .sftp directory")
+	}
+	o, err := os.OpenFile(path.Join(c.BasePath, ".sftp/id_ecdsa"), os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
+	if err != nil {
+		return err
+	}
+	defer o.Close()
+
+	privBytes, err := x509.MarshalPKCS8PrivateKey(key)
+	if err != nil {
+		return err
+	}
+
+	if err := pem.Encode(o, &pem.Block{
+		Type:  "PRIVATE KEY",
+		Bytes: privBytes,
+	}); err != nil {
+		return err
+	}
+	return nil
+}
+
+// generateEd25519PrivateKey generates a ed25519 private key that will be used by the SFTP server.
+func (c *SFTPServer) generateEd25519PrivateKey() error {
+	_, key, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(path.Join(c.BasePath, ".sftp"), 0755); err != nil {
+		return errors.Wrap(err, "sftp/server: could not create .sftp directory")
+	}
+	o, err := os.OpenFile(path.Join(c.BasePath, ".sftp/id_ed25519"), os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
+	if err != nil {
+		return err
+	}
+	defer o.Close()
+
+	privBytes, err := x509.MarshalPKCS8PrivateKey(key)
+	if err != nil {
+		return err
+	}
+
+	if err := pem.Encode(o, &pem.Block{
+		Type:  "PRIVATE KEY",
+		Bytes: privBytes,
+	}); err != nil {
+		return err
+	}
+	return nil
 }
 
 // A function capable of validating user credentials with the Panel API.
@@ -178,6 +298,7 @@ func (c *SFTPServer) passwordCallback(conn ssh.ConnMetadata, pass []byte) (*ssh.
 		IP:            conn.RemoteAddr().String(),
 		SessionID:     conn.SessionID(),
 		ClientVersion: conn.ClientVersion(),
+		Type:          "password",
 	}
 
 	logger := log.WithFields(log.Fields{"subsystem": "sftp", "username": conn.User(), "ip": conn.RemoteAddr().String()})
@@ -188,6 +309,11 @@ func (c *SFTPServer) passwordCallback(conn ssh.ConnMetadata, pass []byte) (*ssh.
 		return nil, &remote.SftpInvalidCredentialsError{}
 	}
 
+	if len(pass) < 1 {
+		logger.Warn("failed to validate user credentials (invalid format)")
+		return nil, &remote.SftpInvalidCredentialsError{}
+	}
+
 	resp, err := c.manager.Client().ValidateSftpCredentials(context.Background(), request)
 	if err != nil {
 		if _, ok := err.(*remote.SftpInvalidCredentialsError); ok {
@@ -209,3 +335,56 @@ func (c *SFTPServer) passwordCallback(conn ssh.ConnMetadata, pass []byte) (*ssh.
 
 	return sshPerm, nil
 }
+
+func (c *SFTPServer) publicKeyCallback(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+	request := remote.SftpAuthRequest{
+		User:          conn.User(),
+		Pass:          "KEKW",
+		IP:            conn.RemoteAddr().String(),
+		SessionID:     conn.SessionID(),
+		ClientVersion: conn.ClientVersion(),
+		Type:          "publicKey",
+	}
+
+	logger := log.WithFields(log.Fields{"subsystem": "sftp", "username": conn.User(), "ip": conn.RemoteAddr().String()})
+	logger.Debug("validating public key for SFTP connection")
+
+	if !validUsernameRegexp.MatchString(request.User) {
+		logger.Warn("failed to validate user credentials (invalid format)")
+		return nil, &remote.SftpInvalidCredentialsError{}
+	}
+
+	resp, err := c.manager.Client().ValidateSftpCredentials(context.Background(), request)
+	if err != nil {
+		if _, ok := err.(*remote.SftpInvalidCredentialsError); ok {
+			logger.Warn("failed to validate user credentials (invalid username or password)")
+		} else {
+			logger.WithField("error", err).Error("encountered an error while trying to validate user credentials")
+		}
+		return nil, err
+	}
+
+	if len(resp.SSHKeys) < 1 {
+		return nil, &remote.SftpInvalidCredentialsError{}
+	}
+
+	for _, k := range resp.SSHKeys {
+		storedPublicKey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(k))
+		if err != nil {
+			return nil, err
+		}
+
+		if !bytes.Equal(key.Marshal(), storedPublicKey.Marshal()) {
+			continue
+		}
+
+		return &ssh.Permissions{
+			Extensions: map[string]string{
+				"uuid":        resp.Server,
+				"user":        conn.User(),
+				"permissions": strings.Join(resp.Permissions, ","),
+			},
+		}, nil
+	}
+	return nil, &remote.SftpInvalidCredentialsError{}
+}