Add password provider to SSH connection (#63)

* Add password provider to SSH connection
This allows to implement a custom password provider (e.g. user input) for
provate keys which require a password

* Fix ssh-agent usage

* rearrange code

* smaller fixes

examples/password/password.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"syscall"
+
+	"github.com/k0sproject/rig"
+	"golang.org/x/crypto/ssh/terminal"
+)
+
+/*
+	This example shows how to use a key password provider
+*/
+
+func main() {
+	user := flag.String("user", "root", "SSH User")
+	host := flag.String("host", "localhost", "Host")
+	flag.Parse()
+	conn := rig.Connection{
+		SSH: &rig.SSH{
+			User:    *user,
+			Address: *host,
+			PasswordCallback: func() (string, error) {
+				fmt.Println("Enter password:")
+				pass, err := terminal.ReadPassword(int(syscall.Stdin))
+				return string(pass), err
+			},
+		},
+	}
+	if err := conn.Connect(); err != nil {
+		panic(err)
+	}
+	defer conn.Disconnect()
+	output, err := conn.ExecOutput("ls -al")
+	if err != nil {
+		panic(err)
+	}
+	println(output)
+}

ssh.go

@@ -32,14 +32,14 @@ import (
 
 // SSH describes an SSH connection
 type SSH struct {
-	Address string `yaml:"address" validate:"required,hostname|ip"`
-	User    string `yaml:"user" validate:"required" default:"root"`
-	Port    int    `yaml:"port" default:"22" validate:"gt=0,lte=65535"`
-	KeyPath string `yaml:"keyPath" validate:"omitempty"`
-	HostKey string `yaml:"hostKey,omitempty"`
-	Bastion *SSH   `yaml:"bastion,omitempty"`
-
-	name string
+	Address          string `yaml:"address" validate:"required,hostname|ip"`
+	User             string `yaml:"user" validate:"required" default:"root"`
+	Port             int    `yaml:"port" default:"22" validate:"gt=0,lte=65535"`
+	KeyPath          string `yaml:"keyPath" validate:"omitempty"`
+	HostKey          string `yaml:"hostKey,omitempty"`
+	Bastion          *SSH   `yaml:"bastion,omitempty"`
+	PasswordCallback PasswordCallback
+	name             string
 
 	isWindows      bool
 	knowOs         bool
@@ -48,6 +48,8 @@ type SSH struct {
 	client *ssh.Client
 }
 
+type PasswordCallback func() (secret string, err error)
+
 const DefaultKeypath = "~/.ssh/id_rsa"
 
 // SetDefaults sets various default values
@@ -131,41 +133,25 @@ func (c *SSH) Connect() error {
 		config.HostKeyCallback = trustedHostKeyCallback(c.HostKey)
 	}
 
-	var pubkeySigners []ssh.Signer
-
-	_, err := os.Stat(c.KeyPath)
-	if err != nil && !c.keypathDefault {
-		return err
-	}
-	if err == nil {
-		var key []byte
-		key, err = os.ReadFile(c.KeyPath)
-		if err != nil {
-			return err
-		}
-		signer, err := ssh.ParsePrivateKey(key)
-		if err != nil {
-			log.Infof("can't parse keyfile %s: %s", c.KeyPath, err.Error())
-		} else {
-			pubkeySigners = append(pubkeySigners, signer)
-		}
-	}
-
 	sshAgentSock := os.Getenv("SSH_AUTH_SOCK")
 	if sshAgentSock != "" {
 		sshAgent, err := net.Dial("unix", sshAgentSock)
 		if err != nil {
 			log.Errorf("can't connect to SSH agent auth socket %s: %s", sshAgentSock, err)
 		} else {
 			signers, err := agent.NewClient(sshAgent).Signers()
-			if err == nil {
-				pubkeySigners = append(pubkeySigners, signers...)
+			if err == nil && len(signers) > 0 {
+				config.Auth = append(config.Auth, ssh.PublicKeys(signers...))
 			}
 		}
 	}
 
-	if len(pubkeySigners) > 0 {
-		config.Auth = append(config.Auth, ssh.PublicKeys(pubkeySigners...))
+	privateKeyAuth, err := c.getPrivateKeys()
+	if err != nil {
+		return err
+	}
+	if len(privateKeyAuth) > 0 {
+		config.Auth = append(config.Auth, privateKeyAuth...)
 	}
 
 	dst := net.JoinHostPort(c.Address, strconv.Itoa(c.Port))
@@ -196,6 +182,49 @@ func (c *SSH) Connect() error {
 	return nil
 }
 
+func (c *SSH) getPrivateKeys() ([]ssh.AuthMethod, error) {
+	result := []ssh.AuthMethod{}
+	_, err := os.Stat(c.KeyPath)
+	if err != nil && !c.keypathDefault {
+		return result, err
+	}
+	if err == nil {
+		var key []byte
+		key, err = os.ReadFile(c.KeyPath)
+		if err != nil {
+			return result, err
+		}
+		signer, err := ssh.ParsePrivateKey(key)
+		if err != nil {
+			if c.PasswordCallback != nil {
+				switch err.(type) {
+				case *ssh.PassphraseMissingError:
+					auth := ssh.PublicKeysCallback(func() ([]ssh.Signer, error) {
+						pass, err := c.PasswordCallback()
+						if err != nil {
+							return nil, fmt.Errorf("password provider failed: %s", err)
+						}
+						signer, err := ssh.ParsePrivateKeyWithPassphrase(key, []byte(pass))
+						if err != nil {
+
+							return nil, err
+						}
+						return []ssh.Signer{signer}, nil
+					})
+
+					result = append(result, auth)
+				default:
+					log.Infof("can't parse keyfile %s: %s", c.KeyPath, err.Error())
+				}
+			} else {
+				log.Infof("can't parse keyfile %s: %s", c.KeyPath, err.Error())
+			}
+		}
+		result = append(result, ssh.PublicKeys(signer))
+	}
+	return result, nil
+}
+
 // Exec executes a command on the host
 func (c *SSH) Exec(cmd string, opts ...exec.Option) error {
 	o := exec.Build(opts...)