starssh/sshagent_auth.go

package starssh

import (
	"errors"
	"fmt"
	"io"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"golang.org/x/crypto/ssh"
	sshagent "golang.org/x/crypto/ssh/agent"
)

var errSSHAgentUnavailable = errors.New("ssh-agent unavailable")
var errRetrySSHAgentAuth = errors.New("retry ssh-agent auth")
var buildSSHAgentAuthMethodFunc = buildSSHAgentAuthMethod

type sshAgentTimeouts struct {
	Dial             time.Duration
	Operation        time.Duration
	Forward          time.Duration
	Endpoint         string
	Resolved         resolvedSSHAgentEndpoint
	Debug            SSHAgentDebugFunc
	SkipFingerprints map[string]struct{}
	SignFailure      func(ssh.PublicKey, error)
}

type sshAgentAuthAttempt struct {
	mu               sync.Mutex
	skipFingerprints map[string]struct{}
	retryRequested   bool
}

var defaultAuthOrder = []AuthMethodKind{
	AuthMethodSSHAgent,
	AuthMethodPrivateKey,
	AuthMethodPassword,
	AuthMethodKeyboardInteractive,
}

func effectiveSSHAgentTimeout(info LoginInput) time.Duration {
	switch {
	case info.SSHAgentTimeout < 0:
		return 0
	case info.SSHAgentTimeout > 0:
		return info.SSHAgentTimeout
	default:
		return defaultSSHAgentTimeout
	}
}

func effectiveSSHAgentTimeouts(info LoginInput) sshAgentTimeouts {
	return sshAgentTimeouts{
		Dial:      effectiveDialTimeout(info),
		Operation: effectiveSSHAgentTimeout(info),
		Forward:   effectiveSSHAgentForwardTimeout(info),
		Endpoint:  info.IdentityAgent,
		Debug:     info.SSHAgentDebug,
	}
}

func effectiveSSHAgentForwardTimeout(info LoginInput) time.Duration {
	if info.SSHAgentForwardTimeout > 0 {
		return info.SSHAgentForwardTimeout
	}
	return 0
}

func buildAuthMethods(info LoginInput) ([]ssh.AuthMethod, func(), error) {
	return buildAuthMethodsWithAgentAttempt(info, nil)
}

func buildAuthMethodsWithAgentAttempt(info LoginInput, agentAttempt *sshAgentAuthAttempt) ([]ssh.AuthMethod, func(), error) {
	order, err := normalizeAuthOrder(info.AuthOrder)
	if err != nil {
		return nil, nil, err
	}

	auth := make([]ssh.AuthMethod, 0, len(order))
	var agentErr error
	var cleanupFuncs []func()

	for _, methodKind := range order {
		switch methodKind {
		case AuthMethodPrivateKey:
			method, err := buildPrivateKeyAuthMethod(info, agentAttempt)
			if err != nil {
				return nil, nil, err
			}
			if method != nil {
				auth = append(auth, method)
			}
		case AuthMethodPassword:
			method := buildPasswordAuthMethod(info.Password, info.PasswordCallback, agentAttempt)
			if method != nil {
				auth = append(auth, method)
			}
		case AuthMethodKeyboardInteractive:
			method := buildKeyboardInteractiveAuthMethod(info.Password, info.PasswordCallback, info.KeyboardInteractiveCallback, agentAttempt)
			if method != nil {
				auth = append(auth, method)
			}
		case AuthMethodSSHAgent:
			if info.DisableSSHAgent {
				continue
			}
			timeouts := effectiveSSHAgentTimeouts(info)
			if agentAttempt != nil {
				timeouts.SkipFingerprints = agentAttempt.skipSnapshot()
				timeouts.SignFailure = agentAttempt.recordSignFailure
			}
			agentMethod, cleanup, err := buildSSHAgentAuthMethodFunc(timeouts)
			if err != nil {
				agentErr = err
				continue
			}
			if agentMethod != nil {
				auth = append(auth, agentMethod)
			}
			if cleanup != nil {
				cleanupFuncs = append(cleanupFuncs, cleanup)
			}
		}
	}

	if len(auth) == 0 {
		if agentErr != nil {
			return nil, nil, fmt.Errorf("no authentication method provided; ssh-agent unavailable: %w", agentErr)
		}
		return nil, nil, errors.New("no authentication method provided: password, private key, or ssh-agent is required")
	}

	return auth, composeCleanup(cleanupFuncs...), nil
}

func normalizeAuthOrder(order []AuthMethodKind) ([]AuthMethodKind, error) {
	if len(order) == 0 {
		return append([]AuthMethodKind(nil), defaultAuthOrder...), nil
	}

	normalized := make([]AuthMethodKind, 0, len(order))
	seen := make(map[AuthMethodKind]struct{}, len(order))
	for _, raw := range order {
		kind := AuthMethodKind(strings.ToLower(strings.TrimSpace(string(raw))))
		if kind == "" {
			return nil, errors.New("auth order contains an empty auth method")
		}
		if !isSupportedAuthMethodKind(kind) {
			return nil, fmt.Errorf("unsupported auth method %q", raw)
		}
		if _, exists := seen[kind]; exists {
			continue
		}
		seen[kind] = struct{}{}
		normalized = append(normalized, kind)
	}

	if len(normalized) == 0 {
		return nil, errors.New("auth order is empty")
	}
	return normalized, nil
}

func isSupportedAuthMethodKind(kind AuthMethodKind) bool {
	switch kind {
	case AuthMethodPrivateKey, AuthMethodPassword, AuthMethodKeyboardInteractive, AuthMethodSSHAgent:
		return true
	default:
		return false
	}
}

func shouldRetrySSHAgentAuth(info LoginInput, order []AuthMethodKind) bool {
	if info.DisableSSHAgent {
		return false
	}
	for _, methodKind := range order {
		if methodKind == AuthMethodSSHAgent {
			return true
		}
	}
	return false
}

func buildPrivateKeyAuthMethod(info LoginInput, agentAttempt *sshAgentAuthAttempt) (ssh.AuthMethod, error) {
	if strings.TrimSpace(info.Prikey) == "" {
		return nil, nil
	}

	pemBytes := []byte(info.Prikey)
	if info.PrikeyPwd == "" {
		signer, err := ssh.ParsePrivateKey(pemBytes)
		if err != nil {
			return nil, err
		}
		return ssh.PublicKeysCallback(privateKeySignersCallback(signer, agentAttempt)), nil
	}

	signer, err := ssh.ParsePrivateKeyWithPassphrase(pemBytes, []byte(info.PrikeyPwd))
	if err != nil {
		return nil, err
	}
	return ssh.PublicKeysCallback(privateKeySignersCallback(signer, agentAttempt)), nil
}

func privateKeySignersCallback(signer ssh.Signer, agentAttempt *sshAgentAuthAttempt) func() ([]ssh.Signer, error) {
	return func() ([]ssh.Signer, error) {
		if err := checkSSHAgentRetryPending(agentAttempt); err != nil {
			return nil, err
		}
		return []ssh.Signer{signer}, nil
	}
}

func buildPasswordAuthMethod(password string, callback func() (string, error), agentAttempt *sshAgentAuthAttempt) ssh.AuthMethod {
	if password == "" && callback == nil {
		return nil
	}
	return ssh.PasswordCallback(func() (string, error) {
		if err := checkSSHAgentRetryPending(agentAttempt); err != nil {
			return "", err
		}
		if password != "" {
			return password, nil
		}
		return callback()
	})
}

func buildKeyboardInteractiveAuthMethod(
	password string,
	passwordCallback func() (string, error),
	challenge ssh.KeyboardInteractiveChallenge,
	agentAttempt *sshAgentAuthAttempt,
) ssh.AuthMethod {
	if challenge != nil {
		return ssh.KeyboardInteractive(func(user, instruction string, questions []string, echos []bool) ([]string, error) {
			if err := checkSSHAgentRetryPending(agentAttempt); err != nil {
				return nil, err
			}
			return challenge(user, instruction, questions, echos)
		})
	}
	if password == "" && passwordCallback == nil {
		return nil
	}

	keyboardInteractiveChallenge := func(user, instruction string, questions []string, echos []bool) ([]string, error) {
		if err := checkSSHAgentRetryPending(agentAttempt); err != nil {
			return nil, err
		}
		if len(questions) == 0 {
			return []string{}, nil
		}

		answer := password
		if answer == "" {
			var err error
			answer, err = passwordCallback()
			if err != nil {
				return nil, err
			}
		}

		answers := make([]string, len(questions))
		for i := range questions {
			answers[i] = answer
		}
		return answers, nil
	}
	return ssh.KeyboardInteractive(keyboardInteractiveChallenge)
}

func buildSSHAgentAuthMethod(timeouts sshAgentTimeouts) (ssh.AuthMethod, func(), error) {
	conn, resolved, err := dialSSHAgentWithDebug("auth", timeouts)
	if err != nil {
		if errors.Is(err, errSSHAgentUnavailable) {
			return nil, nil, nil
		}
		return nil, nil, err
	}
	if conn == nil {
		return nil, nil, nil
	}
	conn = wrapSSHAgentConnWithDeadline(conn, timeouts.Operation)

	started := time.Now()
	signers, err := sshagent.NewClient(conn).Signers()
	err = normalizeSSHAgentError(err)
	logSSHAgentDebug(timeouts.Debug, SSHAgentDebugEvent{
		Step:     "auth",
		Source:   resolved.Source,
		Endpoint: resolved.Endpoint,
		Network:  resolved.Network,
		Phase:    "list",
		Status:   debugStatus(err),
		Duration: time.Since(started),
		KeyCount: len(signers),
		Err:      err,
	})
	if err != nil {
		_ = conn.Close()
		return nil, nil, err
	}
	if len(signers) == 0 {
		_ = conn.Close()
		return nil, nil, errors.New("ssh-agent has no loaded keys")
	}

	timeouts.Resolved = resolved
	orderedSigners := orderSSHAgentSigners(signers)
	filteredSigners := filterSSHAgentSignersForRetry(orderedSigners, timeouts)
	if len(filteredSigners) == 0 {
		_ = conn.Close()
		return nil, nil, errors.New("ssh-agent has no usable keys")
	}

	return ssh.PublicKeys(filteredSigners...), func() {
		_ = conn.Close()
	}, nil
}

func orderSSHAgentSigners(signers []ssh.Signer) []ssh.Signer {
	type orderedSigner struct {
		signer  ssh.Signer
		index   int
		score   int
		comment string
	}

	ordered := make([]orderedSigner, 0, len(signers))
	for index, signer := range signers {
		if signer == nil || signer.PublicKey() == nil {
			continue
		}
		ordered = append(ordered, orderedSigner{
			signer:  signer,
			index:   index,
			score:   sshAgentSignerPriority(signer),
			comment: sshAgentSignerComment(signer),
		})
	}

	sort.SliceStable(ordered, func(i, j int) bool {
		if ordered[i].score != ordered[j].score {
			return ordered[i].score > ordered[j].score
		}
		return ordered[i].index < ordered[j].index
	})

	result := make([]ssh.Signer, 0, len(ordered))
	for _, item := range ordered {
		result = append(result, item.signer)
	}
	return result
}

func sshAgentSignerComment(signer ssh.Signer) string {
	if signer == nil {
		return ""
	}
	if key, ok := signer.PublicKey().(*sshagent.Key); ok {
		return key.Comment
	}
	return ""
}

func sshAgentSignerPriority(signer ssh.Signer) int {
	comment := strings.TrimSpace(sshAgentSignerComment(signer))
	if comment == "" {
		return 0
	}

	score := 0
	if priority, ok := parseSSHAgentSignerPriority(comment); ok {
		score += 100000 + priority*1000
	}

	lower := strings.ToLower(comment)
	if strings.Contains(lower, "current") {
		score += 400
	}
	if strings.Contains(lower, "cardno:") {
		score += 300
	}
	if strings.Contains(lower, "card ") || strings.Contains(lower, " card") || strings.Contains(lower, "card:") {
		score += 100
	}
	if strings.Contains(lower, "openpgp") || strings.Contains(lower, "gpg") {
		score += 50
	}
	return score
}

func parseSSHAgentSignerPriority(comment string) (int, bool) {
	lower := strings.ToLower(comment)
	index := strings.Index(lower, "priority=")
	if index < 0 {
		return 0, false
	}

	value := strings.TrimSpace(comment[index+len("priority="):])
	if value == "" {
		return 0, false
	}

	end := 0
	for end < len(value) {
		ch := value[end]
		if ch == '+' || ch == '-' || (ch >= '0' && ch <= '9') {
			end++
			continue
		}
		break
	}
	if end == 0 {
		return 0, false
	}

	priority, err := strconv.Atoi(value[:end])
	if err != nil {
		return 0, false
	}
	return priority, true
}

func filterSSHAgentSignersForRetry(signers []ssh.Signer, timeouts sshAgentTimeouts) []ssh.Signer {
	filteredSigners := make([]ssh.Signer, 0, len(signers))
	for _, signer := range signers {
		if signer == nil {
			continue
		}
		publicKey := signer.PublicKey()
		if publicKey == nil {
			continue
		}
		if _, skip := timeouts.SkipFingerprints[ssh.FingerprintSHA256(publicKey)]; skip {
			continue
		}
		if timeouts.SignFailure == nil && timeouts.Debug == nil {
			filteredSigners = append(filteredSigners, signer)
			continue
		}
		filteredSigners = append(filteredSigners, wrapSSHAgentSigner(signer, sshAgentSignerOptions{
			Resolved:    timeouts.Resolved,
			Debug:       timeouts.Debug,
			SignFailure: timeouts.SignFailure,
		}))
	}
	return filteredSigners
}

func newSSHAgentAuthAttempt() *sshAgentAuthAttempt {
	return &sshAgentAuthAttempt{
		skipFingerprints: make(map[string]struct{}),
	}
}

func (a *sshAgentAuthAttempt) begin() {
	if a == nil {
		return
	}
	a.mu.Lock()
	defer a.mu.Unlock()
	a.retryRequested = false
}

func (a *sshAgentAuthAttempt) skipSnapshot() map[string]struct{} {
	if a == nil {
		return nil
	}
	a.mu.Lock()
	defer a.mu.Unlock()
	if len(a.skipFingerprints) == 0 {
		return nil
	}
	snapshot := make(map[string]struct{}, len(a.skipFingerprints))
	for fingerprint := range a.skipFingerprints {
		snapshot[fingerprint] = struct{}{}
	}
	return snapshot
}

func (a *sshAgentAuthAttempt) recordSignFailure(publicKey ssh.PublicKey, err error) {
	_ = err
	if a == nil || publicKey == nil {
		return
	}
	a.skipFingerprint(ssh.FingerprintSHA256(publicKey))
}

func (a *sshAgentAuthAttempt) skipFingerprint(fingerprint string) {
	if a == nil {
		return
	}
	a.mu.Lock()
	defer a.mu.Unlock()
	a.retryRequested = true
	if fingerprint != "" {
		a.skipFingerprints[fingerprint] = struct{}{}
	}
}

func (a *sshAgentAuthAttempt) shouldRetry() bool {
	if a == nil {
		return false
	}
	a.mu.Lock()
	defer a.mu.Unlock()
	return a.retryRequested
}

func checkSSHAgentRetryPending(agentAttempt *sshAgentAuthAttempt) error {
	if agentAttempt != nil && agentAttempt.shouldRetry() {
		return errRetrySSHAgentAuth
	}
	return nil
}

type sshAgentRetrySigner struct {
	signer    ssh.Signer
	publicKey ssh.PublicKey
	options   sshAgentSignerOptions
}

type sshAgentRetryAlgorithmSigner struct {
	sshAgentRetrySigner
	algorithmSigner ssh.AlgorithmSigner
}

type sshAgentRetryMultiAlgorithmSigner struct {
	sshAgentRetryAlgorithmSigner
	multiAlgorithmSigner ssh.MultiAlgorithmSigner
}

type sshAgentSignerOptions struct {
	Resolved    resolvedSSHAgentEndpoint
	Debug       SSHAgentDebugFunc
	SignFailure func(ssh.PublicKey, error)
}

func wrapSSHAgentSignerForRetry(signer ssh.Signer, onFailure func(ssh.PublicKey, error)) ssh.Signer {
	return wrapSSHAgentSigner(signer, sshAgentSignerOptions{SignFailure: onFailure})
}

func wrapSSHAgentSigner(signer ssh.Signer, options sshAgentSignerOptions) ssh.Signer {
	publicKey := signer.PublicKey()
	base := sshAgentRetrySigner{
		signer:    signer,
		publicKey: publicKey,
		options:   options,
	}
	if multiAlgorithmSigner, ok := signer.(ssh.MultiAlgorithmSigner); ok {
		return &sshAgentRetryMultiAlgorithmSigner{
			sshAgentRetryAlgorithmSigner: sshAgentRetryAlgorithmSigner{
				sshAgentRetrySigner: base,
				algorithmSigner:     multiAlgorithmSigner,
			},
			multiAlgorithmSigner: multiAlgorithmSigner,
		}
	}
	if algorithmSigner, ok := signer.(ssh.AlgorithmSigner); ok {
		return &sshAgentRetryAlgorithmSigner{
			sshAgentRetrySigner: base,
			algorithmSigner:     algorithmSigner,
		}
	}
	return &base
}

func (s *sshAgentRetrySigner) PublicKey() ssh.PublicKey {
	return s.publicKey
}

func (s *sshAgentRetrySigner) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
	started := time.Now()
	signature, err := s.signer.Sign(rand, data)
	return signature, s.finishSign(started, err)
}

func (s *sshAgentRetrySigner) finishSign(started time.Time, err error) error {
	err = normalizeSSHAgentError(err)
	s.logSignDebug(started, err)
	if err == nil {
		return nil
	}
	if s.options.SignFailure != nil {
		s.options.SignFailure(s.publicKey, err)
		return wrapSSHAgentSignError(err)
	}
	return err
}

func (s *sshAgentRetrySigner) logSignDebug(started time.Time, err error) {
	if s == nil || s.options.Debug == nil {
		return
	}
	logSSHAgentDebug(s.options.Debug, SSHAgentDebugEvent{
		Step:     "auth",
		Source:   s.options.Resolved.Source,
		Endpoint: s.options.Resolved.Endpoint,
		Network:  s.options.Resolved.Network,
		Phase:    "sign",
		Status:   debugStatus(err),
		Duration: time.Since(started),
		Err:      err,
	})
}

func (s *sshAgentRetryAlgorithmSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*ssh.Signature, error) {
	algorithm = preferredSSHAgentSignAlgorithm(s.publicKey, algorithm, nil)
	started := time.Now()
	signature, err := s.algorithmSigner.SignWithAlgorithm(rand, data, algorithm)
	return signature, s.finishSign(started, err)
}

func (s *sshAgentRetryMultiAlgorithmSigner) Algorithms() []string {
	return s.multiAlgorithmSigner.Algorithms()
}

func (s *sshAgentRetryMultiAlgorithmSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*ssh.Signature, error) {
	algorithm = preferredSSHAgentSignAlgorithm(s.publicKey, algorithm, s.multiAlgorithmSigner.Algorithms())
	started := time.Now()
	signature, err := s.multiAlgorithmSigner.SignWithAlgorithm(rand, data, algorithm)
	return signature, s.finishSign(started, err)
}

func preferredSSHAgentSignAlgorithm(publicKey ssh.PublicKey, requested string, algorithms []string) string {
	if publicKey == nil || publicKey.Type() != ssh.KeyAlgoRSA || requested != ssh.KeyAlgoRSA {
		return requested
	}
	if len(algorithms) == 0 {
		return ssh.KeyAlgoRSASHA256
	}
	for _, algorithm := range algorithms {
		if algorithm == ssh.KeyAlgoRSA {
			break
		}
		if algorithm == ssh.KeyAlgoRSASHA256 || algorithm == ssh.KeyAlgoRSASHA512 {
			return algorithm
		}
	}
	return requested
}

func wrapSSHAgentSignError(err error) error {
	if err == nil {
		return nil
	}
	return fmt.Errorf("%w: %v", errRetrySSHAgentAuth, normalizeSSHAgentError(err))
}

func composeCleanup(funcs ...func()) func() {
	if len(funcs) == 0 {
		return nil
	}
	return func() {
		for i := len(funcs) - 1; i >= 0; i-- {
			if funcs[i] != nil {
				funcs[i]()
			}
		}
	}
}