puppeth: handle encrypted ssh keys (closes #15442) (#15443)

* cmd/puppeth: handle encrypted ssh keys

* cmd/puppeth: fix unconvert linter error

vendor/golang.org/x/crypto/ssh/server.go

@@ -14,23 +14,34 @@ import (
 )
 
 // The Permissions type holds fine-grained permissions that are
-// specific to a user or a specific authentication method for a
-// user. Permissions, except for "source-address", must be enforced in
-// the server application layer, after successful authentication. The
-// Permissions are passed on in ServerConn so a server implementation
-// can honor them.
+// specific to a user or a specific authentication method for a user.
+// The Permissions value for a successful authentication attempt is
+// available in ServerConn, so it can be used to pass information from
+// the user-authentication phase to the application layer.
 type Permissions struct {
-	// Critical options restrict default permissions. Common
-	// restrictions are "source-address" and "force-command". If
-	// the server cannot enforce the restriction, or does not
-	// recognize it, the user should not authenticate.
+	// CriticalOptions indicate restrictions to the default
+	// permissions, and are typically used in conjunction with
+	// user certificates. The standard for SSH certificates
+	// defines "force-command" (only allow the given command to
+	// execute) and "source-address" (only allow connections from
+	// the given address). The SSH package currently only enforces
+	// the "source-address" critical option. It is up to server
+	// implementations to enforce other critical options, such as
+	// "force-command", by checking them after the SSH handshake
+	// is successful. In general, SSH servers should reject
+	// connections that specify critical options that are unknown
+	// or not supported.
 	CriticalOptions map[string]string
 
 	// Extensions are extra functionality that the server may
-	// offer on authenticated connections. Common extensions are
-	// "permit-agent-forwarding", "permit-X11-forwarding". Lack of
-	// support for an extension does not preclude authenticating a
-	// user.
+	// offer on authenticated connections. Lack of support for an
+	// extension does not preclude authenticating a user. Common
+	// extensions are "permit-agent-forwarding",
+	// "permit-X11-forwarding". The Go SSH library currently does
+	// not act on any extension, and it is up to server
+	// implementations to honor them. Extensions can be used to
+	// pass data from the authentication callbacks to the server
+	// application layer.
 	Extensions map[string]string
 }
 
@@ -55,9 +66,14 @@ type ServerConfig struct {
 	// attempts to authenticate using a password.
 	PasswordCallback func(conn ConnMetadata, password []byte) (*Permissions, error)
 
-	// PublicKeyCallback, if non-nil, is called when a client attempts public
-	// key authentication. It must return true if the given public key is
-	// valid for the given user. For example, see CertChecker.Authenticate.
+	// PublicKeyCallback, if non-nil, is called when a client
+	// offers a public key for authentication. It must return a nil error
+	// if the given public key can be used to authenticate the
+	// given user. For example, see CertChecker.Authenticate. A
+	// call to this function does not guarantee that the key
+	// offered is in fact used to authenticate. To record any data
+	// depending on the public key, store it inside a
+	// Permissions.Extensions entry.
 	PublicKeyCallback func(conn ConnMetadata, key PublicKey) (*Permissions, error)
 
 	// KeyboardInteractiveCallback, if non-nil, is called when
@@ -147,12 +163,12 @@ type ServerConn struct {
 // Request and NewChannel channels must be serviced, or the connection
 // will hang.
 func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewChannel, <-chan *Request, error) {
-	if config.MaxAuthTries == 0 {
-		config.MaxAuthTries = 6
-	}
-
 	fullConf := *config
 	fullConf.SetDefaults()
+	if fullConf.MaxAuthTries == 0 {
+		fullConf.MaxAuthTries = 6
+	}
+
 	s := &connection{
 		sshConn: sshConn{conn: c},
 	}
@@ -272,12 +288,30 @@ func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
 	return fmt.Errorf("ssh: remote address %v is not allowed because of source-address restriction", addr)
 }
 
+// ServerAuthError implements the error interface. It appends any authentication
+// errors that may occur, and is returned if all of the authentication methods
+// provided by the user failed to authenticate.
+type ServerAuthError struct {
+	// Errors contains authentication errors returned by the authentication
+	// callback methods.
+	Errors []error
+}
+
+func (l ServerAuthError) Error() string {
+	var errs []string
+	for _, err := range l.Errors {
+		errs = append(errs, err.Error())
+	}
+	return "[" + strings.Join(errs, ", ") + "]"
+}
+
 func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
 	sessionID := s.transport.getSessionID()
 	var cache pubKeyCache
 	var perms *Permissions
 
 	authFailures := 0
+	var authErrs []error
 
 userAuthLoop:
 	for {
@@ -296,6 +330,9 @@ userAuthLoop:
 
 		var userAuthReq userAuthRequestMsg
 		if packet, err := s.transport.readPacket(); err != nil {
+			if err == io.EOF {
+				return nil, &ServerAuthError{Errors: authErrs}
+			}
 			return nil, err
 		} else if err = Unmarshal(packet, &userAuthReq); err != nil {
 			return nil, err
@@ -432,6 +469,8 @@ userAuthLoop:
 			authErr = fmt.Errorf("ssh: unknown method %q", userAuthReq.Method)
 		}
 
+		authErrs = append(authErrs, authErr)
+
 		if config.AuthLogCallback != nil {
 			config.AuthLogCallback(s, userAuthReq.Method, authErr)
 		}