authentication.go

/*
Copyright 2016 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package options

import (
	"context"
	"fmt"
	"strings"
	"time"

	"github.com/spf13/pflag"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/util/wait"
	"k8s.io/apiserver/pkg/authentication/authenticatorfactory"
	"k8s.io/apiserver/pkg/authentication/request/headerrequest"
	"k8s.io/apiserver/pkg/server"
	"k8s.io/apiserver/pkg/server/dynamiccertificates"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/rest"
	"k8s.io/client-go/tools/clientcmd"
	"k8s.io/client-go/transport"
	"k8s.io/klog/v2"
	openapicommon "k8s.io/kube-openapi/pkg/common"
)

// DefaultAuthWebhookRetryBackoff is the default backoff parameters for
// both authentication and authorization webhook used by the apiserver.
func DefaultAuthWebhookRetryBackoff() *wait.Backoff {
	return &wait.Backoff{
		Duration: 500 * time.Millisecond,
		Factor:   1.5,
		Jitter:   0.2,
		Steps:    5,
	}
}

type RequestHeaderAuthenticationOptions struct {
	// ClientCAFile is the root certificate bundle to verify client certificates on incoming requests
	// before trusting usernames in headers.
	ClientCAFile string

	UsernameHeaders     []string
	GroupHeaders        []string
	ExtraHeaderPrefixes []string
	AllowedNames        []string
}

func (s *RequestHeaderAuthenticationOptions) Validate() []error {
	allErrors := []error{}

	if err := checkForWhiteSpaceOnly("requestheader-username-headers", s.UsernameHeaders...); err != nil {
		allErrors = append(allErrors, err)
	}
	if err := checkForWhiteSpaceOnly("requestheader-group-headers", s.GroupHeaders...); err != nil {
		allErrors = append(allErrors, err)
	}
	if err := checkForWhiteSpaceOnly("requestheader-extra-headers-prefix", s.ExtraHeaderPrefixes...); err != nil {
		allErrors = append(allErrors, err)
	}
	if err := checkForWhiteSpaceOnly("requestheader-allowed-names", s.AllowedNames...); err != nil {
		allErrors = append(allErrors, err)
	}

	return allErrors
}

func checkForWhiteSpaceOnly(flag string, headerNames ...string) error {
	for _, headerName := range headerNames {
		if len(strings.TrimSpace(headerName)) == 0 {
			return fmt.Errorf("empty value in %q", flag)
		}
	}

	return nil
}

func (s *RequestHeaderAuthenticationOptions) AddFlags(fs *pflag.FlagSet) {
	if s == nil {
		return
	}

	fs.StringSliceVar(&s.UsernameHeaders, "requestheader-username-headers", s.UsernameHeaders, ""+
		"List of request headers to inspect for usernames. X-Remote-User is common.")

	fs.StringSliceVar(&s.GroupHeaders, "requestheader-group-headers", s.GroupHeaders, ""+
		"List of request headers to inspect for groups. X-Remote-Group is suggested.")

	fs.StringSliceVar(&s.ExtraHeaderPrefixes, "requestheader-extra-headers-prefix", s.ExtraHeaderPrefixes, ""+
		"List of request header prefixes to inspect. X-Remote-Extra- is suggested.")

	fs.StringVar(&s.ClientCAFile, "requestheader-client-ca-file", s.ClientCAFile, ""+
		"Root certificate bundle to use to verify client certificates on incoming requests "+
		"before trusting usernames in headers specified by --requestheader-username-headers. "+
		"WARNING: generally do not depend on authorization being already done for incoming requests.")

	fs.StringSliceVar(&s.AllowedNames, "requestheader-allowed-names", s.AllowedNames, ""+
		"List of client certificate common names to allow to provide usernames in headers "+
		"specified by --requestheader-username-headers. If empty, any client certificate validated "+
		"by the authorities in --requestheader-client-ca-file is allowed.")
}

// ToAuthenticationRequestHeaderConfig returns a RequestHeaderConfig config object for these options
// if necessary, nil otherwise.
func (s *RequestHeaderAuthenticationOptions) ToAuthenticationRequestHeaderConfig() (*authenticatorfactory.RequestHeaderConfig, error) {
	if len(s.ClientCAFile) == 0 {
		return nil, nil
	}

	caBundleProvider, err := dynamiccertificates.NewDynamicCAContentFromFile("request-header", s.ClientCAFile)
	if err != nil {
		return nil, err
	}

	return &authenticatorfactory.RequestHeaderConfig{
		UsernameHeaders:     headerrequest.StaticStringSlice(s.UsernameHeaders),
		GroupHeaders:        headerrequest.StaticStringSlice(s.GroupHeaders),
		ExtraHeaderPrefixes: headerrequest.StaticStringSlice(s.ExtraHeaderPrefixes),
		CAContentProvider:   caBundleProvider,
		AllowedClientNames:  headerrequest.StaticStringSlice(s.AllowedNames),
	}, nil
}

// ClientCertAuthenticationOptions provides different options for client cert auth. You should use `GetClientVerifyOptionFn` to
// get the verify options for your authenticator.
type ClientCertAuthenticationOptions struct {
	// ClientCA is the certificate bundle for all the signers that you'll recognize for incoming client certificates
	ClientCA string

	// CAContentProvider are the options for verifying incoming connections using mTLS and directly assigning to users.
	// Generally this is the CA bundle file used to authenticate client certificates
	// If non-nil, this takes priority over the ClientCA file.
	CAContentProvider dynamiccertificates.CAContentProvider
}

// GetClientVerifyOptionFn provides verify options for your authenticator while respecting the preferred order of verifiers.
func (s *ClientCertAuthenticationOptions) GetClientCAContentProvider() (dynamiccertificates.CAContentProvider, error) {
	if s.CAContentProvider != nil {
		return s.CAContentProvider, nil
	}

	if len(s.ClientCA) == 0 {
		return nil, nil
	}

	return dynamiccertificates.NewDynamicCAContentFromFile("client-ca-bundle", s.ClientCA)
}

func (s *ClientCertAuthenticationOptions) AddFlags(fs *pflag.FlagSet) {
	fs.StringVar(&s.ClientCA, "client-ca-file", s.ClientCA, ""+
		"If set, any request presenting a client certificate signed by one of "+
		"the authorities in the client-ca-file is authenticated with an identity "+
		"corresponding to the CommonName of the client certificate.")
}

// DelegatingAuthenticationOptions provides an easy way for composing API servers to delegate their authentication to
// the root kube API server.  The API federator will act as
// a front proxy and direction connections will be able to delegate to the core kube API server
type DelegatingAuthenticationOptions struct {
	// RemoteKubeConfigFile is the file to use to connect to a "normal" kube API server which hosts the
	// TokenAccessReview.authentication.k8s.io endpoint for checking tokens.
	RemoteKubeConfigFile string
	// RemoteKubeConfigFileOptional is specifying whether not specifying the kubeconfig or
	// a missing in-cluster config will be fatal.
	RemoteKubeConfigFileOptional bool

	// CacheTTL is the length of time that a token authentication answer will be cached.
	CacheTTL time.Duration

	ClientCert    ClientCertAuthenticationOptions
	RequestHeader RequestHeaderAuthenticationOptions

	// SkipInClusterLookup indicates missing authentication configuration should not be retrieved from the cluster configmap
	SkipInClusterLookup bool

	// TolerateInClusterLookupFailure indicates failures to look up authentication configuration from the cluster configmap should not be fatal.
	// Setting this can result in an authenticator that will reject all requests.
	TolerateInClusterLookupFailure bool

	// WebhookRetryBackoff specifies the backoff parameters for the authentication webhook retry logic.
	// This allows us to configure the sleep time at each iteration and the maximum number of retries allowed
	// before we fail the webhook call in order to limit the fan out that ensues when the system is degraded.
	WebhookRetryBackoff *wait.Backoff

	// TokenRequestTimeout specifies a time limit for requests made by the authorization webhook client.
	// The default value is set to 10 seconds.
	TokenRequestTimeout time.Duration

	// CustomRoundTripperFn allows for specifying a middleware function for custom HTTP behaviour for the authentication webhook client.
	CustomRoundTripperFn transport.WrapperFunc
}

func NewDelegatingAuthenticationOptions() *DelegatingAuthenticationOptions {
	return &DelegatingAuthenticationOptions{
		// very low for responsiveness, but high enough to handle storms
		CacheTTL:   10 * time.Second,
		ClientCert: ClientCertAuthenticationOptions{},
		RequestHeader: RequestHeaderAuthenticationOptions{
			UsernameHeaders:     []string{"x-remote-user"},
			GroupHeaders:        []string{"x-remote-group"},
			ExtraHeaderPrefixes: []string{"x-remote-extra-"},
		},
		WebhookRetryBackoff: DefaultAuthWebhookRetryBackoff(),
		TokenRequestTimeout: 10 * time.Second,
	}
}

// WithCustomRetryBackoff sets the custom backoff parameters for the authentication webhook retry logic.
func (s *DelegatingAuthenticationOptions) WithCustomRetryBackoff(backoff wait.Backoff) {
	s.WebhookRetryBackoff = &backoff
}

// WithRequestTimeout sets the given timeout for requests made by the authentication webhook client.
func (s *DelegatingAuthenticationOptions) WithRequestTimeout(timeout time.Duration) {
	s.TokenRequestTimeout = timeout
}

// WithCustomRoundTripper allows for specifying a middleware function for custom HTTP behaviour for the authentication webhook client.
func (s *DelegatingAuthenticationOptions) WithCustomRoundTripper(rt transport.WrapperFunc) {
	s.CustomRoundTripperFn = rt
}

func (s *DelegatingAuthenticationOptions) Validate() []error {
	if s == nil {
		return nil
	}

	allErrors := []error{}
	allErrors = append(allErrors, s.RequestHeader.Validate()...)

	if s.WebhookRetryBackoff != nil && s.WebhookRetryBackoff.Steps <= 0 {
		allErrors = append(allErrors, fmt.Errorf("number of webhook retry attempts must be greater than 1, but is: %d", s.WebhookRetryBackoff.Steps))
	}

	return allErrors
}

func (s *DelegatingAuthenticationOptions) AddFlags(fs *pflag.FlagSet) {
	if s == nil {
		return
	}

	var optionalKubeConfigSentence string
	if s.RemoteKubeConfigFileOptional {
		optionalKubeConfigSentence = " This is optional. If empty, all token requests are considered to be anonymous and no client CA is looked up in the cluster."
	}
	fs.StringVar(&s.RemoteKubeConfigFile, "authentication-kubeconfig", s.RemoteKubeConfigFile, ""+
		"kubeconfig file pointing at the 'core' kubernetes server with enough rights to create "+
		"tokenreviews.authentication.k8s.io."+optionalKubeConfigSentence)

	fs.DurationVar(&s.CacheTTL, "authentication-token-webhook-cache-ttl", s.CacheTTL,
		"The duration to cache responses from the webhook token authenticator.")

	s.ClientCert.AddFlags(fs)
	s.RequestHeader.AddFlags(fs)

	fs.BoolVar(&s.SkipInClusterLookup, "authentication-skip-lookup", s.SkipInClusterLookup, ""+
		"If false, the authentication-kubeconfig will be used to lookup missing authentication "+
		"configuration from the cluster.")
	fs.BoolVar(&s.TolerateInClusterLookupFailure, "authentication-tolerate-lookup-failure", s.TolerateInClusterLookupFailure, ""+
		"If true, failures to look up missing authentication configuration from the cluster are not considered fatal. "+
		"Note that this can result in authentication that treats all requests as anonymous.")
}

func (s *DelegatingAuthenticationOptions) ApplyTo(authenticationInfo *server.AuthenticationInfo, servingInfo *server.SecureServingInfo, openAPIConfig *openapicommon.Config) error {
	if s == nil {
		authenticationInfo.Authenticator = nil
		return nil
	}

	cfg := authenticatorfactory.DelegatingAuthenticatorConfig{
		Anonymous:                true,
		CacheTTL:                 s.CacheTTL,
		WebhookRetryBackoff:      s.WebhookRetryBackoff,
		TokenAccessReviewTimeout: s.TokenRequestTimeout,
	}

	client, err := s.getClient()
	if err != nil {
		return fmt.Errorf("failed to get delegated authentication kubeconfig: %v", err)
	}

	// configure token review
	if client != nil {
		cfg.TokenAccessReviewClient = client.AuthenticationV1()
	}

	// get the clientCA information
	clientCASpecified := s.ClientCert != ClientCertAuthenticationOptions{}
	var clientCAProvider dynamiccertificates.CAContentProvider
	if clientCASpecified {
		clientCAProvider, err = s.ClientCert.GetClientCAContentProvider()
		if err != nil {
			return fmt.Errorf("unable to load client CA provider: %v", err)
		}
		cfg.ClientCertificateCAContentProvider = clientCAProvider
		if err = authenticationInfo.ApplyClientCert(cfg.ClientCertificateCAContentProvider, servingInfo); err != nil {
			return fmt.Errorf("unable to assign client CA provider: %v", err)
		}

	} else if !s.SkipInClusterLookup {
		if client == nil {
			klog.Warningf("No authentication-kubeconfig provided in order to lookup client-ca-file in configmap/%s in %s, so client certificate authentication won't work.", authenticationConfigMapName, authenticationConfigMapNamespace)
		} else {
			clientCAProvider, err = dynamiccertificates.NewDynamicCAFromConfigMapController("client-ca", authenticationConfigMapNamespace, authenticationConfigMapName, "client-ca-file", client)
			if err != nil {
				return fmt.Errorf("unable to load configmap based client CA file: %v", err)
			}
			cfg.ClientCertificateCAContentProvider = clientCAProvider
			if err = authenticationInfo.ApplyClientCert(cfg.ClientCertificateCAContentProvider, servingInfo); err != nil {
				return fmt.Errorf("unable to assign configmap based client CA file: %v", err)
			}

		}
	}

	requestHeaderCAFileSpecified := len(s.RequestHeader.ClientCAFile) > 0
	var requestHeaderConfig *authenticatorfactory.RequestHeaderConfig
	if requestHeaderCAFileSpecified {
		requestHeaderConfig, err = s.RequestHeader.ToAuthenticationRequestHeaderConfig()
		if err != nil {
			return fmt.Errorf("unable to create request header authentication config: %v", err)
		}

	} else if !s.SkipInClusterLookup {
		if client == nil {
			klog.Warningf("No authentication-kubeconfig provided in order to lookup requestheader-client-ca-file in configmap/%s in %s, so request-header client certificate authentication won't work.", authenticationConfigMapName, authenticationConfigMapNamespace)
		} else {
			requestHeaderConfig, err = s.createRequestHeaderConfig(client)
			if err != nil {
				if s.TolerateInClusterLookupFailure {
					klog.Warningf("Error looking up in-cluster authentication configuration: %v", err)
					klog.Warning("Continuing without authentication configuration. This may treat all requests as anonymous.")
					klog.Warning("To require authentication configuration lookup to succeed, set --authentication-tolerate-lookup-failure=false")
				} else {
					return fmt.Errorf("unable to load configmap based request-header-client-ca-file: %v", err)
				}
			}
		}
	}
	if requestHeaderConfig != nil {
		cfg.RequestHeaderConfig = requestHeaderConfig
		if err = authenticationInfo.ApplyClientCert(cfg.RequestHeaderConfig.CAContentProvider, servingInfo); err != nil {
			return fmt.Errorf("unable to load request-header-client-ca-file: %v", err)
		}
	}

	// create authenticator
	authenticator, securityDefinitions, err := cfg.New()
	if err != nil {
		return err
	}
	authenticationInfo.Authenticator = authenticator
	if openAPIConfig != nil {
		openAPIConfig.SecurityDefinitions = securityDefinitions
	}

	return nil
}

const (
	authenticationConfigMapNamespace = metav1.NamespaceSystem
	// authenticationConfigMapName is the name of ConfigMap in the kube-system namespace holding the root certificate
	// bundle to use to verify client certificates on incoming requests before trusting usernames in headers specified
	// by --requestheader-username-headers. This is created in the cluster by the kube-apiserver.
	// "WARNING: generally do not depend on authorization being already done for incoming requests.")
	authenticationConfigMapName = "extension-apiserver-authentication"
)

func (s *DelegatingAuthenticationOptions) createRequestHeaderConfig(client kubernetes.Interface) (*authenticatorfactory.RequestHeaderConfig, error) {
	dynamicRequestHeaderProvider, err := newDynamicRequestHeaderController(client)
	if err != nil {
		return nil, fmt.Errorf("unable to create request header authentication config: %v", err)
	}

	//  look up authentication configuration in the cluster and in case of an err defer to authentication-tolerate-lookup-failure flag
	//  We are passing the context to ProxyCerts.RunOnce as it needs to implement RunOnce(ctx) however the
	//  context is not used at all. So passing a empty context shouldn't be a problem
	ctx := context.TODO()
	if err := dynamicRequestHeaderProvider.RunOnce(ctx); err != nil {
		return nil, err
	}

	return &authenticatorfactory.RequestHeaderConfig{
		CAContentProvider:   dynamicRequestHeaderProvider,
		UsernameHeaders:     headerrequest.StringSliceProvider(headerrequest.StringSliceProviderFunc(dynamicRequestHeaderProvider.UsernameHeaders)),
		GroupHeaders:        headerrequest.StringSliceProvider(headerrequest.StringSliceProviderFunc(dynamicRequestHeaderProvider.GroupHeaders)),
		ExtraHeaderPrefixes: headerrequest.StringSliceProvider(headerrequest.StringSliceProviderFunc(dynamicRequestHeaderProvider.ExtraHeaderPrefixes)),
		AllowedClientNames:  headerrequest.StringSliceProvider(headerrequest.StringSliceProviderFunc(dynamicRequestHeaderProvider.AllowedClientNames)),
	}, nil
}

// getClient returns a Kubernetes clientset. If s.RemoteKubeConfigFileOptional is true, nil will be returned
// if no kubeconfig is specified by the user and the in-cluster config is not found.
func (s *DelegatingAuthenticationOptions) getClient() (kubernetes.Interface, error) {
	var clientConfig *rest.Config
	var err error
	if len(s.RemoteKubeConfigFile) > 0 {
		loadingRules := &clientcmd.ClientConfigLoadingRules{ExplicitPath: s.RemoteKubeConfigFile}
		loader := clientcmd.NewNonInteractiveDeferredLoadingClientConfig(loadingRules, &clientcmd.ConfigOverrides{})

		clientConfig, err = loader.ClientConfig()
	} else {
		// without the remote kubeconfig file, try to use the in-cluster config.  Most addon API servers will
		// use this path. If it is optional, ignore errors.
		clientConfig, err = rest.InClusterConfig()
		if err != nil && s.RemoteKubeConfigFileOptional {
			if err != rest.ErrNotInCluster {
				klog.Warningf("failed to read in-cluster kubeconfig for delegated authentication: %v", err)
			}
			return nil, nil
		}
	}
	if err != nil {
		return nil, fmt.Errorf("failed to get delegated authentication kubeconfig: %v", err)
	}

	// set high qps/burst limits since this will effectively limit API server responsiveness
	clientConfig.QPS = 200
	clientConfig.Burst = 400
	// do not set a timeout on the http client, instead use context for cancellation
	// if multiple timeouts were set, the request will pick the smaller timeout to be applied, leaving other useless.
	//
	// see https://github.com/golang/go/blob/a937729c2c2f6950a32bc5cd0f5b88700882f078/src/net/http/client.go#L364
	if s.CustomRoundTripperFn != nil {
		clientConfig.Wrap(s.CustomRoundTripperFn)
	}

	return kubernetes.NewForConfig(clientConfig)
}