authorization.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 (
	"fmt"
	"time"

	"github.com/spf13/pflag"

	"k8s.io/apimachinery/pkg/util/wait"
	"k8s.io/apiserver/pkg/authorization/authorizer"
	"k8s.io/apiserver/pkg/authorization/authorizerfactory"
	"k8s.io/apiserver/pkg/authorization/path"
	"k8s.io/apiserver/pkg/authorization/union"
	"k8s.io/apiserver/pkg/server"
	"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"
)

// DelegatingAuthorizationOptions provides an easy way for composing API servers to delegate their authorization to
// the root kube API server.
// WARNING: never assume that every authenticated incoming request already does authorization.
//
//	The aggregator in the kube API server does this today, but this behaviour is not
//	guaranteed in the future.
type DelegatingAuthorizationOptions struct {
	// RemoteKubeConfigFile is the file to use to connect to a "normal" kube API server which hosts the
	// SubjectAccessReview.authorization.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

	// AllowCacheTTL is the length of time that a successful authorization response will be cached
	AllowCacheTTL time.Duration

	// DenyCacheTTL is the length of time that an unsuccessful authorization response will be cached.
	// You generally want more responsive, "deny, try again" flows.
	DenyCacheTTL time.Duration

	// AlwaysAllowPaths are HTTP paths which are excluded from authorization. They can be plain
	// paths or end in * in which case prefix-match is applied. A leading / is optional.
	AlwaysAllowPaths []string

	// AlwaysAllowGroups are groups which are allowed to take any actions.  In kube, this is system:masters.
	AlwaysAllowGroups []string

	// ClientTimeout specifies a time limit for requests made by SubjectAccessReviews client.
	// The default value is set to 10 seconds.
	ClientTimeout time.Duration

	// WebhookRetryBackoff specifies the backoff parameters for the authorization 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

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

func NewDelegatingAuthorizationOptions() *DelegatingAuthorizationOptions {
	return &DelegatingAuthorizationOptions{
		// very low for responsiveness, but high enough to handle storms
		AllowCacheTTL:       10 * time.Second,
		DenyCacheTTL:        10 * time.Second,
		ClientTimeout:       10 * time.Second,
		WebhookRetryBackoff: DefaultAuthWebhookRetryBackoff(),
		// This allows the kubelet to always get health and readiness without causing an authorization check.
		// This field can be cleared by callers if they don't want this behavior.
		AlwaysAllowPaths: []string{"/healthz", "/readyz", "/livez"},
		// In an authorization call delegated to a kube-apiserver (the expected common-case), system:masters has full
		// authority in a hard-coded authorizer.  This means that our default can reasonably be to skip an authorization
		// check for system:masters.
		// This field can be cleared by callers if they don't want this behavior.
		AlwaysAllowGroups: []string{"system:masters"},
	}
}

// WithAlwaysAllowGroups appends the list of paths to AlwaysAllowGroups
func (s *DelegatingAuthorizationOptions) WithAlwaysAllowGroups(groups ...string) *DelegatingAuthorizationOptions {
	s.AlwaysAllowGroups = append(s.AlwaysAllowGroups, groups...)
	return s
}

// WithAlwaysAllowPaths appends the list of paths to AlwaysAllowPaths
func (s *DelegatingAuthorizationOptions) WithAlwaysAllowPaths(paths ...string) *DelegatingAuthorizationOptions {
	s.AlwaysAllowPaths = append(s.AlwaysAllowPaths, paths...)
	return s
}

// WithClientTimeout sets the given timeout for SAR client used by this authorizer
func (s *DelegatingAuthorizationOptions) WithClientTimeout(timeout time.Duration) {
	s.ClientTimeout = timeout
}

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

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

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

	allErrors := []error{}
	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 *DelegatingAuthorizationOptions) AddFlags(fs *pflag.FlagSet) {
	if s == nil {
		return
	}

	var optionalKubeConfigSentence string
	if s.RemoteKubeConfigFileOptional {
		optionalKubeConfigSentence = " This is optional. If empty, all requests not skipped by authorization are forbidden."
	}
	fs.StringVar(&s.RemoteKubeConfigFile, "authorization-kubeconfig", s.RemoteKubeConfigFile,
		"kubeconfig file pointing at the 'core' kubernetes server with enough rights to create "+
			"subjectaccessreviews.authorization.k8s.io."+optionalKubeConfigSentence)

	fs.DurationVar(&s.AllowCacheTTL, "authorization-webhook-cache-authorized-ttl",
		s.AllowCacheTTL,
		"The duration to cache 'authorized' responses from the webhook authorizer.")

	fs.DurationVar(&s.DenyCacheTTL,
		"authorization-webhook-cache-unauthorized-ttl", s.DenyCacheTTL,
		"The duration to cache 'unauthorized' responses from the webhook authorizer.")

	fs.StringSliceVar(&s.AlwaysAllowPaths, "authorization-always-allow-paths", s.AlwaysAllowPaths,
		"A list of HTTP paths to skip during authorization, i.e. these are authorized without "+
			"contacting the 'core' kubernetes server.")
}

func (s *DelegatingAuthorizationOptions) ApplyTo(c *server.AuthorizationInfo) error {
	if s == nil {
		c.Authorizer = authorizerfactory.NewAlwaysAllowAuthorizer()
		return nil
	}

	client, err := s.getClient()
	if err != nil {
		return err
	}

	c.Authorizer, err = s.toAuthorizer(client)
	return err
}

func (s *DelegatingAuthorizationOptions) toAuthorizer(client kubernetes.Interface) (authorizer.Authorizer, error) {
	var authorizers []authorizer.Authorizer

	if len(s.AlwaysAllowGroups) > 0 {
		authorizers = append(authorizers, authorizerfactory.NewPrivilegedGroups(s.AlwaysAllowGroups...))
	}

	if len(s.AlwaysAllowPaths) > 0 {
		a, err := path.NewAuthorizer(s.AlwaysAllowPaths)
		if err != nil {
			return nil, err
		}
		authorizers = append(authorizers, a)
	}

	if client == nil {
		klog.Warning("No authorization-kubeconfig provided, so SubjectAccessReview of authorization tokens won't work.")
	} else {
		cfg := authorizerfactory.DelegatingAuthorizerConfig{
			SubjectAccessReviewClient: client.AuthorizationV1(),
			AllowCacheTTL:             s.AllowCacheTTL,
			DenyCacheTTL:              s.DenyCacheTTL,
			WebhookRetryBackoff:       s.WebhookRetryBackoff,
		}
		delegatedAuthorizer, err := cfg.New()
		if err != nil {
			return nil, err
		}
		authorizers = append(authorizers, delegatedAuthorizer)
	}

	return union.New(authorizers...), nil
}

func (s *DelegatingAuthorizationOptions) 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 authorization: %v", err)
			}
			return nil, nil
		}
	}
	if err != nil {
		return nil, fmt.Errorf("failed to get delegated authorization kubeconfig: %v", err)
	}

	// set high qps/burst limits since this will effectively limit API server responsiveness
	clientConfig.QPS = 200
	clientConfig.Burst = 400
	clientConfig.Timeout = s.ClientTimeout
	if s.CustomRoundTripperFn != nil {
		clientConfig.Wrap(s.CustomRoundTripperFn)
	}

	return kubernetes.NewForConfig(clientConfig)
}