admission.go

Documentation: k8s.io/kubernetes/plugin/pkg/admission/certificates/subjectrestriction

     1  /*
     2  Copyright 2020 The Kubernetes Authors.
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8      http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  package subjectrestriction
    18  
    19  import (
    20  	"context"
    21  	"fmt"
    22  	"io"
    23  
    24  	certificatesv1beta1 "k8s.io/api/certificates/v1beta1"
    25  	"k8s.io/apiserver/pkg/admission"
    26  	"k8s.io/klog/v2"
    27  	certificatesapi "k8s.io/kubernetes/pkg/apis/certificates"
    28  )
    29  
    30  // PluginName is a string with the name of the plugin
    31  const PluginName = "CertificateSubjectRestriction"
    32  
    33  // Register registers the plugin
    34  func Register(plugins *admission.Plugins) {
    35  	plugins.Register(PluginName, func(config io.Reader) (admission.Interface, error) {
    36  		return NewPlugin(), nil
    37  	})
    38  }
    39  
    40  // Plugin holds state for and implements the admission plugin.
    41  type Plugin struct {
    42  	*admission.Handler
    43  }
    44  
    45  // ValidateInitialization always returns nil.
    46  func (p *Plugin) ValidateInitialization() error {
    47  	return nil
    48  }
    49  
    50  var _ admission.ValidationInterface = &Plugin{}
    51  
    52  // NewPlugin constructs a new instance of the CertificateSubjectRestrictions admission interface.
    53  func NewPlugin() *Plugin {
    54  	return &Plugin{
    55  		Handler: admission.NewHandler(admission.Create),
    56  	}
    57  }
    58  
    59  var csrGroupResource = certificatesapi.Resource("certificatesigningrequests")
    60  
    61  // Validate ensures that if the signerName on a CSR is set to
    62  // `kubernetes.io/kube-apiserver-client`, that its organization (group)
    63  // attribute is not set to `system:masters`.
    64  func (p *Plugin) Validate(_ context.Context, a admission.Attributes, _ admission.ObjectInterfaces) error {
    65  	if a.GetResource().GroupResource() != csrGroupResource || a.GetSubresource() != "" {
    66  		return nil
    67  	}
    68  
    69  	csr, ok := a.GetObject().(*certificatesapi.CertificateSigningRequest)
    70  	if !ok {
    71  		return admission.NewForbidden(a, fmt.Errorf("expected type CertificateSigningRequest, got: %T", a.GetObject()))
    72  	}
    73  
    74  	if csr.Spec.SignerName != certificatesv1beta1.KubeAPIServerClientSignerName {
    75  		return nil
    76  	}
    77  
    78  	csrParsed, err := certificatesapi.ParseCSR(csr.Spec.Request)
    79  	if err != nil {
    80  		return admission.NewForbidden(a, fmt.Errorf("failed to parse CSR: %v", err))
    81  	}
    82  
    83  	for _, group := range csrParsed.Subject.Organization {
    84  		if group == "system:masters" {
    85  			klog.V(4).Infof("CSR %s rejected by admission plugin %s for attempting to use signer %s with system:masters group",
    86  				csr.Name, PluginName, certificatesv1beta1.KubeAPIServerClientSignerName)
    87  			return admission.NewForbidden(a, fmt.Errorf("use of %s signer with system:masters group is not allowed",
    88  				certificatesv1beta1.KubeAPIServerClientSignerName))
    89  		}
    90  	}
    91  
    92  	return nil
    93  }
    94
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