validation.go

/*
Copyright 2023 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 validation

import (
	"errors"
	"fmt"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"time"

	celgo "github.com/google/cel-go/cel"
	"github.com/google/cel-go/common/operators"
	exprpb "google.golang.org/genproto/googleapis/api/expr/v1alpha1"

	v1 "k8s.io/api/authorization/v1"
	"k8s.io/api/authorization/v1beta1"
	"k8s.io/apimachinery/pkg/util/sets"
	utilvalidation "k8s.io/apimachinery/pkg/util/validation"
	"k8s.io/apimachinery/pkg/util/validation/field"
	api "k8s.io/apiserver/pkg/apis/apiserver"
	authenticationcel "k8s.io/apiserver/pkg/authentication/cel"
	authorizationcel "k8s.io/apiserver/pkg/authorization/cel"
	"k8s.io/apiserver/pkg/cel"
	"k8s.io/apiserver/pkg/cel/environment"
	"k8s.io/apiserver/pkg/features"
	utilfeature "k8s.io/apiserver/pkg/util/feature"
	"k8s.io/client-go/util/cert"
)

// ValidateAuthenticationConfiguration validates a given AuthenticationConfiguration.
func ValidateAuthenticationConfiguration(c *api.AuthenticationConfiguration, disallowedIssuers []string) field.ErrorList {
	root := field.NewPath("jwt")
	var allErrs field.ErrorList

	// We allow 0 authenticators in the authentication configuration.
	// This allows us to support scenarios where the API server is initially set up without
	// any authenticators and then authenticators are added later via dynamic config.

	if len(c.JWT) > 64 {
		allErrs = append(allErrs, field.TooMany(root, len(c.JWT), 64))
		return allErrs
	}

	seenIssuers := sets.New[string]()
	seenDiscoveryURLs := sets.New[string]()
	for i, a := range c.JWT {
		fldPath := root.Index(i)
		_, errs := validateJWTAuthenticator(a, fldPath, sets.New(disallowedIssuers...), utilfeature.DefaultFeatureGate.Enabled(features.StructuredAuthenticationConfiguration))
		allErrs = append(allErrs, errs...)

		if seenIssuers.Has(a.Issuer.URL) {
			allErrs = append(allErrs, field.Duplicate(fldPath.Child("issuer").Child("url"), a.Issuer.URL))
		}
		seenIssuers.Insert(a.Issuer.URL)

		if len(a.Issuer.DiscoveryURL) > 0 {
			if seenDiscoveryURLs.Has(a.Issuer.DiscoveryURL) {
				allErrs = append(allErrs, field.Duplicate(fldPath.Child("issuer").Child("discoveryURL"), a.Issuer.DiscoveryURL))
			}
			seenDiscoveryURLs.Insert(a.Issuer.DiscoveryURL)
		}
	}

	return allErrs
}

// CompileAndValidateJWTAuthenticator validates a given JWTAuthenticator and returns a CELMapper with the compiled
// CEL expressions for claim mappings and validation rules.
// This is exported for use in oidc package.
func CompileAndValidateJWTAuthenticator(authenticator api.JWTAuthenticator, disallowedIssuers []string) (authenticationcel.CELMapper, field.ErrorList) {
	return validateJWTAuthenticator(authenticator, nil, sets.New(disallowedIssuers...), utilfeature.DefaultFeatureGate.Enabled(features.StructuredAuthenticationConfiguration))
}

func validateJWTAuthenticator(authenticator api.JWTAuthenticator, fldPath *field.Path, disallowedIssuers sets.Set[string], structuredAuthnFeatureEnabled bool) (authenticationcel.CELMapper, field.ErrorList) {
	var allErrs field.ErrorList

	compiler := authenticationcel.NewCompiler(environment.MustBaseEnvSet(environment.DefaultCompatibilityVersion()))
	state := &validationState{}

	allErrs = append(allErrs, validateIssuer(authenticator.Issuer, disallowedIssuers, fldPath.Child("issuer"))...)
	allErrs = append(allErrs, validateClaimValidationRules(compiler, state, authenticator.ClaimValidationRules, fldPath.Child("claimValidationRules"), structuredAuthnFeatureEnabled)...)
	allErrs = append(allErrs, validateClaimMappings(compiler, state, authenticator.ClaimMappings, fldPath.Child("claimMappings"), structuredAuthnFeatureEnabled)...)
	allErrs = append(allErrs, validateUserValidationRules(compiler, state, authenticator.UserValidationRules, fldPath.Child("userValidationRules"), structuredAuthnFeatureEnabled)...)

	return state.mapper, allErrs
}

type validationState struct {
	mapper                 authenticationcel.CELMapper
	usesEmailClaim         bool
	usesEmailVerifiedClaim bool
}

func validateIssuer(issuer api.Issuer, disallowedIssuers sets.Set[string], fldPath *field.Path) field.ErrorList {
	var allErrs field.ErrorList

	allErrs = append(allErrs, validateIssuerURL(issuer.URL, disallowedIssuers, fldPath.Child("url"))...)
	allErrs = append(allErrs, validateIssuerDiscoveryURL(issuer.URL, issuer.DiscoveryURL, fldPath.Child("discoveryURL"))...)
	allErrs = append(allErrs, validateAudiences(issuer.Audiences, issuer.AudienceMatchPolicy, fldPath.Child("audiences"), fldPath.Child("audienceMatchPolicy"))...)
	allErrs = append(allErrs, validateCertificateAuthority(issuer.CertificateAuthority, fldPath.Child("certificateAuthority"))...)

	return allErrs
}

func validateIssuerURL(issuerURL string, disallowedIssuers sets.Set[string], fldPath *field.Path) field.ErrorList {
	if len(issuerURL) == 0 {
		return field.ErrorList{field.Required(fldPath, "URL is required")}
	}

	return validateURL(issuerURL, disallowedIssuers, fldPath)
}

func validateIssuerDiscoveryURL(issuerURL, issuerDiscoveryURL string, fldPath *field.Path) field.ErrorList {
	var allErrs field.ErrorList

	if len(issuerDiscoveryURL) == 0 {
		return nil
	}

	if len(issuerURL) > 0 && strings.TrimRight(issuerURL, "/") == strings.TrimRight(issuerDiscoveryURL, "/") {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerDiscoveryURL, "discoveryURL must be different from URL"))
	}

	// issuerDiscoveryURL is not an issuer URL and does not need to validated against any set of disallowed issuers
	allErrs = append(allErrs, validateURL(issuerDiscoveryURL, nil, fldPath)...)
	return allErrs
}

func validateURL(issuerURL string, disallowedIssuers sets.Set[string], fldPath *field.Path) field.ErrorList {
	var allErrs field.ErrorList

	if disallowedIssuers.Has(issuerURL) {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerURL, fmt.Sprintf("URL must not overlap with disallowed issuers: %s", sets.List(disallowedIssuers))))
	}

	u, err := url.Parse(issuerURL)
	if err != nil {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerURL, err.Error()))
		return allErrs
	}
	if u.Scheme != "https" {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerURL, "URL scheme must be https"))
	}
	if u.User != nil {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerURL, "URL must not contain a username or password"))
	}
	if len(u.RawQuery) > 0 {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerURL, "URL must not contain a query"))
	}
	if len(u.Fragment) > 0 {
		allErrs = append(allErrs, field.Invalid(fldPath, issuerURL, "URL must not contain a fragment"))
	}

	return allErrs
}

func validateAudiences(audiences []string, audienceMatchPolicy api.AudienceMatchPolicyType, fldPath, audienceMatchPolicyFldPath *field.Path) field.ErrorList {
	var allErrs field.ErrorList

	if len(audiences) == 0 {
		allErrs = append(allErrs, field.Required(fldPath, fmt.Sprintf(atLeastOneRequiredErrFmt, fldPath)))
		return allErrs
	}

	seenAudiences := sets.NewString()
	for i, audience := range audiences {
		fldPath := fldPath.Index(i)
		if len(audience) == 0 {
			allErrs = append(allErrs, field.Required(fldPath, "audience can't be empty"))
		}
		if seenAudiences.Has(audience) {
			allErrs = append(allErrs, field.Duplicate(fldPath, audience))
		}
		seenAudiences.Insert(audience)
	}

	if len(audiences) > 1 && audienceMatchPolicy != api.AudienceMatchPolicyMatchAny {
		allErrs = append(allErrs, field.Invalid(audienceMatchPolicyFldPath, audienceMatchPolicy, "audienceMatchPolicy must be MatchAny for multiple audiences"))
	}
	if len(audiences) == 1 && (len(audienceMatchPolicy) > 0 && audienceMatchPolicy != api.AudienceMatchPolicyMatchAny) {
		allErrs = append(allErrs, field.Invalid(audienceMatchPolicyFldPath, audienceMatchPolicy, "audienceMatchPolicy must be empty or MatchAny for single audience"))
	}

	return allErrs
}

func validateCertificateAuthority(certificateAuthority string, fldPath *field.Path) field.ErrorList {
	var allErrs field.ErrorList

	if len(certificateAuthority) == 0 {
		return allErrs
	}
	_, err := cert.NewPoolFromBytes([]byte(certificateAuthority))
	if err != nil {
		allErrs = append(allErrs, field.Invalid(fldPath, "<omitted>", err.Error()))
	}

	return allErrs
}

func validateClaimValidationRules(compiler authenticationcel.Compiler, state *validationState, rules []api.ClaimValidationRule, fldPath *field.Path, structuredAuthnFeatureEnabled bool) field.ErrorList {
	var allErrs field.ErrorList

	seenClaims := sets.NewString()
	seenExpressions := sets.NewString()
	var compilationResults []authenticationcel.CompilationResult

	for i, rule := range rules {
		fldPath := fldPath.Index(i)

		if len(rule.Expression) > 0 && !structuredAuthnFeatureEnabled {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("expression"), rule.Expression, "expression is not supported when StructuredAuthenticationConfiguration feature gate is disabled"))
		}

		switch {
		case len(rule.Claim) > 0 && len(rule.Expression) > 0:
			allErrs = append(allErrs, field.Invalid(fldPath, rule.Claim, "claim and expression can't both be set"))
		case len(rule.Claim) == 0 && len(rule.Expression) == 0:
			allErrs = append(allErrs, field.Required(fldPath, "claim or expression is required"))
		case len(rule.Claim) > 0:
			if len(rule.Message) > 0 {
				allErrs = append(allErrs, field.Invalid(fldPath.Child("message"), rule.Message, "message can't be set when claim is set"))
			}
			if seenClaims.Has(rule.Claim) {
				allErrs = append(allErrs, field.Duplicate(fldPath.Child("claim"), rule.Claim))
			}
			seenClaims.Insert(rule.Claim)
		case len(rule.Expression) > 0:
			if len(rule.RequiredValue) > 0 {
				allErrs = append(allErrs, field.Invalid(fldPath.Child("requiredValue"), rule.RequiredValue, "requiredValue can't be set when expression is set"))
			}
			if seenExpressions.Has(rule.Expression) {
				allErrs = append(allErrs, field.Duplicate(fldPath.Child("expression"), rule.Expression))
				continue
			}
			seenExpressions.Insert(rule.Expression)

			compilationResult, err := compileClaimsCELExpression(compiler, &authenticationcel.ClaimValidationCondition{
				Expression: rule.Expression,
				Message:    rule.Message,
			}, fldPath.Child("expression"))

			if err != nil {
				allErrs = append(allErrs, err)
				continue
			}
			if compilationResult != nil {
				compilationResults = append(compilationResults, *compilationResult)
			}
		}
	}

	if structuredAuthnFeatureEnabled && len(compilationResults) > 0 {
		state.mapper.ClaimValidationRules = authenticationcel.NewClaimsMapper(compilationResults)
		state.usesEmailVerifiedClaim = state.usesEmailVerifiedClaim || anyUsesEmailVerifiedClaim(compilationResults)
	}

	return allErrs
}

func validateClaimMappings(compiler authenticationcel.Compiler, state *validationState, m api.ClaimMappings, fldPath *field.Path, structuredAuthnFeatureEnabled bool) field.ErrorList {
	var allErrs field.ErrorList

	if !structuredAuthnFeatureEnabled {
		if len(m.Username.Expression) > 0 {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("username").Child("expression"), m.Username.Expression, "expression is not supported when StructuredAuthenticationConfiguration feature gate is disabled"))
		}
		if len(m.Groups.Expression) > 0 {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("groups").Child("expression"), m.Groups.Expression, "expression is not supported when StructuredAuthenticationConfiguration feature gate is disabled"))
		}
		if len(m.UID.Claim) > 0 || len(m.UID.Expression) > 0 {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("uid"), "", "uid claim mapping is not supported when StructuredAuthenticationConfiguration feature gate is disabled"))
		}
		if len(m.Extra) > 0 {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("extra"), "", "extra claim mapping is not supported when StructuredAuthenticationConfiguration feature gate is disabled"))
		}
	}

	compilationResult, err := validatePrefixClaimOrExpression(compiler, m.Username, fldPath.Child("username"), true)
	if err != nil {
		allErrs = append(allErrs, err...)
	} else if compilationResult != nil && structuredAuthnFeatureEnabled {
		state.usesEmailClaim = state.usesEmailClaim || usesEmailClaim(compilationResult.AST)
		state.usesEmailVerifiedClaim = state.usesEmailVerifiedClaim || usesEmailVerifiedClaim(compilationResult.AST)
		state.mapper.Username = authenticationcel.NewClaimsMapper([]authenticationcel.CompilationResult{*compilationResult})
	}

	compilationResult, err = validatePrefixClaimOrExpression(compiler, m.Groups, fldPath.Child("groups"), false)
	if err != nil {
		allErrs = append(allErrs, err...)
	} else if compilationResult != nil && structuredAuthnFeatureEnabled {
		state.mapper.Groups = authenticationcel.NewClaimsMapper([]authenticationcel.CompilationResult{*compilationResult})
	}

	switch {
	case len(m.UID.Claim) > 0 && len(m.UID.Expression) > 0:
		allErrs = append(allErrs, field.Invalid(fldPath.Child("uid"), "", "claim and expression can't both be set"))
	case len(m.UID.Expression) > 0:
		compilationResult, err := compileClaimsCELExpression(compiler, &authenticationcel.ClaimMappingExpression{
			Expression: m.UID.Expression,
		}, fldPath.Child("uid").Child("expression"))

		if err != nil {
			allErrs = append(allErrs, err)
		} else if structuredAuthnFeatureEnabled && compilationResult != nil {
			state.mapper.UID = authenticationcel.NewClaimsMapper([]authenticationcel.CompilationResult{*compilationResult})
		}
	}

	var extraCompilationResults []authenticationcel.CompilationResult
	seenExtraKeys := sets.NewString()

	for i, mapping := range m.Extra {
		fldPath := fldPath.Child("extra").Index(i)
		// Key should be namespaced to the authenticator or authenticator/authorizer pair making use of them.
		// For instance: "example.org/foo" instead of "foo".
		// xref: https://github.com/kubernetes/kubernetes/blob/3825e206cb162a7ad7431a5bdf6a065ae8422cf7/staging/src/k8s.io/apiserver/pkg/authentication/user/user.go#L31-L41
		// IsDomainPrefixedPath checks for non-empty key and that the key is prefixed with a domain name.
		allErrs = append(allErrs, utilvalidation.IsDomainPrefixedPath(fldPath.Child("key"), mapping.Key)...)
		if mapping.Key != strings.ToLower(mapping.Key) {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("key"), mapping.Key, "key must be lowercase"))
		}
		if seenExtraKeys.Has(mapping.Key) {
			allErrs = append(allErrs, field.Duplicate(fldPath.Child("key"), mapping.Key))
			continue
		}
		seenExtraKeys.Insert(mapping.Key)

		if len(mapping.ValueExpression) == 0 {
			allErrs = append(allErrs, field.Required(fldPath.Child("valueExpression"), "valueExpression is required"))
			continue
		}

		compilationResult, err := compileClaimsCELExpression(compiler, &authenticationcel.ExtraMappingExpression{
			Key:        mapping.Key,
			Expression: mapping.ValueExpression,
		}, fldPath.Child("valueExpression"))

		if err != nil {
			allErrs = append(allErrs, err)
			continue
		}

		if compilationResult != nil {
			extraCompilationResults = append(extraCompilationResults, *compilationResult)
		}
	}

	if structuredAuthnFeatureEnabled && len(extraCompilationResults) > 0 {
		state.mapper.Extra = authenticationcel.NewClaimsMapper(extraCompilationResults)
		state.usesEmailVerifiedClaim = state.usesEmailVerifiedClaim || anyUsesEmailVerifiedClaim(extraCompilationResults)
	}

	if structuredAuthnFeatureEnabled && state.usesEmailClaim && !state.usesEmailVerifiedClaim {
		allErrs = append(allErrs, field.Invalid(fldPath.Child("username", "expression"), m.Username.Expression,
			"claims.email_verified must be used in claimMappings.username.expression or claimMappings.extra[*].valueExpression or claimValidationRules[*].expression when claims.email is used in claimMappings.username.expression"))
	}

	return allErrs
}

func usesEmailClaim(ast *celgo.Ast) bool {
	return hasSelectExp(ast.Expr(), "claims", "email")
}

func anyUsesEmailVerifiedClaim(results []authenticationcel.CompilationResult) bool {
	for _, result := range results {
		if usesEmailVerifiedClaim(result.AST) {
			return true
		}
	}
	return false
}

func usesEmailVerifiedClaim(ast *celgo.Ast) bool {
	return hasSelectExp(ast.Expr(), "claims", "email_verified")
}

func hasSelectExp(exp *exprpb.Expr, operand, field string) bool {
	if exp == nil {
		return false
	}
	switch e := exp.ExprKind.(type) {
	case *exprpb.Expr_ConstExpr,
		*exprpb.Expr_IdentExpr:
		return false
	case *exprpb.Expr_SelectExpr:
		s := e.SelectExpr
		if s == nil {
			return false
		}
		if isIdentOperand(s.Operand, operand) && s.Field == field {
			return true
		}
		return hasSelectExp(s.Operand, operand, field)
	case *exprpb.Expr_CallExpr:
		c := e.CallExpr
		if c == nil {
			return false
		}
		if c.Target == nil && c.Function == operators.OptSelect && len(c.Args) == 2 &&
			isIdentOperand(c.Args[0], operand) && isConstField(c.Args[1], field) {
			return true
		}
		for _, arg := range c.Args {
			if hasSelectExp(arg, operand, field) {
				return true
			}
		}
		return hasSelectExp(c.Target, operand, field)
	case *exprpb.Expr_ListExpr:
		l := e.ListExpr
		if l == nil {
			return false
		}
		for _, element := range l.Elements {
			if hasSelectExp(element, operand, field) {
				return true
			}
		}
		return false
	case *exprpb.Expr_StructExpr:
		s := e.StructExpr
		if s == nil {
			return false
		}
		for _, entry := range s.Entries {
			if hasSelectExp(entry.GetMapKey(), operand, field) {
				return true
			}
			if hasSelectExp(entry.Value, operand, field) {
				return true
			}
		}
		return false
	case *exprpb.Expr_ComprehensionExpr:
		c := e.ComprehensionExpr
		if c == nil {
			return false
		}
		return hasSelectExp(c.IterRange, operand, field) ||
			hasSelectExp(c.AccuInit, operand, field) ||
			hasSelectExp(c.LoopCondition, operand, field) ||
			hasSelectExp(c.LoopStep, operand, field) ||
			hasSelectExp(c.Result, operand, field)
	default:
		return false
	}
}

func isIdentOperand(exp *exprpb.Expr, operand string) bool {
	if len(operand) == 0 {
		return false // sanity check against default values
	}
	id := exp.GetIdentExpr() // does not panic even if exp is nil
	return id != nil && id.Name == operand
}

func isConstField(exp *exprpb.Expr, field string) bool {
	if len(field) == 0 {
		return false // sanity check against default values
	}
	c := exp.GetConstExpr()                        // does not panic even if exp is nil
	return c != nil && c.GetStringValue() == field // does not panic even if c is not a string
}

func validatePrefixClaimOrExpression(compiler authenticationcel.Compiler, mapping api.PrefixedClaimOrExpression, fldPath *field.Path, claimOrExpressionRequired bool) (*authenticationcel.CompilationResult, field.ErrorList) {
	var allErrs field.ErrorList

	var compilationResult *authenticationcel.CompilationResult
	switch {
	case len(mapping.Expression) > 0 && len(mapping.Claim) > 0:
		allErrs = append(allErrs, field.Invalid(fldPath, "", "claim and expression can't both be set"))
	case len(mapping.Expression) == 0 && len(mapping.Claim) == 0 && claimOrExpressionRequired:
		allErrs = append(allErrs, field.Required(fldPath, "claim or expression is required"))
	case len(mapping.Expression) > 0:
		var err *field.Error

		if mapping.Prefix != nil {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("prefix"), *mapping.Prefix, "prefix can't be set when expression is set"))
		}
		compilationResult, err = compileClaimsCELExpression(compiler, &authenticationcel.ClaimMappingExpression{
			Expression: mapping.Expression,
		}, fldPath.Child("expression"))

		if err != nil {
			allErrs = append(allErrs, err)
		}

	case len(mapping.Claim) > 0:
		if mapping.Prefix == nil {
			allErrs = append(allErrs, field.Required(fldPath.Child("prefix"), "prefix is required when claim is set. It can be set to an empty string to disable prefixing"))
		}
	}

	return compilationResult, allErrs
}

func validateUserValidationRules(compiler authenticationcel.Compiler, state *validationState, rules []api.UserValidationRule, fldPath *field.Path, structuredAuthnFeatureEnabled bool) field.ErrorList {
	var allErrs field.ErrorList
	var compilationResults []authenticationcel.CompilationResult

	if len(rules) > 0 && !structuredAuthnFeatureEnabled {
		allErrs = append(allErrs, field.Invalid(fldPath, "", "user validation rules are not supported when StructuredAuthenticationConfiguration feature gate is disabled"))
	}

	seenExpressions := sets.NewString()
	for i, rule := range rules {
		fldPath := fldPath.Index(i)

		if len(rule.Expression) == 0 {
			allErrs = append(allErrs, field.Required(fldPath.Child("expression"), "expression is required"))
			continue
		}

		if seenExpressions.Has(rule.Expression) {
			allErrs = append(allErrs, field.Duplicate(fldPath.Child("expression"), rule.Expression))
			continue
		}
		seenExpressions.Insert(rule.Expression)

		compilationResult, err := compileUserCELExpression(compiler, &authenticationcel.UserValidationCondition{
			Expression: rule.Expression,
			Message:    rule.Message,
		}, fldPath.Child("expression"))

		if err != nil {
			allErrs = append(allErrs, err)
			continue
		}

		if compilationResult != nil {
			compilationResults = append(compilationResults, *compilationResult)
		}
	}

	if structuredAuthnFeatureEnabled && len(compilationResults) > 0 {
		state.mapper.UserValidationRules = authenticationcel.NewUserMapper(compilationResults)
	}

	return allErrs
}

func compileClaimsCELExpression(compiler authenticationcel.Compiler, expression authenticationcel.ExpressionAccessor, fldPath *field.Path) (*authenticationcel.CompilationResult, *field.Error) {
	compilationResult, err := compiler.CompileClaimsExpression(expression)
	if err != nil {
		return nil, convertCELErrorToValidationError(fldPath, expression.GetExpression(), err)
	}
	return &compilationResult, nil
}

func compileUserCELExpression(compiler authenticationcel.Compiler, expression authenticationcel.ExpressionAccessor, fldPath *field.Path) (*authenticationcel.CompilationResult, *field.Error) {
	compilationResult, err := compiler.CompileUserExpression(expression)
	if err != nil {
		return nil, convertCELErrorToValidationError(fldPath, expression.GetExpression(), err)
	}
	return &compilationResult, nil
}

// ValidateAuthorizationConfiguration validates a given AuthorizationConfiguration.
func ValidateAuthorizationConfiguration(fldPath *field.Path, c *api.AuthorizationConfiguration, knownTypes sets.String, repeatableTypes sets.String) field.ErrorList {
	allErrs := field.ErrorList{}

	if len(c.Authorizers) == 0 {
		allErrs = append(allErrs, field.Required(fldPath.Child("authorizers"), "at least one authorization mode must be defined"))
	}

	seenAuthorizerTypes := sets.NewString()
	seenAuthorizerNames := sets.NewString()
	for i, a := range c.Authorizers {
		fldPath := fldPath.Child("authorizers").Index(i)
		aType := string(a.Type)
		if aType == "" {
			allErrs = append(allErrs, field.Required(fldPath.Child("type"), ""))
			continue
		}
		if !knownTypes.Has(aType) {
			allErrs = append(allErrs, field.NotSupported(fldPath.Child("type"), aType, knownTypes.List()))
			continue
		}
		if seenAuthorizerTypes.Has(aType) && !repeatableTypes.Has(aType) {
			allErrs = append(allErrs, field.Duplicate(fldPath.Child("type"), aType))
			continue
		}
		seenAuthorizerTypes.Insert(aType)

		if len(a.Name) == 0 {
			allErrs = append(allErrs, field.Required(fldPath.Child("name"), ""))
		} else if seenAuthorizerNames.Has(a.Name) {
			allErrs = append(allErrs, field.Duplicate(fldPath.Child("name"), a.Name))
		} else if errs := utilvalidation.IsDNS1123Subdomain(a.Name); len(errs) != 0 {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("name"), a.Name, fmt.Sprintf("authorizer name is invalid: %s", strings.Join(errs, ", "))))
		}
		seenAuthorizerNames.Insert(a.Name)

		switch a.Type {
		case api.TypeWebhook:
			if a.Webhook == nil {
				allErrs = append(allErrs, field.Required(fldPath.Child("webhook"), "required when type=Webhook"))
				continue
			}
			allErrs = append(allErrs, ValidateWebhookConfiguration(fldPath, a.Webhook)...)
		default:
			if a.Webhook != nil {
				allErrs = append(allErrs, field.Invalid(fldPath.Child("webhook"), "non-null", "may only be specified when type=Webhook"))
			}
		}
	}

	return allErrs
}

func ValidateWebhookConfiguration(fldPath *field.Path, c *api.WebhookConfiguration) field.ErrorList {
	allErrs := field.ErrorList{}

	if c.Timeout.Duration == 0 {
		allErrs = append(allErrs, field.Required(fldPath.Child("timeout"), ""))
	} else if c.Timeout.Duration > 30*time.Second || c.Timeout.Duration < 0 {
		allErrs = append(allErrs, field.Invalid(fldPath.Child("timeout"), c.Timeout.Duration.String(), "must be > 0s and <= 30s"))
	}

	if c.AuthorizedTTL.Duration == 0 {
		allErrs = append(allErrs, field.Required(fldPath.Child("authorizedTTL"), ""))
	} else if c.AuthorizedTTL.Duration < 0 {
		allErrs = append(allErrs, field.Invalid(fldPath.Child("authorizedTTL"), c.AuthorizedTTL.Duration.String(), "must be > 0s"))
	}

	if c.UnauthorizedTTL.Duration == 0 {
		allErrs = append(allErrs, field.Required(fldPath.Child("unauthorizedTTL"), ""))
	} else if c.UnauthorizedTTL.Duration < 0 {
		allErrs = append(allErrs, field.Invalid(fldPath.Child("unauthorizedTTL"), c.UnauthorizedTTL.Duration.String(), "must be > 0s"))
	}

	switch c.SubjectAccessReviewVersion {
	case "":
		allErrs = append(allErrs, field.Required(fldPath.Child("subjectAccessReviewVersion"), ""))
	case "v1":
		_ = &v1.SubjectAccessReview{}
	case "v1beta1":
		_ = &v1beta1.SubjectAccessReview{}
	default:
		allErrs = append(allErrs, field.NotSupported(fldPath.Child("subjectAccessReviewVersion"), c.SubjectAccessReviewVersion, []string{"v1", "v1beta1"}))
	}

	switch c.MatchConditionSubjectAccessReviewVersion {
	case "":
		if len(c.MatchConditions) > 0 {
			allErrs = append(allErrs, field.Required(fldPath.Child("matchConditionSubjectAccessReviewVersion"), "required if match conditions are specified"))
		}
	case "v1":
		_ = &v1.SubjectAccessReview{}
	default:
		allErrs = append(allErrs, field.NotSupported(fldPath.Child("matchConditionSubjectAccessReviewVersion"), c.MatchConditionSubjectAccessReviewVersion, []string{"v1"}))
	}

	switch c.FailurePolicy {
	case "":
		allErrs = append(allErrs, field.Required(fldPath.Child("failurePolicy"), ""))
	case api.FailurePolicyNoOpinion, api.FailurePolicyDeny:
	default:
		allErrs = append(allErrs, field.NotSupported(fldPath.Child("failurePolicy"), c.FailurePolicy, []string{"NoOpinion", "Deny"}))
	}

	switch c.ConnectionInfo.Type {
	case "":
		allErrs = append(allErrs, field.Required(fldPath.Child("connectionInfo", "type"), ""))
	case api.AuthorizationWebhookConnectionInfoTypeInCluster:
		if c.ConnectionInfo.KubeConfigFile != nil {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("connectionInfo", "kubeConfigFile"), *c.ConnectionInfo.KubeConfigFile, "can only be set when type=KubeConfigFile"))
		}
	case api.AuthorizationWebhookConnectionInfoTypeKubeConfigFile:
		if c.ConnectionInfo.KubeConfigFile == nil || *c.ConnectionInfo.KubeConfigFile == "" {
			allErrs = append(allErrs, field.Required(fldPath.Child("connectionInfo", "kubeConfigFile"), ""))
		} else if !filepath.IsAbs(*c.ConnectionInfo.KubeConfigFile) {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("connectionInfo", "kubeConfigFile"), *c.ConnectionInfo.KubeConfigFile, "must be an absolute path"))
		} else if info, err := os.Stat(*c.ConnectionInfo.KubeConfigFile); err != nil {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("connectionInfo", "kubeConfigFile"), *c.ConnectionInfo.KubeConfigFile, fmt.Sprintf("error loading file: %v", err)))
		} else if !info.Mode().IsRegular() {
			allErrs = append(allErrs, field.Invalid(fldPath.Child("connectionInfo", "kubeConfigFile"), *c.ConnectionInfo.KubeConfigFile, "must be a regular file"))
		}
	default:
		allErrs = append(allErrs, field.NotSupported(fldPath.Child("connectionInfo", "type"), c.ConnectionInfo, []string{api.AuthorizationWebhookConnectionInfoTypeInCluster, api.AuthorizationWebhookConnectionInfoTypeKubeConfigFile}))
	}

	_, errs := compileMatchConditions(c.MatchConditions, fldPath, utilfeature.DefaultFeatureGate.Enabled(features.StructuredAuthorizationConfiguration))
	allErrs = append(allErrs, errs...)

	return allErrs
}

// ValidateAndCompileMatchConditions validates a given webhook's matchConditions.
// This is exported for use in authz package.
func ValidateAndCompileMatchConditions(matchConditions []api.WebhookMatchCondition) (*authorizationcel.CELMatcher, field.ErrorList) {
	return compileMatchConditions(matchConditions, nil, utilfeature.DefaultFeatureGate.Enabled(features.StructuredAuthorizationConfiguration))
}

func compileMatchConditions(matchConditions []api.WebhookMatchCondition, fldPath *field.Path, structuredAuthzFeatureEnabled bool) (*authorizationcel.CELMatcher, field.ErrorList) {
	var allErrs field.ErrorList
	// should fail when match conditions are used without feature enabled
	if len(matchConditions) > 0 && !structuredAuthzFeatureEnabled {
		allErrs = append(allErrs, field.Invalid(fldPath.Child("matchConditions"), "", "matchConditions are not supported when StructuredAuthorizationConfiguration feature gate is disabled"))
	}
	if len(matchConditions) > 64 {
		allErrs = append(allErrs, field.TooMany(fldPath.Child("matchConditions"), len(matchConditions), 64))
		return nil, allErrs
	}

	compiler := authorizationcel.NewCompiler(environment.MustBaseEnvSet(environment.DefaultCompatibilityVersion()))
	seenExpressions := sets.NewString()
	var compilationResults []authorizationcel.CompilationResult

	for i, condition := range matchConditions {
		fldPath := fldPath.Child("matchConditions").Index(i).Child("expression")
		if len(strings.TrimSpace(condition.Expression)) == 0 {
			allErrs = append(allErrs, field.Required(fldPath, ""))
			continue
		}
		if seenExpressions.Has(condition.Expression) {
			allErrs = append(allErrs, field.Duplicate(fldPath, condition.Expression))
			continue
		}
		seenExpressions.Insert(condition.Expression)
		compilationResult, err := compileMatchConditionsExpression(fldPath, compiler, condition.Expression)
		if err != nil {
			allErrs = append(allErrs, err)
			continue
		}
		compilationResults = append(compilationResults, compilationResult)
	}
	if len(compilationResults) == 0 {
		return nil, allErrs
	}
	return &authorizationcel.CELMatcher{
		CompilationResults: compilationResults,
	}, allErrs
}

func compileMatchConditionsExpression(fldPath *field.Path, compiler authorizationcel.Compiler, expression string) (authorizationcel.CompilationResult, *field.Error) {
	authzExpression := &authorizationcel.SubjectAccessReviewMatchCondition{
		Expression: expression,
	}
	compilationResult, err := compiler.CompileCELExpression(authzExpression)
	if err != nil {
		return compilationResult, convertCELErrorToValidationError(fldPath, authzExpression.GetExpression(), err)
	}
	return compilationResult, nil
}

func convertCELErrorToValidationError(fldPath *field.Path, expression string, err error) *field.Error {
	var celErr *cel.Error
	if errors.As(err, &celErr) {
		switch celErr.Type {
		case cel.ErrorTypeRequired:
			return field.Required(fldPath, celErr.Detail)
		case cel.ErrorTypeInvalid:
			return field.Invalid(fldPath, expression, celErr.Detail)
		default:
			return field.InternalError(fldPath, celErr)
		}
	}
	return field.InternalError(fldPath, fmt.Errorf("error is not cel error: %w", err))
}