GitLab Delegated Joining (#22705)

* Add type for GitLab ProvisionToken

* Add default behaviour for domain

* Add IDTokenClaims for GitLab

* Add gitlab token source and token validator

* Thread GitLab support through auth and tbot packages

* Adjust cluster name fetching in token validator

* Initialize GitLab token validator in auth

* Improve comment on `sub`

* Working GitLab CI delegated joining

* Add additional token rule fields

* Add checking for new configuration fields

* add additional test cases for validation of gitlab config struct

* Add TestAuth_RegisterUsingToken_GitLab

* Add tests for IDTokenSource

* Fix imports

* Add tests for GitLab Token Validator

* Fix some comments that were incomplete

* Add license headers

lib/auth/join_gitlab.go

@@ -0,0 +1,95 @@
+/*
+Copyright 2022 Gravitational, Inc.
+
+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 auth
+
+import (
+	"context"
+
+	"github.com/gravitational/trace"
+	"github.com/sirupsen/logrus"
+
+	"github.com/gravitational/teleport/api/types"
+	"github.com/gravitational/teleport/lib/gitlab"
+)
+
+type gitlabIDTokenValidator interface {
+	Validate(
+		ctx context.Context, domain string, token string,
+	) (*gitlab.IDTokenClaims, error)
+}
+
+func (a *Server) checkGitLabJoinRequest(ctx context.Context, req *types.RegisterUsingTokenRequest) (*gitlab.IDTokenClaims, error) {
+	if req.IDToken == "" {
+		return nil, trace.BadParameter("IDToken not provided for gitlab join request")
+	}
+	pt, err := a.GetToken(ctx, req.Token)
+	if err != nil {
+		return nil, trace.Wrap(err)
+	}
+
+	token, ok := pt.(*types.ProvisionTokenV2)
+	if !ok {
+		return nil, trace.BadParameter("gitlab join method only supports ProvisionTokenV2, '%T' was provided", pt)
+	}
+
+	claims, err := a.gitlabIDTokenValidator.Validate(
+		ctx, token.Spec.GitLab.Domain, req.IDToken,
+	)
+	if err != nil {
+		return nil, trace.Wrap(err)
+	}
+
+	log.WithFields(logrus.Fields{
+		"claims": claims,
+		"token":  pt.GetName(),
+	}).Info("GitLab CI run trying to join cluster")
+
+	return claims, trace.Wrap(checkGitLabAllowRules(token, claims))
+}
+
+func checkGitLabAllowRules(token *types.ProvisionTokenV2, claims *gitlab.IDTokenClaims) error {
+	// If a single rule passes, accept the IDToken
+	for _, rule := range token.Spec.GitLab.Allow {
+		// Please consider keeping these field validators in the same order they
+		// are defined within the ProvisionTokenSpecV2GitLab proto spec.
+		if rule.Sub != "" && claims.Sub != rule.Sub {
+			continue
+		}
+		if rule.Ref != "" && claims.Ref != rule.Ref {
+			continue
+		}
+		if rule.RefType != "" && claims.RefType != rule.RefType {
+			continue
+		}
+		if rule.NamespacePath != "" && claims.NamespacePath != rule.NamespacePath {
+			continue
+		}
+		if rule.ProjectPath != "" && claims.ProjectPath != rule.ProjectPath {
+			continue
+		}
+		if rule.PipelineSource != "" && claims.PipelineSource != rule.PipelineSource {
+			continue
+		}
+		if rule.Environment != "" && claims.Environment != rule.Environment {
+			continue
+		}
+		// All provided rules met.
+		return nil
+	}
+
+	return trace.AccessDenied("id token claims did not match any allow rules")
+}