How to access dataset of an identity in cognito federated identities from lambda

Let me start with the overall description of what I'm trying to achieve. I'm building a serverless API using Lambda, Cognito (Federated Identities), API Gateway etc. I'm using aws_iam as the authorizer in API Gateway. In some endpoints, I need to access for example user e-mail or username or whatever so I can send it back in the response (also data of users who did not make the request). I guess I'm looking for some kind of "admin" access to the identity pool so I can retrieve data based on cognitoIdentityId.

Now in my case, this data is stored in a dataset in Cognito. The question is, how can I access this data from my Lambda function (node.js)? Is this a good approach at all? Should I use something else instead of datasets? Is there a working example somewhere?

I will be happy to provide more details if necessary.

Thanks

EDIT #1:

here is the code of my lambda function:

module.exports.getDataSet = (event, context, callback) => {
    console.log("event: " + JSON.stringify(event));

    AWS.config.credentials = new AWS.CognitoIdentityCredentials({
        IdentityPoolId: IDENTITY_POOL_ID
    });

    try {
        AWS.config.credentials.get(function() {
            var client = new AWS.CognitoSync();

            var params = {
                DatasetName: 'userinfo',
                IdentityId: event.requestContext.identity.cognitoIdentityId,
                IdentityPoolId: IDENTITY_POOL_ID
            };
            client.listRecords(params, function (err, data) {
                if (err) {
                    console.log(JSON.stringify(err));
                } else {
                    console.log(data);    
                }
            });
        });
    } catch (ex) {
        callback(ex);
    }
};

and this is what i get in err when calling listRecords:

{ "message": "Missing credentials in config", "code": "CredentialsError", "time": "2017-05-26T08:42:39.298Z", "requestId": "46712a9b-41ef-11e7-9e3c-074afafb3349", "statusCode": 400, "retryable": false, "retryDelay": 21.688148977111666, "originalError": { "message": "Could not load credentials from CognitoIdentityCredentials", "code": "CredentialsError", "time": "2017-05-26T08:42:39.298Z", "requestId": "46712a9b-41ef-11e7-9e3c-074afafb3349", "statusCode": 400, "retryable": false, "retryDelay": 21.688148977111666, "originalError": { "message": "Unauthenticated access is not supported for this identity pool.", "code": "NotAuthorizedException", "time": "2017-05-26T08:42:39.298Z", "requestId": "46712a9b-41ef-11e7-9e3c-074afafb3349", "statusCode": 400, "retryable": false, "retryDelay": 21.688148977111666 } } }

EDIT #2:

solved by removing

AWS.config.credentials = new AWS.CognitoIdentityCredentials({
    IdentityPoolId: IDENTITY_POOL_ID
});

from the code and adding the AmazonCognitoReadOnly policy to the role that invokes the lambda.