bucket-policy-parser.go

/*
 * Minio Cloud Storage, (C) 2015, 2016 Minio, 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.
 */

// This file implements AWS Access Policy Language parser in
// accordance with http://docs.aws.amazon.com/AmazonS3/latest/dev/access-policy-language-overview.html
package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"regexp"
	"sort"
	"strings"
)

const (
	// AWSResourcePrefix - bucket policy resource prefix.
	AWSResourcePrefix = "arn:aws:s3:::"
)

// supportedActionMap - lists all the actions supported by minio.
var supportedActionMap = map[string]struct{}{
	"s3:GetObject":                  {},
	"s3:ListBucket":                 {},
	"s3:PutObject":                  {},
	"s3:GetBucketLocation":          {},
	"s3:DeleteObject":               {},
	"s3:AbortMultipartUpload":       {},
	"s3:ListBucketMultipartUploads": {},
	"s3:ListMultipartUploadParts":   {},
}

// supported Conditions type.
var supportedConditionsType = map[string]struct{}{
	"StringEquals":    {},
	"StringNotEquals": {},
}

// Validate s3:prefix, s3:max-keys are present if not
// supported keys for the conditions.
var supportedConditionsKey = map[string]struct{}{
	"s3:prefix":   {},
	"s3:max-keys": {},
}

// User - canonical users list.
type policyUser struct {
	AWS []string
}

// Statement - minio policy statement
type policyStatement struct {
	Sid        string
	Effect     string
	Principal  policyUser                   `json:"Principal"`
	Actions    []string                     `json:"Action"`
	Resources  []string                     `json:"Resource"`
	Conditions map[string]map[string]string `json:"Condition"`
}

// BucketPolicy - minio policy collection
type BucketPolicy struct {
	Version    string            // date in 0000-00-00 format
	Statements []policyStatement `json:"Statement"`
}

// supportedEffectMap - supported effects.
var supportedEffectMap = map[string]struct{}{
	"Allow": {},
	"Deny":  {},
}

// isValidActions - are actions valid.
func isValidActions(actions []string) (err error) {
	// Statement actions cannot be empty.
	if len(actions) == 0 {
		err = errors.New("Action list cannot be empty.")
		return err
	}
	for _, action := range actions {
		if _, ok := supportedActionMap[action]; !ok {
			err = errors.New("Unsupported action found: ‘" + action + "’, please validate your policy document.")
			return err
		}
	}
	return nil
}

// isValidEffect - is effect valid.
func isValidEffect(effect string) error {
	// Statement effect cannot be empty.
	if len(effect) == 0 {
		err := errors.New("Policy effect cannot be empty.")
		return err
	}
	_, ok := supportedEffectMap[effect]
	if !ok {
		err := errors.New("Unsupported Effect found: ‘" + effect + "’, please validate your policy document.")
		return err
	}
	return nil
}

// isValidResources - are valid resources.
func isValidResources(resources []string) (err error) {
	// Statement resources cannot be empty.
	if len(resources) == 0 {
		err = errors.New("Resource list cannot be empty.")
		return err
	}
	for _, resource := range resources {
		if !strings.HasPrefix(resource, AWSResourcePrefix) {
			err = errors.New("Unsupported resource style found: ‘" + resource + "’, please validate your policy document.")
			return err
		}
		resourceSuffix := strings.SplitAfter(resource, AWSResourcePrefix)[1]
		if len(resourceSuffix) == 0 || strings.HasPrefix(resourceSuffix, "/") {
			err = errors.New("Invalid resource style found: ‘" + resource + "’, please validate your policy document.")
			return err
		}
	}
	return nil
}

// isValidPrincipals - are valid principals.
func isValidPrincipals(principals []string) (err error) {
	// Statement principal should have a value.
	if len(principals) == 0 {
		err = errors.New("Principal cannot be empty.")
		return err
	}
	for _, principal := range principals {
		// Minio does not support or implement IAM, "*" is the only valid value.
		// Amazon s3 doc on principals: http://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements.html#Principal
		if principal != "*" {
			err = fmt.Errorf("Unsupported principal style found: ‘%s’, please validate your policy document.", principal)
			return err
		}
	}
	return nil
}

// isValidConditions - are valid conditions.
func isValidConditions(conditions map[string]map[string]string) (err error) {
	// Returns true if string 'a' is found in the list.
	findString := func(a string, list []string) bool {
		for _, b := range list {
			if b == a {
				return true
			}
		}
		return false
	}
	conditionKeyVal := make(map[string][]string)
	// Verify conditions should be valid.
	// Validate if stringEquals, stringNotEquals are present
	// if not throw an error.
	for conditionType := range conditions {
		_, validType := supportedConditionsType[conditionType]
		if !validType {
			err = fmt.Errorf("Unsupported condition type '%s', please validate your policy document.", conditionType)
			return err
		}
		for key := range conditions[conditionType] {
			_, validKey := supportedConditionsKey[key]
			if !validKey {
				err = fmt.Errorf("Unsupported condition key '%s', please validate your policy document.", conditionType)
				return err
			}
			conditionArray, ok := conditionKeyVal[key]
			if ok && findString(conditions[conditionType][key], conditionArray) {
				err = fmt.Errorf("Ambigious condition values for key '%s', please validate your policy document.", key)
				return err
			}
			conditionKeyVal[key] = append(conditionKeyVal[key], conditions[conditionType][key])
		}
	}
	return nil
}

// List of actions for which prefixes are not allowed.
var invalidPrefixActions = map[string]struct{}{
	"s3:GetBucketLocation":          {},
	"s3:ListBucket":                 {},
	"s3:ListBucketMultipartUploads": {},
	// Add actions which do not honor prefixes.
}

// checkBucketPolicyResources validates Resources in unmarshalled bucket policy structure.
// First valation of Resources done for given set of Actions.
// Later its validated for recursive Resources.
func checkBucketPolicyResources(bucket string, bucketPolicy BucketPolicy) APIErrorCode {
	// Validate statements for special actions and collect resources
	// for others to validate nesting.
	var resourceMap = make(map[string]struct{})
	for _, statement := range bucketPolicy.Statements {
		for _, action := range statement.Actions {
			for _, resource := range statement.Resources {
				resourcePrefix := strings.SplitAfter(resource, AWSResourcePrefix)[1]
				if _, ok := invalidPrefixActions[action]; ok {
					// Resource prefix is not equal to bucket for
					// prefix invalid actions, reject them.
					if resourcePrefix != bucket {
						return ErrMalformedPolicy
					}
				} else {
					// For all other actions validate if resourcePrefix begins
					// with bucket name, if not reject them.
					if strings.Split(resourcePrefix, "/")[0] != bucket {
						return ErrMalformedPolicy
					}
					// All valid resources collect them separately to verify nesting.
					resourceMap[resourcePrefix] = struct{}{}
				}
			}
		}
	}

	var resources []string
	for resource := range resourceMap {
		resources = append(resources, resource)
	}

	// Sort strings as shorter first.
	sort.Strings(resources)

	for len(resources) > 1 {
		var resource string
		resource, resources = resources[0], resources[1:]
		resourceRegex := regexp.MustCompile(resource)
		// Loop through all resources, if one of them matches with
		// previous shorter one, it means we have detected
		// nesting. Reject such rules.
		for _, otherResource := range resources {
			if resourceRegex.MatchString(otherResource) {
				return ErrMalformedPolicy
			}
		}
	}

	// No errors found.
	return ErrNone
}

// parseBucketPolicy - parses and validates if bucket policy is of
// proper JSON and follows allowed restrictions with policy standards.
func parseBucketPolicy(bucketPolicyBuf []byte) (policy BucketPolicy, err error) {
	if err = json.Unmarshal(bucketPolicyBuf, &policy); err != nil {
		return BucketPolicy{}, err
	}

	// Policy version cannot be empty.
	if len(policy.Version) == 0 {
		err = errors.New("Policy version cannot be empty.")
		return BucketPolicy{}, err
	}

	// Policy statements cannot be empty.
	if len(policy.Statements) == 0 {
		err = errors.New("Policy statement cannot be empty.")
		return BucketPolicy{}, err
	}

	// Loop through all policy statements and validate entries.
	for _, statement := range policy.Statements {
		// Statement effect should be valid.
		if err := isValidEffect(statement.Effect); err != nil {
			return BucketPolicy{}, err
		}
		// Statement principal should be supported format.
		if err := isValidPrincipals(statement.Principal.AWS); err != nil {
			return BucketPolicy{}, err
		}
		// Statement actions should be valid.
		if err := isValidActions(statement.Actions); err != nil {
			return BucketPolicy{}, err
		}
		// Statement resources should be valid.
		if err := isValidResources(statement.Resources); err != nil {
			return BucketPolicy{}, err
		}
		// Statement conditions should be valid.
		if err := isValidConditions(statement.Conditions); err != nil {
			return BucketPolicy{}, err
		}
	}

	// Separate deny and allow statements, so that we can apply deny
	// statements in the beginning followed by Allow statements.
	var denyStatements []policyStatement
	var allowStatements []policyStatement
	for _, statement := range policy.Statements {
		if statement.Effect == "Deny" {
			denyStatements = append(denyStatements, statement)
			continue
		}
		// else if statement.Effect == "Allow"
		allowStatements = append(allowStatements, statement)
	}

	// Deny statements are enforced first once matched.
	policy.Statements = append(denyStatements, allowStatements...)

	// Return successfully parsed policy structure.
	return policy, nil
}