minio/cmd/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.
 */

// Package cmd 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 cmd

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"sort"
	"strings"

	"github.com/minio/minio-go/pkg/set"
)

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

// supportedActionMap - lists all the actions supported by minio.
var supportedActionMap = set.CreateStringSet("*", "*", "s3:*", "s3:GetObject",
	"s3:ListBucket", "s3:PutObject", "s3:GetBucketLocation", "s3:DeleteObject",
	"s3:AbortMultipartUpload", "s3:ListBucketMultipartUploads", "s3:ListMultipartUploadParts")

// supported Conditions type.
var supportedConditionsType = set.CreateStringSet("StringEquals", "StringNotEquals")

// Validate s3:prefix, s3:max-keys are present if not
// supported keys for the conditions.
var supportedConditionsKey = set.CreateStringSet("s3:prefix", "s3:max-keys")

// supportedEffectMap - supported effects.
var supportedEffectMap = set.CreateStringSet("Allow", "Deny")

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

// bucketPolicy - collection of various bucket policy statements.
type bucketPolicy struct {
	Version    string            // date in YYYY-MM-DD format
	Statements []policyStatement `json:"Statement"`
}

// Stringer implementation for the bucket policies.
func (b bucketPolicy) String() string {
	bbytes, err := json.Marshal(&b)
	if err != nil {
		errorIf(err, "Unable to unmarshal bucket policy into JSON %#v", b)
		return ""
	}
	return string(bbytes)
}

// isValidActions - are actions valid.
func isValidActions(actions set.StringSet) (err error) {
	// Statement actions cannot be empty.
	if len(actions) == 0 {
		err = errors.New("Action list cannot be empty.")
		return err
	}
	if unsupportedActions := actions.Difference(supportedActionMap); !unsupportedActions.IsEmpty() {
		err = fmt.Errorf("Unsupported actions found: ‘%#v’, please validate your policy document.", unsupportedActions)
		return err
	}
	return nil
}

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

// isValidResources - are valid resources.
func isValidResources(resources set.StringSet) (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
}

// Parse principals parses a incoming json. Handles cases for
// these three combinations.
// - "Principal": "*",
// - "Principal": { "AWS" : "*" }
// - "Principal": { "AWS" : [ "*" ]}
func parsePrincipals(principal interface{}) set.StringSet {
	principals, ok := principal.(map[string]interface{})
	if !ok {
		var principalStr string
		principalStr, ok = principal.(string)
		if ok {
			return set.CreateStringSet(principalStr)
		}
	} // else {
	var principalStrs []string
	for _, p := range principals {
		principalStr, isStr := p.(string)
		if !isStr {
			principalsAdd, isInterface := p.([]interface{})
			if !isInterface {
				principalStrsAddr, isStrs := p.([]string)
				if !isStrs {
					continue
				}
				principalStrs = append(principalStrs, principalStrsAddr...)
			} else {
				for _, pa := range principalsAdd {
					var pstr string
					pstr, isStr = pa.(string)
					if !isStr {
						continue
					}
					principalStrs = append(principalStrs, pstr)
				}
			}
			continue
		} // else {
		principalStrs = append(principalStrs, principalStr)
	}
	return set.CreateStringSet(principalStrs...)
}

// isValidPrincipals - are valid principals.
func isValidPrincipals(principal interface{}) (err error) {
	principals := parsePrincipals(principal)
	// Statement principal should have a value.
	if len(principals) == 0 {
		err = errors.New("Principal cannot be empty.")
		return err
	}
	if unsuppPrincipals := principals.Difference(set.CreateStringSet([]string{"*"}...)); !unsuppPrincipals.IsEmpty() {
		// 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
		err = fmt.Errorf("Unsupported principals found: ‘%#v’, please validate your policy document.", unsuppPrincipals)
		return err
	}
	return nil
}

// isValidConditions - are valid conditions.
func isValidConditions(conditions map[string]map[string]set.StringSet) (err error) {
	// Verify conditions should be valid.
	// Validate if stringEquals, stringNotEquals are present
	// if not throw an error.
	conditionKeyVal := make(map[string]set.StringSet)
	for conditionType := range conditions {
		if !supportedConditionsType.Contains(conditionType) {
			err = fmt.Errorf("Unsupported condition type '%s', please validate your policy document.", conditionType)
			return err
		}
		for key, value := range conditions[conditionType] {
			if !supportedConditionsKey.Contains(key) {
				err = fmt.Errorf("Unsupported condition key '%s', please validate your policy document.", conditionType)
				return err
			}
			conditionVal, ok := conditionKeyVal[key]
			if ok && !value.Intersection(conditionVal).IsEmpty() {
				err = fmt.Errorf("Ambigious condition values for key '%s', please validate your policy document.", key)
				return err
			}
			conditionKeyVal[key] = value
		}
	}
	return nil
}

// List of actions for which prefixes are not allowed.
var invalidPrefixActions = set.StringSet{
	"s3:GetBucketLocation":          {},
	"s3:ListBucket":                 {},
	"s3:ListBucketMultipartUploads": {},
	// Add actions which do not honor prefixes.
}

// resourcePrefix - provides the prefix removing any wildcards.
func resourcePrefix(resource string) string {
	if strings.HasSuffix(resource, "*") {
		resource = strings.TrimSuffix(resource, "*")
	}
	return resource
}

// 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 = set.NewStringSet()
	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.Add(resourcePrefix)
				}
			}
		}
	}

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

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

	for len(resources) > 1 {
		var resource string
		resource, resources = resources[0], resources[1:]
		// 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 {
			// Common prefix reject such rules.
			if strings.HasPrefix(otherResource, resource) {
				return ErrPolicyNesting
			}
		}
	}

	// 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(bucketPolicyReader io.Reader, policy *bucketPolicy) (err error) {
	// Parse bucket policy reader.
	decoder := json.NewDecoder(bucketPolicyReader)
	if err = decoder.Decode(&policy); err != nil {
		return err
	}

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

	// Policy statements cannot be empty.
	if len(policy.Statements) == 0 {
		err = errors.New("Policy statement cannot be empty.")
		return 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 err
		}
		// Statement principal should be supported format.
		if err := isValidPrincipals(statement.Principal); err != nil {
			return err
		}
		// Statement actions should be valid.
		if err := isValidActions(statement.Actions); err != nil {
			return err
		}
		// Statement resources should be valid.
		if err := isValidResources(statement.Resources); err != nil {
			return err
		}
		// Statement conditions should be valid.
		if err := isValidConditions(statement.Conditions); err != nil {
			return 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 nil
}