styx/policy/policy.go

package policy

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"html/template"
	"io"
	"net/http"
	"os"

	"github.com/go-viper/mapstructure/v2"
	"github.com/open-policy-agent/opa/v1/rego"
	regoprint "github.com/open-policy-agent/opa/v1/topdown/print"

	"git.maze.io/maze/styx/logger"
	proxy "git.maze.io/maze/styx/proxy"
)

const DefaultPackageName = "styx"

var ErrNoResult = errors.New("policy: no result")

type Policy struct {
	name    string
	options []func(*rego.Rego)
}

func New(name, pkg string) (*Policy, error) {
	p := &Policy{
		name:    name,
		options: newRego(rego.Load([]string{name}, nil), pkg),
	}
	if _, err := p.Query(&Input{}); err != nil {
		return nil, err
	}
	return p, nil
}

func NewFromString(module, pkg string) (*Policy, error) {
	p := &Policy{
		name:    "<inline>",
		options: newRego(rego.Module("styx", module), pkg),
	}
	if _, err := p.Query(&Input{}); err != nil {
		return nil, err
	}
	return p, nil
}

func newRego(option func(*rego.Rego), pkg string) []func(*rego.Rego) {
	if pkg == "" {
		pkg = DefaultPackageName
	}
	return []func(*rego.Rego){
		rego.Dump(os.Stderr),
		rego.Query("data." + pkg),
		rego.Strict(true),
		rego.Function2(&rego.Function{
			Name:             "styx.in_domains",
			Decl:             domainContainsDecl,
			Nondeterministic: true,
		}, domainContainsImpl),
		rego.Function2(&rego.Function{
			Name:             "styx.in_networks",
			Decl:             networkContainsDecl,
			Nondeterministic: true,
		}, networkContainsImpl),
		rego.PrintHook(printHook{}),
		option,
	}
}

type printHook struct{}

func (printHook) Print(ctx regoprint.Context, message string) error {
	logger.StandardLog.Values(logger.Values{
		"where": fmt.Sprintf("%s:%d,%d", ctx.Location.File, ctx.Location.Row, ctx.Location.Col),
		"from":  string(ctx.Location.Text),
	}).Debug(message)
	return nil
}

type Result struct {
	// Reject signals explicit rejection.
	Reject int `json:"reject" mapstructure:"reject"`

	// Permit signals explicit permission.
	Permit *bool `json:"permit" mapstructure:"permit"`

	// Redirect to this URL.
	Redirect string `json:"redirect" mapstructure:"redirect"`

	// Template to render as response body.
	Template string `json:"template" mapstructure:"template"`

	// Errors contains error messages.
	Errors []string `json:"errors" mapstructure:"errors,omitempty"`
}

func (r *Result) Response(ctx proxy.Context) (*http.Response, error) {
	for _, text := range r.Errors {
		logger.StandardLog.Values(logger.Values{
			"id":     ctx.ID(),
			"client": ctx.RemoteAddr().String(),
		}).Err(errors.New(text)).Warn("Error from policy")
	}

	switch {
	case r.Redirect != "":
		response := proxy.NewResponse(http.StatusFound, nil, ctx.Request())
		response.Header.Set("Server", "styx")
		response.Header.Set(proxy.HeaderLocation, r.Redirect)
		return response, nil

	case r.Template != "":
		b := new(bytes.Buffer)
		t, err := template.New("policy").ParseFiles(r.Template)
		if err != nil {
			return nil, err
		}
		if err = t.Execute(b, map[string]any{"context": ctx}); err != nil {
			return nil, err
		}

		response := proxy.NewResponse(http.StatusFound, io.NopCloser(b), ctx.Request())
		response.Header.Set("Server", "styx")
		response.Header.Set(proxy.HeaderContentType, "text/html")
		return response, nil

	case r.Reject > 0:
		body := io.NopCloser(bytes.NewBufferString(http.StatusText(r.Reject)))
		response := proxy.NewResponse(r.Reject, body, ctx.Request())
		response.Header.Set(proxy.HeaderContentType, "text/plain")
		return response, nil

	case r.Permit != nil && !*r.Permit:
		body := io.NopCloser(bytes.NewBufferString(http.StatusText(http.StatusForbidden)))
		response := proxy.NewResponse(http.StatusForbidden, body, ctx.Request())
		response.Header.Set(proxy.HeaderContentType, "text/plain")
		return response, nil

	default:
		return nil, nil
	}
}

func (p *Policy) Query(input *Input) (*Result, error) {
	/*
		e := json.NewEncoder(os.Stdout)
		e.SetIndent("", "  ")
		e.Encode(doc)
	*/

	log := logger.StandardLog.Value("policy", p.name)
	log.Trace("Evaluating policy")

	r := rego.New(append(p.options, rego.Input(input))...)

	ctx := context.Background()
	/*
		query, err := p.rego.PrepareForEval(ctx)
		if err != nil {
			return nil, err
		}
		rs, err := query.Eval(ctx, rego.EvalInput(input))
		if err != nil {
			return nil, err
		}
	*/
	rs, err := r.Eval(ctx)
	if err != nil {
		return nil, err
	}
	if len(rs) == 0 || len(rs[0].Expressions) == 0 {
		return nil, ErrNoResult
	}
	result := &Result{}
	for _, expr := range rs[0].Expressions {
		if m, ok := expr.Value.(map[string]any); ok {
			log.Values(m).Trace("Policy result expression")
			if err = mapstructure.Decode(m, result); err != nil {
				return nil, err
			}
		}
	}
	return result, nil
}