/*
 *
 * Copyright 2020 gRPC authors.
 *
 * 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 xds contains non-user facing functionality of the xds credentials.
package xds

import (
	"context"
	"crypto/tls"
	"crypto/x509"
	"errors"
	"fmt"
	"strings"
	"sync"

	"google.golang.org/grpc/attributes"
	"google.golang.org/grpc/credentials/tls/certprovider"
	"google.golang.org/grpc/internal"
	xdsinternal "google.golang.org/grpc/internal/xds"
	"google.golang.org/grpc/resolver"
)

func init() {
	internal.GetXDSHandshakeInfoForTesting = GetHandshakeInfo
}

// handshakeAttrKey is the type used as the key to store HandshakeInfo in
// the Attributes field of resolver.Address.
type handshakeAttrKey struct{}

// SetHandshakeInfo returns a copy of addr in which the Attributes field is
// updated with hInfo.
func SetHandshakeInfo(addr resolver.Address, hInfo *HandshakeInfo) resolver.Address {
	addr.Attributes = addr.Attributes.WithValues(handshakeAttrKey{}, hInfo)
	return addr
}

// GetHandshakeInfo returns a pointer to the HandshakeInfo stored in attr.
func GetHandshakeInfo(attr *attributes.Attributes) *HandshakeInfo {
	v := attr.Value(handshakeAttrKey{})
	hi, _ := v.(*HandshakeInfo)
	return hi
}

// HandshakeInfo wraps all the security configuration required by client and
// server handshake methods in xds credentials. The xDS implementation will be
// responsible for populating these fields.
//
// Safe for concurrent access.
type HandshakeInfo struct {
	mu                sync.Mutex
	rootProvider      certprovider.Provider
	identityProvider  certprovider.Provider
	sanMatchers       []xdsinternal.StringMatcher // Only on the client side.
	requireClientCert bool                        // Only on server side.
}

// SetRootCertProvider updates the root certificate provider.
func (hi *HandshakeInfo) SetRootCertProvider(root certprovider.Provider) {
	hi.mu.Lock()
	hi.rootProvider = root
	hi.mu.Unlock()
}

// SetIdentityCertProvider updates the identity certificate provider.
func (hi *HandshakeInfo) SetIdentityCertProvider(identity certprovider.Provider) {
	hi.mu.Lock()
	hi.identityProvider = identity
	hi.mu.Unlock()
}

// SetSANMatchers updates the list of SAN matchers.
func (hi *HandshakeInfo) SetSANMatchers(sanMatchers []xdsinternal.StringMatcher) {
	hi.mu.Lock()
	hi.sanMatchers = sanMatchers
	hi.mu.Unlock()
}

// SetRequireClientCert updates whether a client cert is required during the
// ServerHandshake(). A value of true indicates that we are performing mTLS.
func (hi *HandshakeInfo) SetRequireClientCert(require bool) {
	hi.mu.Lock()
	hi.requireClientCert = require
	hi.mu.Unlock()
}

// UseFallbackCreds returns true when fallback credentials are to be used based
// on the contents of the HandshakeInfo.
func (hi *HandshakeInfo) UseFallbackCreds() bool {
	if hi == nil {
		return true
	}

	hi.mu.Lock()
	defer hi.mu.Unlock()
	return hi.identityProvider == nil && hi.rootProvider == nil
}

// GetSANMatchersForTesting returns the SAN matchers stored in HandshakeInfo.
// To be used only for testing purposes.
func (hi *HandshakeInfo) GetSANMatchersForTesting() []xdsinternal.StringMatcher {
	hi.mu.Lock()
	defer hi.mu.Unlock()
	return append([]xdsinternal.StringMatcher{}, hi.sanMatchers...)
}

// ClientSideTLSConfig constructs a tls.Config to be used in a client-side
// handshake based on the contents of the HandshakeInfo.
func (hi *HandshakeInfo) ClientSideTLSConfig(ctx context.Context) (*tls.Config, error) {
	hi.mu.Lock()
	// On the client side, rootProvider is mandatory. IdentityProvider is
	// optional based on whether the client is doing TLS or mTLS.
	if hi.rootProvider == nil {
		return nil, errors.New("xds: CertificateProvider to fetch trusted roots is missing, cannot perform TLS handshake. Please check configuration on the management server")
	}
	// Since the call to KeyMaterial() can block, we read the providers under
	// the lock but call the actual function after releasing the lock.
	rootProv, idProv := hi.rootProvider, hi.identityProvider
	hi.mu.Unlock()

	// InsecureSkipVerify needs to be set to true because we need to perform
	// custom verification to check the SAN on the received certificate.
	// Currently the Go stdlib does complete verification of the cert (which
	// includes hostname verification) or none. We are forced to go with the
	// latter and perform the normal cert validation ourselves.
	cfg := &tls.Config{
		InsecureSkipVerify: true,
		NextProtos:         []string{"h2"},
	}

	km, err := rootProv.KeyMaterial(ctx)
	if err != nil {
		return nil, fmt.Errorf("xds: fetching trusted roots from CertificateProvider failed: %v", err)
	}
	cfg.RootCAs = km.Roots

	if idProv != nil {
		km, err := idProv.KeyMaterial(ctx)
		if err != nil {
			return nil, fmt.Errorf("xds: fetching identity certificates from CertificateProvider failed: %v", err)
		}
		cfg.Certificates = km.Certs
	}
	return cfg, nil
}

// ServerSideTLSConfig constructs a tls.Config to be used in a server-side
// handshake based on the contents of the HandshakeInfo.
func (hi *HandshakeInfo) ServerSideTLSConfig(ctx context.Context) (*tls.Config, error) {
	cfg := &tls.Config{
		ClientAuth: tls.NoClientCert,
		NextProtos: []string{"h2"},
	}
	hi.mu.Lock()
	// On the server side, identityProvider is mandatory. RootProvider is
	// optional based on whether the server is doing TLS or mTLS.
	if hi.identityProvider == nil {
		return nil, errors.New("xds: CertificateProvider to fetch identity certificate is missing, cannot perform TLS handshake. Please check configuration on the management server")
	}
	// Since the call to KeyMaterial() can block, we read the providers under
	// the lock but call the actual function after releasing the lock.
	rootProv, idProv := hi.rootProvider, hi.identityProvider
	if hi.requireClientCert {
		cfg.ClientAuth = tls.RequireAndVerifyClientCert
	}
	hi.mu.Unlock()

	// identityProvider is mandatory on the server side.
	km, err := idProv.KeyMaterial(ctx)
	if err != nil {
		return nil, fmt.Errorf("xds: fetching identity certificates from CertificateProvider failed: %v", err)
	}
	cfg.Certificates = km.Certs

	if rootProv != nil {
		km, err := rootProv.KeyMaterial(ctx)
		if err != nil {
			return nil, fmt.Errorf("xds: fetching trusted roots from CertificateProvider failed: %v", err)
		}
		cfg.ClientCAs = km.Roots
	}
	return cfg, nil
}

// MatchingSANExists returns true if the SANs contained in cert match the
// criteria enforced by the list of SAN matchers in HandshakeInfo.
//
// If the list of SAN matchers in the HandshakeInfo is empty, this function
// returns true for all input certificates.
func (hi *HandshakeInfo) MatchingSANExists(cert *x509.Certificate) bool {
	hi.mu.Lock()
	defer hi.mu.Unlock()
	if len(hi.sanMatchers) == 0 {
		return true
	}

	// SANs can be specified in any of these four fields on the parsed cert.
	for _, san := range cert.DNSNames {
		if hi.matchSAN(san, true) {
			return true
		}
	}
	for _, san := range cert.EmailAddresses {
		if hi.matchSAN(san, false) {
			return true
		}
	}
	for _, san := range cert.IPAddresses {
		if hi.matchSAN(san.String(), false) {
			return true
		}
	}
	for _, san := range cert.URIs {
		if hi.matchSAN(san.String(), false) {
			return true
		}
	}
	return false
}

// Caller must hold mu.
func (hi *HandshakeInfo) matchSAN(san string, isDNS bool) bool {
	for _, matcher := range hi.sanMatchers {
		if em := matcher.ExactMatch(); em != "" && isDNS {
			// This is a special case which is documented in the xDS protos.
			// If the DNS SAN is a wildcard entry, and the match criteria is
			// `exact`, then we need to perform DNS wildcard matching
			// instead of regular string comparison.
			if dnsMatch(em, san) {
				return true
			}
			continue
		}
		if matcher.Match(san) {
			return true
		}
	}
	return false
}

// dnsMatch implements a DNS wildcard matching algorithm based on RFC2828 and
// grpc-java's implementation in `OkHostnameVerifier` class.
//
// NOTE: Here the `host` argument is the one from the set of string matchers in
// the xDS proto and the `san` argument is a DNS SAN from the certificate, and
// this is the one which can potentially contain a wildcard pattern.
func dnsMatch(host, san string) bool {
	// Add trailing "." and turn them into absolute domain names.
	if !strings.HasSuffix(host, ".") {
		host += "."
	}
	if !strings.HasSuffix(san, ".") {
		san += "."
	}
	// Domain names are case-insensitive.
	host = strings.ToLower(host)
	san = strings.ToLower(san)

	// If san does not contain a wildcard, do exact match.
	if !strings.Contains(san, "*") {
		return host == san
	}

	// Wildcard dns matching rules
	// - '*' is only permitted in the left-most label and must be the only
	//   character in that label. For example, *.example.com is permitted, while
	//   *a.example.com, a*.example.com, a*b.example.com, a.*.example.com are
	//   not permitted.
	// - '*' matches a single domain name component. For example, *.example.com
	//   matches test.example.com but does not match sub.test.example.com.
	// - Wildcard patterns for single-label domain names are not permitted.
	if san == "*." || !strings.HasPrefix(san, "*.") || strings.Contains(san[1:], "*") {
		return false
	}
	// Optimization: at this point, we know that the san contains a '*' and
	// is the first domain component of san. So, the host name must be at
	// least as long as the san to be able to match.
	if len(host) < len(san) {
		return false
	}
	// Hostname must end with the non-wildcard portion of san.
	if !strings.HasSuffix(host, san[1:]) {
		return false
	}
	// At this point we know that the hostName and san share the same suffix
	// (the non-wildcard portion of san). Now, we just need to make sure
	// that the '*' does not match across domain components.
	hostPrefix := strings.TrimSuffix(host, san[1:])
	return !strings.Contains(hostPrefix, ".")
}

// NewHandshakeInfo returns a new instance of HandshakeInfo with the given root
// and identity certificate providers.
func NewHandshakeInfo(root, identity certprovider.Provider) *HandshakeInfo {
	return &HandshakeInfo{rootProvider: root, identityProvider: identity}
}