metropolis/pkg/pki/certificate.go

// Copyright 2020 The Monogon Project Authors.
//
// SPDX-License-Identifier: Apache-2.0
//
// 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 pki

import (
	"context"
	"crypto/ed25519"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"fmt"
	"net"

	"go.etcd.io/etcd/clientv3"

	"source.monogon.dev/metropolis/pkg/fileargs"
)

// Namespace represents some path in etcd where certificate/CA data will be
// stored. Creating a namespace via Namespaced then permits the consumer of
// this library to start creating certificates within this namespace.
type Namespace struct {
	prefix string
}

// Namespaced creates a namespace for storing certificate data in etcd at a
// given 'path' prefix.
func Namespaced(prefix string) Namespace {
	return Namespace{
		prefix: prefix,
	}
}

// Certificate is the promise of a Certificate being available to the caller.
// In this case, Certificate refers to a pair of x509 certificate and
// corresponding private key.  Certificates can be stored in etcd, and their
// issuers might also be store on etcd. As such, this type's methods contain
// references to an etcd KV client.  This Certificate type is agnostic to
// usage, but mostly geared towards Kubernetes certificates.
type Certificate struct {
	namespace *Namespace

	// issuer is the Issuer that will generate this certificate if one doesn't
	// yet exist or etcd, or the requested certificate is volatile (not to be
	// stored on etcd).
	Issuer Issuer
	// name is a unique key for storing the certificate in etcd. If empty,
	// certificate is 'volatile', will not be stored on etcd, and every
	// .Ensure() call will generate a new pair.
	name string
	// template is an x509 certificate definition that will be used to generate
	// the certificate when issuing it.
	template x509.Certificate
	// key is the private key for which the certificate should emitted, or nil
	// if the key should be generated. The private key is required (vs. the
	// private one) because the Certificate might be attempted to be issued via
	// self-signing.
	key ed25519.PrivateKey
}

func (n *Namespace) etcdPath(f string, args ...interface{}) string {
	return n.prefix + fmt.Sprintf(f, args...)
}

// New creates a new Certificate, or to be more precise, a promise that a
// certificate will exist once Ensure is called.  Issuer must be a valid
// certificate issuer (SelfSigned or another Certificate). Name must be unique
// among all certificates, or empty (which will cause the certificate to be
// volatile, ie. not stored in etcd).
func (n *Namespace) New(issuer Issuer, name string, template x509.Certificate) *Certificate {
	return &Certificate{
		namespace: n,
		Issuer:    issuer,
		name:      name,
		template:  template,
	}
}

// Client makes a Kubernetes PKI-compatible client certificate template.
// Directly derived from Kubernetes PKI requirements documented at
//   https://kubernetes.io/docs/setup/best-practices/certificates/#configure-certificates-manually
func Client(identity string, groups []string) x509.Certificate {
	return x509.Certificate{
		Subject: pkix.Name{
			CommonName:   identity,
			Organization: groups,
		},
		KeyUsage:    x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
	}
}

// Server makes a Kubernetes PKI-compatible server certificate template.
func Server(dnsNames []string, ips []net.IP) x509.Certificate {
	return x509.Certificate{
		Subject:     pkix.Name{},
		KeyUsage:    x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		DNSNames:    dnsNames,
		IPAddresses: ips,
	}
}

// CA makes a Certificate that can sign other certificates.
func CA(cn string) x509.Certificate {
	return x509.Certificate{
		Subject: pkix.Name{
			CommonName: cn,
		},
		IsCA:        true,
		KeyUsage:    x509.KeyUsageCertSign | x509.KeyUsageCRLSign | x509.KeyUsageDigitalSignature,
		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageOCSPSigning},
	}
}

func (c *Certificate) etcdPaths() (cert, key string) {
	return c.namespace.etcdPath("%s-cert.der", c.name), c.namespace.etcdPath("%s-key.der", c.name)
}

func (c *Certificate) UseExistingKey(key ed25519.PrivateKey) {
	c.key = key
}

// ensure returns a DER-encoded x509 certificate and internally encoded bare
// ed25519 key for a given Certificate, in memory (if volatile), loading it
// from etcd, or creating and saving it on etcd if needed.
// This function is safe to call in parallel from multiple etcd clients
// (including across machines), but it will error in case a concurrent
// certificate generation happens. These errors are, however, safe to retry -
// as long as all the certificate creators (ie., Metropolis nodes) run the same
// version of this code.
//
// TODO(q3k): in the future, this should be handled better - especially as we
// introduce new certificates, or worse, change the issuance chain. As a
// stopgap measure, an explicit per-certificate or even global lock can be
// implemented.  And, even before that, we can handle concurrency errors in a
// smarter way.
func (c *Certificate) ensure(ctx context.Context, kv clientv3.KV) (cert, key []byte, err error) {
	if c.name == "" {
		// Volatile certificate - generate.
		// TODO(q3k): cache internally?
		cert, key, err = c.Issuer.Issue(ctx, c, kv)
		if err != nil {
			err = fmt.Errorf("failed to issue: %w", err)
			return
		}
		return
	}

	certPath, keyPath := c.etcdPaths()

	// Try loading certificate and key from etcd.
	certRes, err := kv.Get(ctx, certPath)
	if err != nil {
		err = fmt.Errorf("failed to get certificate from etcd: %w", err)
		return
	}
	keyRes, err := kv.Get(ctx, keyPath)
	if err != nil {
		err = fmt.Errorf("failed to get key from etcd: %w", err)
		return
	}

	if len(certRes.Kvs) == 1 && len(keyRes.Kvs) == 1 {
		// Certificate and key exists in etcd, return that.
		cert = certRes.Kvs[0].Value
		key = keyRes.Kvs[0].Value

		err = nil
		// TODO(q3k): check for expiration
		return
	}

	// No certificate found - issue one.
	cert, key, err = c.Issuer.Issue(ctx, c, kv)
	if err != nil {
		err = fmt.Errorf("failed to issue: %w", err)
		return
	}

	// Save to etcd in transaction. This ensures that no partial writes happen,
	// and that we haven't been raced to the save.
	res, err := kv.Txn(ctx).
		If(
			clientv3.Compare(clientv3.CreateRevision(certPath), "=", 0),
			clientv3.Compare(clientv3.CreateRevision(keyPath), "=", 0),
		).
		Then(
			clientv3.OpPut(certPath, string(cert)),
			clientv3.OpPut(keyPath, string(key)),
		).Commit()
	if err != nil {
		err = fmt.Errorf("failed to write newly issued certificate: %w", err)
	} else if !res.Succeeded {
		err = fmt.Errorf("certificate issuance transaction failed: concurrent write")
	}

	return
}

// Ensure returns an x509 DER-encoded (but not PEM-encoded) certificate and key
// for a given Certificate.  If the certificate is volatile, each call to
// Ensure will cause a new certificate to be generated.  Otherwise, it will be
// retrieved from etcd, or generated and stored there if needed.
func (c *Certificate) Ensure(ctx context.Context, kv clientv3.KV) (cert, key []byte, err error) {
	cert, key, err = c.ensure(ctx, kv)
	if err != nil {
		return nil, nil, err
	}
	key, err = x509.MarshalPKCS8PrivateKey(ed25519.PrivateKey(key))
	if err != nil {
		err = fmt.Errorf("could not marshal private key (data corruption?): %w", err)
		return
	}
	return cert, key, err
}

// FilesystemCertificate is a fileargs.FileArgs wrapper which will contain PEM
// encoded certificate material when Mounted. This construct is useful when
// dealing with services that want to access etcd-backed certificates as files
// available locally.
// Paths to the available files are considered opaque and should not be leaked
// outside of the struct. Further restrictions on access to these files might
// be imposed in the future.
type FilesystemCertificate struct {
	*fileargs.FileArgs
	// CACertPath is the full path at which the CA certificate is available.
	// Read only.
	CACertPath string
	// CertPath is the full path at which the certificate is available. Read
	// only.
	CertPath string
	// KeyPath is the full path at which the key is available. Read only.
	KeyPath string
}

// Mount returns a locally mounted FilesystemCertificate for this Certificate,
// which allows services to access this Certificate via local filesystem
// access.
// The embeded fileargs.FileArgs can also be used to add additional file-backed
// data under the same mount by calling ArgPath.
// The returned FilesystemCertificate must be Closed in order to prevent a
// system mount leak.
func (c *Certificate) Mount(ctx context.Context, kv clientv3.KV) (*FilesystemCertificate, error) {
	fa, err := fileargs.New()
	if err != nil {
		return nil, fmt.Errorf("when creating fileargs mount: %w", err)
	}
	fs := &FilesystemCertificate{FileArgs: fa}

	cert, key, err := c.Ensure(ctx, kv)
	if err != nil {
		return nil, fmt.Errorf("when issuing certificate: %w", err)
	}

	cacert, err := c.Issuer.CACertificate(ctx, kv)
	if err != nil {
		return nil, fmt.Errorf("when getting issuer CA: %w", err)
	}
	// cacert will be null if this is a self-signed certificate.
	if cacert == nil {
		cacert = cert
	}

	fs.CACertPath = fs.ArgPath("ca.crt", pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: cacert}))
	fs.CertPath = fs.ArgPath("tls.crt", pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: cert}))
	fs.KeyPath = fs.ArgPath("tls.key", pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: key}))

	return fs, nil
}