caddytls: Refactor certificate selection policies (close #1575)

Certificate selection used to be a module, but this seems unnecessary,
especially since the built-in CustomSelectionPolicy allows quite complex
selection logic on a number of fields in certs. If we need to extend
that logic, we can, but I don't think there are SO many possibilities
that we need modules.

This update also allows certificate selection to choose between multiple
matching certs based on client compatibility and makes a number of other
improvements in the default cert selection logic, both here and in the
latest CertMagic.

The hardest part of this was the conn policy consolidation logic
(Caddyfile only, of course). We have to merge connection policies that
we can easily combine, because if two certs are manually loaded in a
Caddyfile site block, that produces two connection policies, and each
cert is tagged with a different tag, meaning only the first would ever
be selected. So given the same matchers, we can merge the two, but this
required improving the Tag selection logic to support multiple tags to
choose from, hence "tags" changed to "any_tag" or "all_tags" (but we
use any_tag in our Caddyfile logic).

Combining conn policies with conflicting settings is impossible, so
that should return an error if two policies with the exact same matchers
have non-empty settings that are not the same (the one exception being
any_tag which we can merge because the logic for them is to OR them).

It was a bit complicated. It seems to work in numerous tests I've
conducted, but we'll see how it pans out in the release candidates.

2 files changed, 72 insertions, 48 deletions

diff --git a/modules/caddytls/certselection.go b/modules/caddytls/certselection.go
index 88b7e0b..8a24034 100644
--- a/modules/caddytls/certselection.go
+++ b/modules/caddytls/certselection.go
@@ -20,65 +20,104 @@ import (
 	"fmt"
 	"math/big"
 
-	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/certmagic"
 )
 
-func init() {
-	caddy.RegisterModule(CustomCertSelectionPolicy{})
-}
-
 // CustomCertSelectionPolicy represents a policy for selecting the certificate
 // used to complete a handshake when there may be multiple options. All fields
 // specified must match the candidate certificate for it to be chosen.
 // This was needed to solve https://github.com/caddyserver/caddy/issues/2588.
 type CustomCertSelectionPolicy struct {
-	SerialNumber        *big.Int           `json:"serial_number,omitempty"`
-	SubjectOrganization string             `json:"subject_organization,omitempty"`
-	PublicKeyAlgorithm  PublicKeyAlgorithm `json:"public_key_algorithm,omitempty"`
-	Tag                 string             `json:"tag,omitempty"`
-}
+	// The certificate must have one of these serial numbers.
+	SerialNumber []*big.Int `json:"serial_number,omitempty"`
 
-// CaddyModule returns the Caddy module information.
-func (CustomCertSelectionPolicy) CaddyModule() caddy.ModuleInfo {
-	return caddy.ModuleInfo{
-		ID:  "tls.certificate_selection.custom",
-		New: func() caddy.Module { return new(CustomCertSelectionPolicy) },
-	}
+	// The certificate must have one of these organization names.
+	SubjectOrganization []string `json:"subject_organization,omitempty"`
+
+	// The certificate must use this public key algorithm.
+	PublicKeyAlgorithm PublicKeyAlgorithm `json:"public_key_algorithm,omitempty"`
+
+	// The certificate must have at least one of the tags in the list.
+	AnyTag []string `json:"any_tag,omitempty"`
+
+	// The certificate must have all of the tags in the list.
+	AllTags []string `json:"all_tags,omitempty"`
 }
 
-// SelectCertificate implements certmagic.CertificateSelector.
-func (p CustomCertSelectionPolicy) SelectCertificate(_ *tls.ClientHelloInfo, choices []certmagic.Certificate) (certmagic.Certificate, error) {
+// SelectCertificate implements certmagic.CertificateSelector. It
+// only chooses a certificate that at least meets the criteria in
+// p. It then chooses the first non-expired certificate that is
+// compatible with the client. If none are valid, it chooses the
+// first viable candidate anyway.
+func (p CustomCertSelectionPolicy) SelectCertificate(hello *tls.ClientHelloInfo, choices []certmagic.Certificate) (certmagic.Certificate, error) {
+	viable := make([]certmagic.Certificate, 0, len(choices))
+
+nextChoice:
 	for _, cert := range choices {
-		if p.SerialNumber != nil && cert.SerialNumber.Cmp(p.SerialNumber) != 0 {
-			continue
+		if len(p.SerialNumber) > 0 {
+			var found bool
+			for _, sn := range p.SerialNumber {
+				if cert.Leaf.SerialNumber.Cmp(sn) == 0 {
+					found = true
+					break
+				}
+			}
+			if !found {
+				continue
+			}
+		}
+
+		if len(p.SubjectOrganization) > 0 {
+			var found bool
+			for _, subjOrg := range p.SubjectOrganization {
+				for _, org := range cert.Leaf.Subject.Organization {
+					if subjOrg == org {
+						found = true
+						break
+					}
+				}
+			}
+			if !found {
+				continue
+			}
 		}
 
 		if p.PublicKeyAlgorithm != PublicKeyAlgorithm(x509.UnknownPublicKeyAlgorithm) &&
-			PublicKeyAlgorithm(cert.PublicKeyAlgorithm) != p.PublicKeyAlgorithm {
+			PublicKeyAlgorithm(cert.Leaf.PublicKeyAlgorithm) != p.PublicKeyAlgorithm {
 			continue
 		}
 
-		if p.SubjectOrganization != "" {
-			var matchOrg bool
-			for _, org := range cert.Subject.Organization {
-				if p.SubjectOrganization == org {
-					matchOrg = true
+		if len(p.AnyTag) > 0 {
+			var found bool
+			for _, tag := range p.AnyTag {
+				if cert.HasTag(tag) {
+					found = true
 					break
 				}
 			}
-			if !matchOrg {
+			if !found {
 				continue
 			}
 		}
 
-		if p.Tag != "" && !cert.HasTag(p.Tag) {
-			continue
+		if len(p.AllTags) > 0 {
+			for _, tag := range p.AllTags {
+				if !cert.HasTag(tag) {
+					continue nextChoice
+				}
+			}
 		}
 
-		return cert, nil
+		// this certificate at least meets the policy's requirements,
+		// but we still have to check expiration and compatibility
+		viable = append(viable, cert)
 	}
-	return certmagic.Certificate{}, fmt.Errorf("no certificates matched custom selection policy")
+
+	if len(viable) == 0 {
+		return certmagic.Certificate{}, fmt.Errorf("no certificates matched custom selection policy")
+	}
+
+	return certmagic.DefaultCertificateSelector(hello, viable)
 }
 
 // Interface guard
diff --git a/modules/caddytls/connpolicy.go b/modules/caddytls/connpolicy.go
index 52ccdd9..4fd8112 100644
--- a/modules/caddytls/connpolicy.go
+++ b/modules/caddytls/connpolicy.go
@@ -18,12 +18,10 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/base64"
-	"encoding/json"
 	"fmt"
 	"strings"
 
 	"github.com/caddyserver/caddy/v2"
-	"github.com/caddyserver/certmagic"
 	"github.com/go-acme/lego/v3/challenge/tlsalpn01"
 )
 
@@ -46,15 +44,6 @@ func (cp ConnectionPolicies) Provision(ctx caddy.Context) error {
 			cp[i].matchers = append(cp[i].matchers, modIface.(ConnectionMatcher))
 		}
 
-		// certificate selector
-		if pol.CertSelection != nil {
-			val, err := ctx.LoadModule(pol, "CertSelection")
-			if err != nil {
-				return fmt.Errorf("loading certificate selection module: %s", err)
-			}
-			cp[i].certSelector = val.(certmagic.CertificateSelector)
-		}
-
 		// enable HTTP/2 by default
 		if len(pol.ALPN) == 0 {
 			pol.ALPN = append(pol.ALPN, defaultALPN...)
@@ -123,7 +112,7 @@ type ConnectionPolicy struct {
 
 	// How to choose a certificate if more than one matched
 	// the given ServerName (SNI) value.
-	CertSelection json.RawMessage `json:"certificate_selection,omitempty" caddy:"namespace=tls.certificate_selection inline_key=policy"`
+	CertSelection *CustomCertSelectionPolicy `json:"certificate_selection,omitempty"`
 
 	// The list of cipher suites to support. Caddy's
 	// defaults are modern and secure.
@@ -151,8 +140,6 @@ type ConnectionPolicy struct {
 	DefaultSNI string `json:"default_sni,omitempty"`
 
 	matchers     []ConnectionMatcher
-	certSelector certmagic.CertificateSelector
-
 	stdTLSConfig *tls.Config
 }
 
@@ -184,9 +171,7 @@ func (p *ConnectionPolicy) buildStandardTLSConfig(ctx caddy.Context) error {
 			// more at handshake-time, but I don't know how to practically pre-build
 			// a certmagic config for each combination of conn policy + automation policy...
 			cfg := *tlsApp.getConfigForName(hello.ServerName)
-			if p.certSelector != nil {
-				cfg.CertSelection = p.certSelector
-			}
+			cfg.CertSelection = p.CertSelection
 			cfg.DefaultServerName = p.DefaultSNI
 			return cfg.GetCertificate(hello)
 		},