control/controlclient: select newer certificate

If multiple certificates match when selecting a certificate, use the one
issued the most recently (as determined by the NotBefore timestamp).
This also adds some tests for the function that performs that
comparison.

Updates tailscale/coral#6

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

control/controlclient/sign_supported.go

@@ -18,6 +18,7 @@
 	"errors"
 	"fmt"
 	"sync"
+	"time"
 
 	"github.com/tailscale/certstore"
 	"tailscale.com/tailcfg"
@@ -73,23 +74,46 @@ func isSubjectInChain(subject string, chain []*x509.Certificate) bool {
 	return false
 }
 
-func selectIdentityFromSlice(subject string, ids []certstore.Identity) (certstore.Identity, []*x509.Certificate) {
+func selectIdentityFromSlice(subject string, ids []certstore.Identity, now time.Time) (certstore.Identity, []*x509.Certificate) {
+	var bestCandidate struct {
+		id    certstore.Identity
+		chain []*x509.Certificate
+	}
+
 	for _, id := range ids {
 		chain, err := id.CertificateChain()
 		if err != nil {
 			continue
 		}
 
+		if len(chain) < 1 {
+			continue
+		}
+
 		if !isSupportedCertificate(chain[0]) {
 			continue
 		}
 
-		if isSubjectInChain(subject, chain) {
-			return id, chain
+		if now.Before(chain[0].NotBefore) || now.After(chain[0].NotAfter) {
+			// Certificate is not valid at this time
+			continue
 		}
+
+		if !isSubjectInChain(subject, chain) {
+			continue
+		}
+
+		// Select the most recently issued certificate. If there is a tie, pick
+		// one arbitrarily.
+		if len(bestCandidate.chain) > 0 && bestCandidate.chain[0].NotBefore.After(chain[0].NotBefore) {
+			continue
+		}
+
+		bestCandidate.id = id
+		bestCandidate.chain = chain
 	}
 
-	return nil, nil
+	return bestCandidate.id, bestCandidate.chain
 }
 
 // findIdentity locates an identity from the Windows or Darwin certificate
@@ -105,7 +129,7 @@ func findIdentity(subject string, st certstore.Store) (certstore.Identity, []*x5
 		return nil, nil, err
 	}
 
-	selected, chain := selectIdentityFromSlice(subject, ids)
+	selected, chain := selectIdentityFromSlice(subject, ids, time.Now())
 
 	for _, id := range ids {
 		if id != selected {

control/controlclient/sign_supported_test.go

@@ -0,0 +1,238 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows && cgo
+// +build windows,cgo
+
+package controlclient
+
+import (
+	"crypto"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"errors"
+	"reflect"
+	"testing"
+	"time"
+
+	"github.com/tailscale/certstore"
+)
+
+const (
+	testRootCommonName = "testroot"
+	testRootSubject    = "CN=testroot"
+)
+
+type testIdentity struct {
+	chain []*x509.Certificate
+}
+
+func makeChain(rootCommonName string, notBefore, notAfter time.Time) []*x509.Certificate {
+	return []*x509.Certificate{
+		{
+			NotBefore:          notBefore,
+			NotAfter:           notAfter,
+			PublicKeyAlgorithm: x509.RSA,
+		},
+		{
+			Subject: pkix.Name{
+				CommonName: rootCommonName,
+			},
+			PublicKeyAlgorithm: x509.RSA,
+		},
+	}
+}
+
+func (t *testIdentity) Certificate() (*x509.Certificate, error) {
+	return t.chain[0], nil
+}
+
+func (t *testIdentity) CertificateChain() ([]*x509.Certificate, error) {
+	return t.chain, nil
+}
+
+func (t *testIdentity) Signer() (crypto.Signer, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (t *testIdentity) Delete() error {
+	return errors.New("not implemented")
+}
+
+func (t *testIdentity) Close() {}
+
+func TestSelectIdentityFromSlice(t *testing.T) {
+	var times []time.Time
+	for _, ts := range []string{
+		"2000-01-01T00:00:00Z",
+		"2001-01-01T00:00:00Z",
+		"2002-01-01T00:00:00Z",
+		"2003-01-01T00:00:00Z",
+	} {
+		tm, err := time.Parse(time.RFC3339, ts)
+		if err != nil {
+			t.Fatal(err)
+		}
+		times = append(times, tm)
+	}
+
+	tests := []struct {
+		name    string
+		subject string
+		ids     []certstore.Identity
+		now     time.Time
+		// wantIndex is an index into ids, or -1 for nil.
+		wantIndex int
+	}{
+		{
+			name:    "single unexpired identity",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[2]),
+				},
+			},
+			now:       times[1],
+			wantIndex: 0,
+		},
+		{
+			name:    "single expired identity",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[1]),
+				},
+			},
+			now:       times[2],
+			wantIndex: -1,
+		},
+		{
+			name:    "unrelated ids",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain("something", times[0], times[2]),
+				},
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[2]),
+				},
+				&testIdentity{
+					chain: makeChain("else", times[0], times[2]),
+				},
+			},
+			now:       times[1],
+			wantIndex: 1,
+		},
+		{
+			name:    "expired with unrelated ids",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain("something", times[0], times[3]),
+				},
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[1]),
+				},
+				&testIdentity{
+					chain: makeChain("else", times[0], times[3]),
+				},
+			},
+			now:       times[2],
+			wantIndex: -1,
+		},
+		{
+			name:    "one expired",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[1]),
+				},
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[1], times[3]),
+				},
+			},
+			now:       times[2],
+			wantIndex: 1,
+		},
+		{
+			name:    "two certs both unexpired",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[3]),
+				},
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[1], times[3]),
+				},
+			},
+			now:       times[2],
+			wantIndex: 1,
+		},
+		{
+			name:    "two unexpired one expired",
+			subject: testRootSubject,
+			ids: []certstore.Identity{
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[3]),
+				},
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[1], times[3]),
+				},
+				&testIdentity{
+					chain: makeChain(testRootCommonName, times[0], times[1]),
+				},
+			},
+			now:       times[2],
+			wantIndex: 1,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotId, gotChain := selectIdentityFromSlice(tt.subject, tt.ids, tt.now)
+
+			if gotId == nil && gotChain != nil {
+				t.Error("id is nil: got non-nil chain, want nil chain")
+				return
+			}
+			if gotId != nil && gotChain == nil {
+				t.Error("id is not nil: got nil chain, want non-nil chain")
+				return
+			}
+			if tt.wantIndex == -1 {
+				if gotId != nil {
+					t.Error("got non-nil id, want nil id")
+				}
+				return
+			}
+			if gotId == nil {
+				t.Error("got nil id, want non-nil id")
+				return
+			}
+			if gotId != tt.ids[tt.wantIndex] {
+				found := -1
+				for i := range tt.ids {
+					if tt.ids[i] == gotId {
+						found = i
+						break
+					}
+				}
+				if found == -1 {
+					t.Errorf("got unknown id, want id at index %v", tt.wantIndex)
+				} else {
+					t.Errorf("got id at index %v, want id at index %v", found, tt.wantIndex)
+				}
+			}
+
+			tid, ok := tt.ids[tt.wantIndex].(*testIdentity)
+			if !ok {
+				t.Error("got non-testIdentity, want testIdentity")
+				return
+			}
+
+			if !reflect.DeepEqual(tid.chain, gotChain) {
+				t.Errorf("got unknown chain, want chain from id at index %v", tt.wantIndex)
+			}
+		})
+	}
+}