prober: expand certificate verification logic in the TLS prober

TLS prober now checks validity period for all server certificates
and verifies OCSP revocation status for the leaf cert.

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

prober/tls.go

@@ -5,18 +5,28 @@
 package prober
 
 import (
+	"bytes"
 	"context"
 	"crypto/tls"
+	"crypto/x509"
 	"fmt"
+	"io"
 	"net"
+	"net/http"
 	"time"
+
+	"github.com/pkg/errors"
+	"golang.org/x/crypto/ocsp"
+	"tailscale.com/util/multierr"
 )
 
+const expiresSoon = 7 * 24 * time.Hour // 7 days from now
+
 // TLS returns a Probe that healthchecks a TLS endpoint.
 //
-// The ProbeFunc connects to hostname, does a TLS handshake, verifies
-// that the hostname matches the presented certificate, and that the
-// certificate expires in more than 7 days from the probe time.
+// The ProbeFunc connects to a hostname (host:port string), does a TLS
+// handshake, verifies that the hostname matches the presented certificate,
+// checks certificate validity time and OCSP revocation status.
 func TLS(hostname string) ProbeFunc {
 	return func(ctx context.Context) error {
 		return probeTLS(ctx, hostname)
@@ -24,23 +34,108 @@ func TLS(hostname string) ProbeFunc {
 }
 
 func probeTLS(ctx context.Context, hostname string) error {
-	var d net.Dialer
-	conn, err := tls.DialWithDialer(&d, "tcp", hostname+":443", nil)
+	host, _, err := net.SplitHostPort(hostname)
+	if err != nil {
+		return err
+	}
+
+	dialer := &tls.Dialer{Config: &tls.Config{ServerName: host}}
+	conn, err := dialer.DialContext(ctx, "tcp", hostname)
 	if err != nil {
 		return fmt.Errorf("connecting to %q: %w", hostname, err)
 	}
-	if err := conn.Handshake(); err != nil {
-		return fmt.Errorf("TLS handshake error with %q: %w", hostname, err)
-	}
-	if err := conn.VerifyHostname(hostname); err != nil {
-		return fmt.Errorf("Host %q TLS verification failed: %w", hostname, err)
-	}
+	defer conn.Close()
 
-	latestAllowedExpiration := time.Now().Add(7 * 24 * time.Hour) // 7 days from now
-	if expires := conn.ConnectionState().PeerCertificates[0].NotAfter; latestAllowedExpiration.After(expires) {
-		left := expires.Sub(time.Now())
-		return fmt.Errorf("TLS certificate for %q expires in %v", hostname, left)
-	}
-
-	return nil
+	tlsConnState := conn.(*tls.Conn).ConnectionState()
+	return validateConnState(ctx, &tlsConnState)
+}
+
+// validateConnState verifies certificate validity time in all certificates
+// returned by the TLS server and checks OCSP revocation status for the
+// leaf cert.
+func validateConnState(ctx context.Context, cs *tls.ConnectionState) (returnerr error) {
+	var errs []error
+	defer func() {
+		returnerr = multierr.New(errs...)
+	}()
+	latestAllowedExpiration := time.Now().Add(expiresSoon)
+
+	var leafCert *x509.Certificate
+	var issuerCert *x509.Certificate
+	var leafAuthorityKeyID string
+	// PeerCertificates will never be len == 0 on the client side
+	for i, cert := range cs.PeerCertificates {
+		if i == 0 {
+			leafCert = cert
+			leafAuthorityKeyID = string(cert.AuthorityKeyId)
+		}
+		if i > 0 {
+			if leafAuthorityKeyID == string(cert.SubjectKeyId) {
+				issuerCert = cert
+			}
+		}
+
+		// Do not check certificate validity period for self-signed certs.
+		// The practical reason is to avoid raising alerts for expiring
+		// DERP metaCert certificates that are returned as part of regular
+		// TLS handshake.
+		if string(cert.SubjectKeyId) == string(cert.AuthorityKeyId) {
+			continue
+		}
+
+		if time.Now().Before(cert.NotBefore) {
+			errs = append(errs, fmt.Errorf("one of the certs has NotBefore in the future (%v): %v", cert.NotBefore, cert.Subject))
+		}
+		if latestAllowedExpiration.After(cert.NotAfter) {
+			left := cert.NotAfter.Sub(time.Now())
+			errs = append(errs, fmt.Errorf("one of the certs expires in %v: %v", left, cert.Subject))
+		}
+	}
+
+	if len(leafCert.OCSPServer) == 0 {
+		errs = append(errs, fmt.Errorf("no OCSP server presented in leaf cert for %v", leafCert.Subject))
+		return
+	}
+
+	ocspResp, err := getOCSPResponse(ctx, leafCert.OCSPServer[0], leafCert, issuerCert)
+	if err != nil {
+		errs = append(errs, errors.Wrapf(err, "OCSP verification failed for %v", leafCert.Subject))
+		return
+	}
+
+	if ocspResp.Status == ocsp.Unknown {
+		errs = append(errs, fmt.Errorf("unknown OCSP verification status for %v", leafCert.Subject))
+	}
+
+	if ocspResp.Status == ocsp.Revoked {
+		errs = append(errs, fmt.Errorf("cert for %v has been revoked on %v, reason: %v", leafCert.Subject, ocspResp.RevokedAt, ocspResp.RevocationReason))
+	}
+	return
+}
+
+func getOCSPResponse(ctx context.Context, ocspServer string, leafCert, issuerCert *x509.Certificate) (*ocsp.Response, error) {
+	reqb, err := ocsp.CreateRequest(leafCert, issuerCert, nil)
+	if err != nil {
+		return nil, errors.Wrap(err, "could not create OCSP request")
+	}
+	hreq, err := http.NewRequestWithContext(ctx, "POST", ocspServer, bytes.NewReader(reqb))
+	if err != nil {
+		return nil, errors.Wrap(err, "could not create OCSP POST request")
+	}
+	hreq.Header.Add("Content-Type", "application/ocsp-request")
+	hreq.Header.Add("Accept", "application/ocsp-response")
+	hresp, err := http.DefaultClient.Do(hreq)
+	if err != nil {
+		return nil, errors.Wrap(err, "OCSP request failed")
+	}
+	defer hresp.Body.Close()
+	if hresp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("ocsp: non-200 status code from OCSP server: %s", hresp.Status)
+	}
+	lr := io.LimitReader(hresp.Body, 10<<20) // 10MB
+	ocspB, err := io.ReadAll(lr)
+	if err != nil {
+		return nil, err
+	}
+	return ocsp.ParseResponse(ocspB, issuerCert)
 }