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derp/derpserver,cmd/derper: don't mutate cert provider's shared tls.Certificate (#20478)

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ModifyTLSConfigToAddMetaCert (and its inline copy in cmd/derper) appended
the DERP meta cert directly to the *tls.Certificate returned by the
underlying GetCertificate. autocert returns a certificate sharing a cached
chain slice (and, on the TLS-ALPN token path, the same pointer) across
concurrent handshakes, so the in-place append was a data race and could
grow the served chain unboundedly.

Return a shallow copy with the meta cert appended to a fresh backing
array instead, and have cmd/derper reuse ModifyTLSConfigToAddMetaCert
rather than duplicating the wrapper.

Fixes #20352

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>
This commit is contained in:
Mike O'Driscoll
2026-07-15 17:16:40 -04:00
committed by GitHub
parent bef2cd8088
commit bf7d815631
4 changed files with 88 additions and 18 deletions
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cmd/derper/cert.go
+5 -6
View File
@@ -35,10 +35,9 @@ var unsafeHostnameCharacters = regexp.MustCompile(`[^a-zA-Z0-9-\.]`)
type certProvider interface {
// TLSConfig creates a new TLS config suitable for net/http.Server servers.
//
// The returned Config must have a GetCertificate function set and that
// function must return a unique *tls.Certificate for each call. The
// returned *tls.Certificate will be mutated by the caller to append to the
// (*tls.Certificate).Certificate field.
// The returned Config must have a GetCertificate function set. The
// *tls.Certificate values it returns may be shared and cached, so
// callers must not mutate them.
TLSConfig() *tls.Config
// HTTPHandler handle ACME related request, if any.
HTTPHandler(fallback http.Handler) http.Handler
@@ -157,8 +156,8 @@ func (m *manualCertManager) getCertificate(hi *tls.ClientHelloInfo) (*tls.Certif
return nil, fmt.Errorf("cert mismatch with hostname: %q", hi.ServerName)
}
// Return a shallow copy of the cert so the caller can append to its
// Certificate field.
// Return a shallow copy of the cert with a capacity-clamped chain
// so callers can never mutate the manager's long-lived certificate.
certCopy := new(tls.Certificate)
*certCopy = *m.cert
certCopy.Certificate = certCopy.Certificate[:len(certCopy.Certificate):len(certCopy.Certificate)]
cmd/derper/derper.go
+1 -9
View File
@@ -354,15 +354,7 @@ func main() {
log.Fatalf("derper: can not start cert provider: %v", err)
}
httpsrv.TLSConfig = certManager.TLSConfig()
getCert := httpsrv.TLSConfig.GetCertificate
httpsrv.TLSConfig.GetCertificate = func(hi *tls.ClientHelloInfo) (*tls.Certificate, error) {
cert, err := getCert(hi)
if err != nil {
return nil, err
}
cert.Certificate = append(cert.Certificate, s.MetaCert())
return cert, nil
}
s.ModifyTLSConfigToAddMetaCert(httpsrv.TLSConfig)
// Disable TLS 1.0 and 1.1, which are obsolete and have security issues.
httpsrv.TLSConfig.MinVersion = tls.VersionTLS12
httpsrv.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {