mirror of https://github.com/tailscale/tailscale/
control/controlclient: sign RegisterRequest (#1549)
control/controlclient: sign RegisterRequest Some customers wish to verify eligibility for devices to join their tailnets using machine identity certificates. TLS client certs could potentially fulfill this role but the initial customer for this feature has technical requirements that prevent their use. Instead, the certificate is loaded from the Windows local machine certificate store and uses its RSA public key to sign the RegisterRequest message. There is room to improve the flexibility of this feature in future and it is currently only tested on Windows (although Darwin theoretically works too), but this offers a reasonable starting place for now. Updates tailscale/coral#6 Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>pull/1593/head
Adrian Dewhurst
3 years ago
committed by
GitHub
parent
672731ac6f
commit
04dd6d1dae
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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package controlclient
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import (
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"crypto"
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"errors"
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"fmt"
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"time"
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"tailscale.com/types/wgkey"
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)
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var (
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errNoCertStore = errors.New("no certificate store")
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errCertificateNotConfigured = errors.New("no certificate subject configured")
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)
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// HashRegisterRequest generates the hash required sign or verify a
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// tailcfg.RegisterRequest with tailcfg.SignatureV1.
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func HashRegisterRequest(ts time.Time, serverURL string, deviceCert []byte, serverPubKey, machinePubKey wgkey.Key) []byte {
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h := crypto.SHA256.New()
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// hash.Hash.Write never returns an error, so we don't check for one here.
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fmt.Fprintf(h, "%s%s%s%s%s",
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ts.UTC().Format(time.RFC3339), serverURL, deviceCert, serverPubKey, machinePubKey)
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return h.Sum(nil)
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}
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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build windows,cgo
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// darwin,cgo is also supported by certstore but machineCertificateSubject will
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// need to be loaded by a different mechanism, so this is not currently enabled
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// on darwin.
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package controlclient
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import (
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"crypto"
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"crypto/rsa"
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"crypto/x509"
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"errors"
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"fmt"
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"sync"
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"github.com/github/certstore"
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"tailscale.com/tailcfg"
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"tailscale.com/types/wgkey"
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"tailscale.com/util/winutil"
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)
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var getMachineCertificateSubjectOnce struct {
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sync.Once
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v string // Subject of machine certificate to search for
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}
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// getMachineCertificateSubject returns the exact name of a Subject that needs
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// to be present in an identity's certificate chain to sign a RegisterRequest,
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// formatted as per pkix.Name.String(). The Subject may be that of the identity
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// itself, an intermediate CA or the root CA.
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//
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// If getMachineCertificateSubject() returns "" then no lookup will occur and
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// each RegisterRequest will be unsigned.
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//
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// Example: "CN=Tailscale Inc Test Root CA,OU=Tailscale Inc Test Certificate Authority,O=Tailscale Inc,ST=ON,C=CA"
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func getMachineCertificateSubject() string {
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getMachineCertificateSubjectOnce.Do(func() {
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getMachineCertificateSubjectOnce.v = winutil.GetRegString("MachineCertificateSubject", "")
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})
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return getMachineCertificateSubjectOnce.v
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}
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var (
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errNoMatch = errors.New("no matching certificate")
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errBadRequest = errors.New("malformed request")
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)
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// findIdentity locates an identity from the Windows or Darwin certificate
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// store. It returns the first certificate with a matching Subject anywhere in
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// its certificate chain, so it is possible to search for the leaf certificate,
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// intermediate CA or root CA. If err is nil then the returned identity will
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// never be nil (if no identity is found, the error errNoMatch will be
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// returned). If an identity is returned then its certificate chain is also
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// returned.
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func findIdentity(subject string, st certstore.Store) (certstore.Identity, []*x509.Certificate, error) {
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ids, err := st.Identities()
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if err != nil {
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return nil, nil, err
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}
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var selected certstore.Identity
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var chain []*x509.Certificate
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for _, id := range ids {
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chain, err = id.CertificateChain()
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if err != nil {
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continue
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}
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if chain[0].PublicKeyAlgorithm != x509.RSA {
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continue
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}
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for _, c := range chain {
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if c.Subject.String() == subject {
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selected = id
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break
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}
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}
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}
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for _, id := range ids {
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if id != selected {
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id.Close()
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}
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}
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if selected == nil {
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return nil, nil, errNoMatch
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}
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return selected, chain, nil
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}
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// signRegisterRequest looks for a suitable machine identity from the local
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// system certificate store, and if one is found, signs the RegisterRequest
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// using that identity's public key. In addition to the signature, the full
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// certificate chain is included so that the control server can validate the
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// certificate from a copy of the root CA's certificate.
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func signRegisterRequest(req *tailcfg.RegisterRequest, serverURL string, serverPubKey, machinePubKey wgkey.Key) (err error) {
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defer func() {
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if err != nil {
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err = fmt.Errorf("signRegisterRequest: %w", err)
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}
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}()
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if req.Timestamp == nil {
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return errBadRequest
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}
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machineCertificateSubject := getMachineCertificateSubject()
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if machineCertificateSubject == "" {
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return errCertificateNotConfigured
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}
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st, err := certstore.Open(certstore.System)
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if err != nil {
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return fmt.Errorf("open cert store: %w", err)
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}
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defer st.Close()
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id, chain, err := findIdentity(machineCertificateSubject, st)
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if err != nil {
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return fmt.Errorf("find identity: %w", err)
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}
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defer id.Close()
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signer, err := id.Signer()
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if err != nil {
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return fmt.Errorf("create signer: %w", err)
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}
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cl := 0
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for _, c := range chain {
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cl += len(c.Raw)
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}
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req.DeviceCert = make([]byte, 0, cl)
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for _, c := range chain {
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req.DeviceCert = append(req.DeviceCert, c.Raw...)
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}
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h := HashRegisterRequest(req.Timestamp.UTC(), serverURL, req.DeviceCert, serverPubKey, machinePubKey)
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req.Signature, err = signer.Sign(nil, h, &rsa.PSSOptions{
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SaltLength: rsa.PSSSaltLengthEqualsHash,
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Hash: crypto.SHA256,
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})
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if err != nil {
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return fmt.Errorf("sign: %w", err)
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}
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req.SignatureType = tailcfg.SignatureV1
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return nil
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}
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@ -0,0 +1,17 @@
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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build !windows !cgo
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package controlclient
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import (
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"tailscale.com/tailcfg"
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"tailscale.com/types/wgkey"
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)
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// signRegisterRequest on non-supported platforms always returns errNoCertStore.
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func signRegisterRequest(req *tailcfg.RegisterRequest, serverURL string, serverPubKey, machinePubKey wgkey.Key) error {
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return errNoCertStore
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}
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