feature/tpm: protect all TPM handle operations with a mutex (#17708)

In particular on Windows, the `transport.TPMCloser` we get is not safe
for concurrent use. This is especially noticeable because
`tpm.attestationKey.Clone` uses the same open handle as the original
key. So wrap the operations on ak.tpm with a mutex and make a deep copy
with a new connection in Clone.

Updates #15830
Updates #17662
Updates #17644

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

feature/tpm/attestation_test.go

@@ -10,6 +10,8 @@ import (
 	"crypto/rand"
 	"crypto/sha256"
 	"encoding/json"
+	"runtime"
+	"sync"
 	"testing"
 )
 
@@ -62,6 +64,37 @@ func TestAttestationKeySign(t *testing.T) {
 	}
 }
 
+func TestAttestationKeySignConcurrent(t *testing.T) {
+	skipWithoutTPM(t)
+	ak, err := newAttestationKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := ak.Close(); err != nil {
+			t.Errorf("ak.Close: %v", err)
+		}
+	})
+
+	data := []byte("secrets")
+	digest := sha256.Sum256(data)
+
+	wg := sync.WaitGroup{}
+	for range runtime.GOMAXPROCS(-1) {
+		wg.Go(func() {
+			// Check signature/validation round trip.
+			sig, err := ak.Sign(rand.Reader, digest[:], crypto.SHA256)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !ecdsa.VerifyASN1(ak.Public().(*ecdsa.PublicKey), digest[:], sig) {
+				t.Errorf("ecdsa.VerifyASN1 failed")
+			}
+		})
+	}
+	wg.Wait()
+}
+
 func TestAttestationKeyUnmarshal(t *testing.T) {
 	skipWithoutTPM(t)
 	ak, err := newAttestationKey()
@@ -96,3 +129,36 @@ func TestAttestationKeyUnmarshal(t *testing.T) {
 		t.Error("unmarshalled public key is not the same as the original public key")
 	}
 }
+
+func TestAttestationKeyClone(t *testing.T) {
+	skipWithoutTPM(t)
+	ak, err := newAttestationKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ak2 := ak.Clone()
+	if ak2 == nil {
+		t.Fatal("Clone failed")
+	}
+	t.Cleanup(func() {
+		if err := ak2.Close(); err != nil {
+			t.Errorf("ak2.Close: %v", err)
+		}
+	})
+	// Close the original key, ak2 should remain open and usable.
+	if err := ak.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	data := []byte("secrets")
+	digest := sha256.Sum256(data)
+	// Check signature/validation round trip using cloned key.
+	sig, err := ak2.Sign(rand.Reader, digest[:], crypto.SHA256)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !ecdsa.VerifyASN1(ak2.Public().(*ecdsa.PublicKey), digest[:], sig) {
+		t.Errorf("ecdsa.VerifyASN1 failed")
+	}
+}