feature/tpm: use withSRK to probe TPM availability (#17627)

On some platforms e.g. ChromeOS the owner hierarchy might not always be
available to us. To avoid stale sealing exceptions later we probe to
confirm it's working rather than rely solely on family indicator status.

Updates #17622

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

feature/tpm/tpm.go

@@ -59,7 +59,22 @@ func tpmSupported() bool {
 	if hi == nil {
 		return false
 	}
-	return hi.FamilyIndicator == "2.0"
+	if hi.FamilyIndicator != "2.0" {
+		return false
+	}
+
+	tpm, err := open()
+	if err != nil {
+		return false
+	}
+	defer tpm.Close()
+
+	if err := withSRK(logger.Discard, tpm, func(srk tpm2.AuthHandle) error {
+		return nil
+	}); err != nil {
+		return false
+	}
+	return true
 }
 
 var verboseTPM = envknob.RegisterBool("TS_DEBUG_TPM")

feature/tpm/tpm_test.go

@@ -146,6 +146,18 @@ func BenchmarkInfo(b *testing.B) {
 	b.StopTimer()
 }
 
+func BenchmarkTPMSupported(b *testing.B) {
+	b.StopTimer()
+	skipWithoutTPM(b)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		if !tpmSupported() {
+			b.Fatalf("tpmSupported returned false")
+		}
+	}
+	b.StopTimer()
+}
+
 func BenchmarkStore(b *testing.B) {
 	skipWithoutTPM(b)
 	b.StopTimer()