Handle invalid SafetyNet results

Fix #4253

app/proguard-rules.pro

@@ -25,9 +25,7 @@
 # Snet
 -keepclassmembers class com.topjohnwu.magisk.ui.safetynet.SafetyNetHelper { *; }
 -keep,allowobfuscation interface com.topjohnwu.magisk.ui.safetynet.SafetyNetHelper$Callback
--keepclassmembers class * implements com.topjohnwu.magisk.ui.safetynet.SafetyNetHelper$Callback {
-  void onResponse(java.lang.String);
-}
+-keepclassmembers class * implements com.topjohnwu.magisk.ui.safetynet.SafetyNetHelper$Callback { *; }
 
 # Stub
 -keep class com.topjohnwu.magisk.core.App { <init>(java.lang.Object); }

app/src/main/java/com/topjohnwu/magisk/core/Const.kt

@@ -29,7 +29,7 @@ object Const {
     const val MAGISK_LOG = "/cache/magisk.log"
 
     // Versions
-    const val SNET_EXT_VER = 16
+    const val SNET_EXT_VER = 17
     const val SNET_REVISION = "22.0"
     const val BOOTCTL_REVISION = "22.0"
 

app/src/main/java/com/topjohnwu/magisk/ui/safetynet/CheckSafetyNetEvent.kt

@@ -31,10 +31,9 @@ import java.io.ByteArrayInputStream
 import java.io.File
 import java.io.IOException
 import java.lang.reflect.InvocationHandler
-import java.security.GeneralSecurityException
+import java.lang.reflect.Proxy
 import java.security.SecureRandom
 import java.security.Signature
-import java.security.cert.X509Certificate
 
 class CheckSafetyNetEvent(
     private val callback: (SafetyNetResult) -> Unit = {}
@@ -42,19 +41,17 @@ class CheckSafetyNetEvent(
 
     private val svc get() = ServiceLocator.networkService
 
-    private lateinit var apk: File
-    private lateinit var dex: File
+    private lateinit var jar: File
     private lateinit var nonce: ByteArray
 
     override fun invoke(context: Context) {
-        apk = File("${context.filesDir.parent}/snet", "snet.jar")
-        dex = File(apk.parent, "snet.dex")
+        jar = File("${context.filesDir.parent}/snet", "snet.jar")
 
         scope.launch(Dispatchers.IO) {
             attest(context) {
                 // Download and retry
-                Shell.sh("rm -rf " + apk.parent).exec()
-                apk.parentFile?.mkdir()
+                Shell.sh("rm -rf " + jar.parent).exec()
+                jar.parentFile?.mkdir()
                 withContext(Dispatchers.Main) {
                     showDialog(context)
                 }
@@ -65,25 +62,24 @@ class CheckSafetyNetEvent(
     private suspend fun attest(context: Context, onError: suspend (Exception) -> Unit) {
         val helper: SafetyNetHelper
         try {
-            val loader = createClassLoader(apk)
+            val loader = createClassLoader(jar)
 
             // Scan through the dex and find our helper class
             var clazz: Class<*>? = null
             loop@for (dex in loader.getDexFiles()) {
                 for (name in dex.entries()) {
-                    if (name.startsWith("x.")) {
-                        val cls = loader.loadClass(name)
-                        if (InvocationHandler::class.java.isAssignableFrom(cls)) {
-                            clazz = cls
-                            break@loop
-                        }
+                    val cls = loader.loadClass(name)
+                    if (InvocationHandler::class.java.isAssignableFrom(cls)) {
+                        clazz = cls
+                        break@loop
                     }
                 }
             }
-            clazz ?: throw Exception("Cannot find SafetyNetHelper class")
+            clazz ?: throw Exception("Cannot find SafetyNetHelper implementation")
 
-            helper = clazz.getMethod("get", Class::class.java, Context::class.java, Any::class.java)
-                .invoke(null, SafetyNetHelper::class.java, context, this) as SafetyNetHelper
+            helper = Proxy.newProxyInstance(
+                loader, arrayOf(SafetyNetHelper::class.java),
+                clazz.newInstance() as InvocationHandler) as SafetyNetHelper
 
             if (helper.version != Const.SNET_EXT_VER)
                 throw Exception("snet extension version mismatch")
@@ -95,7 +91,7 @@ class CheckSafetyNetEvent(
         val random = SecureRandom()
         nonce = ByteArray(24)
         random.nextBytes(nonce)
-        helper.attest(nonce)
+        helper.attest(context, nonce, this)
     }
 
     // All of these fields are whitelisted
@@ -114,7 +110,7 @@ class CheckSafetyNetEvent(
             }
         }
         try {
-            svc.fetchSafetynet().byteStream().writeTo(apk)
+            svc.fetchSafetynet().byteStream().writeTo(jar)
             attest(context, abort)
         } catch (e: IOException) {
             abort(e)
@@ -147,7 +143,7 @@ class CheckSafetyNetEvent(
             Base64.decode(this, Base64.URL_SAFE)
     }
 
-    private fun String.parseJws(): SafetyNetResponse? {
+    private fun String.parseJws(): SafetyNetResponse {
         val jws = split('.')
         val secondDot = lastIndexOf('.')
         val rawHeader = String(jws[0].decode())
@@ -156,7 +152,8 @@ class CheckSafetyNetEvent(
         val signedBytes = substring(0, secondDot).toByteArray()
 
         val moshi = Moshi.Builder().build()
-        val header = moshi.adapter(JwsHeader::class.java).fromJson(rawHeader) ?: return null
+        val header = moshi.adapter(JwsHeader::class.java).fromJson(rawHeader)
+            ?: error("Invalid JWS header")
 
         val alg = when (header.algorithm) {
             "RS256" -> "SHA256withRSA"
@@ -165,41 +162,30 @@ class CheckSafetyNetEvent(
                 signature = ASN1Primitive.fromByteArray(signature).getEncoded(ASN1Encoding.DER)
                 "SHA256withECDSA"
             }
-            else -> return null
+            else -> error("Unsupported algorithm: ${header.algorithm}")
         }
 
         // Verify signature
-        val certB64 = header.certificates?.first() ?: return null
-        val certDer = certB64.decode()
-        val bis = ByteArrayInputStream(certDer)
-        val cert: X509Certificate
-        try {
-            cert = CryptoUtils.readCertificate(bis)
-            val verifier = Signature.getInstance(alg)
-            verifier.initVerify(cert.publicKey)
-            verifier.update(signedBytes)
-            if (!verifier.verify(signature))
-                return null
-        } catch (e: GeneralSecurityException) {
-            Timber.e(e)
-            return null
-        }
+        val certB64 = header.certificates?.first() ?: error("Cannot find certificate in JWS")
+        val bis = ByteArrayInputStream(certB64.decode())
+        val cert = CryptoUtils.readCertificate(bis)
+        val verifier = Signature.getInstance(alg)
+        verifier.initVerify(cert.publicKey)
+        verifier.update(signedBytes)
+        if (!verifier.verify(signature))
+            error("Signature mismatch")
 
         // Verify hostname
-        val hostNameVerifier = JsseDefaultHostnameAuthorizer(setOf())
-        try {
-            if (!hostNameVerifier.verify("attest.android.com", cert))
-                return null
-        } catch (e: IOException) {
-            Timber.e(e)
-            return null
-        }
+        val hostnameVerifier = JsseDefaultHostnameAuthorizer(setOf())
+        if (!hostnameVerifier.verify("attest.android.com", cert))
+            error("Hostname mismatch")
 
-        val response = moshi.adapter(SafetyNetResponse::class.java).fromJson(payload) ?: return null
+        val response = moshi.adapter(SafetyNetResponse::class.java).fromJson(payload)
+            ?: error("Invalid SafetyNet response")
 
         // Verify results
         if (!response.nonce.decode().contentEquals(nonce))
-            return null
+            error("nonce mismatch")
 
         return response
     }
@@ -207,7 +193,10 @@ class CheckSafetyNetEvent(
     override fun onResponse(response: String?) {
         if (response != null) {
             scope.launch(Dispatchers.Default) {
-                val res = response.parseJws()
+                val res = runCatching { response.parseJws() }.getOrElse {
+                    Timber.e(it)
+                    INVALID_RESPONSE
+                }
                 withContext(Dispatchers.Main) {
                     callback(SafetyNetResult(res))
                 }
@@ -231,3 +220,6 @@ data class SafetyNetResponse(
     val basicIntegrity: Boolean,
     val evaluationType: String = ""
 )
+
+// Special instance to indicate invalid SafetyNet response
+val INVALID_RESPONSE = SafetyNetResponse("", ctsProfileMatch = false, basicIntegrity = false)

app/src/main/java/com/topjohnwu/magisk/ui/safetynet/SafetyNetHelper.kt

@@ -1,10 +1,12 @@
 package com.topjohnwu.magisk.ui.safetynet
 
+import android.content.Context
+
 interface SafetyNetHelper {
 
     val version: Int
 
-    fun attest(nonce: ByteArray)
+    fun attest(context: Context, nonce: ByteArray, callback: Callback)
 
     interface Callback {
         fun onResponse(response: String?)

app/src/main/java/com/topjohnwu/magisk/ui/safetynet/SafetynetViewModel.kt

@@ -6,7 +6,7 @@ import com.topjohnwu.magisk.R
 import com.topjohnwu.magisk.arch.BaseViewModel
 import com.topjohnwu.magisk.utils.set
 
-data class SafetyNetResult(
+class SafetyNetResult(
     val response: SafetyNetResponse? = null,
     val dismiss: Boolean = false
 )
@@ -42,37 +42,43 @@ class SafetynetViewModel : BaseViewModel() {
 
     init {
         cachedResult?.also {
-            handleResponse(SafetyNetResult(it))
+            handleResult(SafetyNetResult(it))
         } ?: attest()
     }
 
     private fun attest() {
         isChecking = true
-        CheckSafetyNetEvent {
-            handleResponse(it)
-        }.publish()
+        CheckSafetyNetEvent(::handleResult).publish()
     }
 
     fun reset() = attest()
 
-    private fun handleResponse(response: SafetyNetResult) {
+    private fun handleResult(result: SafetyNetResult) {
         isChecking = false
 
-        if (response.dismiss) {
+        if (result.dismiss) {
             back()
             return
         }
 
-        response.response?.apply {
-            val result = ctsProfileMatch && basicIntegrity
+        result.response?.apply {
             cachedResult = this
-            ctsState = ctsProfileMatch
-            basicIntegrityState = basicIntegrity
-            evalType = if (evaluationType.contains("HARDWARE")) "HARDWARE" else "BASIC"
-            isSuccess = result
-            safetyNetTitle =
-                if (result) R.string.safetynet_attest_success
-                else R.string.safetynet_attest_failure
+            if (this === INVALID_RESPONSE) {
+                isSuccess = false
+                ctsState = false
+                basicIntegrityState = false
+                evalType = "N/A"
+                safetyNetTitle = R.string.safetynet_res_invalid
+            } else {
+                val success = ctsProfileMatch && basicIntegrity
+                isSuccess = success
+                ctsState = ctsProfileMatch
+                basicIntegrityState = basicIntegrity
+                evalType = if (evaluationType.contains("HARDWARE")) "HARDWARE" else "BASIC"
+                safetyNetTitle =
+                    if (success) R.string.safetynet_attest_success
+                    else R.string.safetynet_attest_failure
+            }
         } ?: {
             isSuccess = false
             ctsState = false