Implement initial policy for memory tag checks.

System apps and the system_server receive async tag checks, while all other
app processes have it disabled. Developers may enable async tag checks
per application with:

$ adb shell am compat 135772972 <app.name>

Bug: 135772972
Change-Id: I154623941eec8e79af347453fbca1b062346c85b
Merged-In: I154623941eec8e79af347453fbca1b062346c85b

2 files changed, 14 insertions, 3 deletions

diff --git a/core/java/com/android/internal/os/Zygote.java b/core/java/com/android/internal/os/Zygote.java
index 5f196a0e4c1c..f7ac8e801f36 100644
--- a/core/java/com/android/internal/os/Zygote.java
+++ b/core/java/com/android/internal/os/Zygote.java
@@ -977,4 +977,9 @@ public final class Zygote {
      */
     @FastNative
     public static native int nativeParseSigChld(byte[] in, int length, int[] out);
+
+    /**
+     * Returns whether the hardware supports memory tagging (ARM MTE).
+     */
+    public static native boolean nativeSupportsMemoryTagging();
 }
diff --git a/core/java/com/android/internal/os/ZygoteInit.java b/core/java/com/android/internal/os/ZygoteInit.java
index 300f71af5dd5..9d7d9625a944 100644
--- a/core/java/com/android/internal/os/ZygoteInit.java
+++ b/core/java/com/android/internal/os/ZygoteInit.java
@@ -781,9 +781,15 @@ public class ZygoteInit {
             Zygote.applyDebuggerSystemProperty(parsedArgs);
             Zygote.applyInvokeWithSystemProperty(parsedArgs);
 
-            /* Enable pointer tagging in the system server unconditionally. Hardware support for
-             * this is present in all ARMv8 CPUs; this flag has no effect on other platforms. */
-            parsedArgs.mRuntimeFlags |= Zygote.MEMORY_TAG_LEVEL_TBI;
+            if (Zygote.nativeSupportsMemoryTagging()) {
+                /* The system server is more privileged than regular app processes, so it has async
+                 * tag checks enabled on hardware that supports memory tagging. */
+                parsedArgs.mRuntimeFlags |= Zygote.MEMORY_TAG_LEVEL_ASYNC;
+            } else {
+                /* Enable pointer tagging in the system server. Hardware support for this is present
+                 * in all ARMv8 CPUs; this flag has no effect on other platforms. */
+                parsedArgs.mRuntimeFlags |= Zygote.MEMORY_TAG_LEVEL_TBI;
+            }
 
             if (shouldProfileSystemServer()) {
                 parsedArgs.mRuntimeFlags |= Zygote.PROFILE_SYSTEM_SERVER;