tunnel: avoid race between shutdown and stats

wgTurnOff can block for a while, in which case, calling getStatistics
will use a stale handle and stale tunnel. Not only that, but wgGetConfig
might return null, in which case string.split throws.

    java.lang.NullPointerException: Attempt to invoke virtual method
    'java.lang.String[] java.lang.String.split(java.lang.String)' on a null
    at com.wireguard.android.backend.GoBackend.getStatistics

Reported-by: tomt@adslweb.co.uk
Link: https://lists.zx2c4.com/pipermail/wireguard/2021-May/006709.html
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java b/tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java
index c7381487..8b9213db 100644
--- a/tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java
+++ b/tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java
@@ -68,7 +68,7 @@ public final class GoBackend implements Backend {
         alwaysOnCallback = cb;
     }
 
-    private static native String wgGetConfig(int handle);
+    @Nullable private static native String wgGetConfig(int handle);
 
     private static native int wgGetSocketV4(int handle);
 
@@ -115,10 +115,11 @@ public final class GoBackend implements Backend {
     @Override
     public Statistics getStatistics(final Tunnel tunnel) {
         final Statistics stats = new Statistics();
-        if (tunnel != currentTunnel) {
+        if (tunnel != currentTunnel || currentTunnelHandle == -1)
             return stats;
-        }
         final String config = wgGetConfig(currentTunnelHandle);
+        if (config == null)
+            return stats;
         Key key = null;
         long rx = 0;
         long tx = 0;
@@ -294,11 +295,11 @@ public final class GoBackend implements Backend {
                 Log.w(TAG, "Tunnel already down");
                 return;
             }
-
-            wgTurnOff(currentTunnelHandle);
+            int handleToClose = currentTunnelHandle;
             currentTunnel = null;
             currentTunnelHandle = -1;
             currentConfig = null;
+            wgTurnOff(handleToClose);
         }
 
         tunnel.onStateChange(state);