Only allow access to log file via explicitly created URIs

Since ContentProviders are public and permissions don't seem to work any
other application could access the log file.  With this token system
only URIs we explicitly created can be accessed.

src/frontends/android/AndroidManifest.xml

@@ -62,6 +62,9 @@
         <provider
             android:name=".data.LogContentProvider"
             android:authorities="org.strongswan.android.content.log" >
+            <!-- android:grantUriPermissions="true" combined with a custom permission does
+                 not work (probably too many indirections with ACTION_SEND) so we secure
+                 this provider with a custom ticketing system -->
         </provider>
     </application>
 

src/frontends/android/src/org/strongswan/android/data/LogContentProvider.java

@@ -17,6 +17,9 @@ package org.strongswan.android.data;
 
 import java.io.File;
 import java.io.FileNotFoundException;
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.strongswan.android.logic.CharonVpnService;
 
@@ -26,11 +29,15 @@ import android.database.Cursor;
 import android.database.MatrixCursor;
 import android.net.Uri;
 import android.os.ParcelFileDescriptor;
+import android.os.SystemClock;
 import android.provider.OpenableColumns;
 
 public class LogContentProvider extends ContentProvider
 {
 	private static final String AUTHORITY = "org.strongswan.android.content.log";
+	/* an Uri is valid for 30 minutes */
+	private static final long URI_VALIDITY = 30 * 60 * 1000;
+	private static ConcurrentHashMap<Uri, Long> mUris = new ConcurrentHashMap<Uri, Long>();
 	private File mLogFile;
 
 	public LogContentProvider()
@@ -50,7 +57,17 @@ public class LogContentProvider extends ContentProvider
 	 */
 	public static Uri createContentUri()
 	{
-		Uri uri = Uri.parse("content://"+ AUTHORITY + "/" + CharonVpnService.LOG_FILE);
+		SecureRandom random;
+		try
+		{
+			random = SecureRandom.getInstance("SHA1PRNG");
+		}
+		catch (NoSuchAlgorithmException e)
+		{
+			return null;
+		}
+		Uri uri = Uri.parse("content://" + AUTHORITY + "/" + random.nextLong());
+		mUris.put(uri, SystemClock.uptimeMillis());
 		return uri;
 	}
 
@@ -71,6 +88,11 @@ public class LogContentProvider extends ContentProvider
 		{
 			return null;
 		}
+		Long timestamp = mUris.get(uri);
+		if (timestamp == null)
+		{	/* don't check the validity as this information is not really private */
+			return null;
+		}
 		MatrixCursor cursor = new MatrixCursor(projection, 1);
 		if (OpenableColumns.DISPLAY_NAME.equals(cursor.getColumnName(0)))
 		{
@@ -90,7 +112,17 @@ public class LogContentProvider extends ContentProvider
 	@Override
 	public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException
 	{
-		return ParcelFileDescriptor.open(mLogFile, ParcelFileDescriptor.MODE_CREATE | ParcelFileDescriptor.MODE_READ_ONLY);
+		Long timestamp = mUris.get(uri);
+		if (timestamp != null)
+		{
+			long elapsed = SystemClock.uptimeMillis() - timestamp;
+			if (elapsed > 0 && elapsed < URI_VALIDITY)
+			{	/* we fail if clock wrapped, should happen rarely though */
+				return ParcelFileDescriptor.open(mLogFile, ParcelFileDescriptor.MODE_CREATE | ParcelFileDescriptor.MODE_READ_ONLY);
+			}
+			mUris.remove(uri);
+		}
+		return super.openFile(uri, mode);
 	}
 
 	@Override