Add protection against migrating to a nested folder

osu.Game.Tests/NonVisual/CustomDataDirectoryTest.cs

@@ -220,6 +220,30 @@ namespace osu.Game.Tests.NonVisual
             }
         }
 
+        [Test]
+        public void TestMigrationToNestedTargetFails()
+        {
+            using (HeadlessGameHost host = new CleanRunHeadlessGameHost(nameof(TestMigrationToSameTargetFails)))
+            {
+                try
+                {
+                    var osu = loadOsu(host);
+
+                    Assert.DoesNotThrow(() => osu.Migrate(customPath));
+
+                    string subFolder = Path.Combine(customPath, "sub");
+
+                    Directory.CreateDirectory(subFolder);
+
+                    Assert.Throws<ArgumentException>(() => osu.Migrate(subFolder));
+                }
+                finally
+                {
+                    host.Exit();
+                }
+            }
+        }
+
         private OsuGameBase loadOsu(GameHost host)
         {
             var osu = new OsuGameBase();

osu.Game/IO/OsuStorage.cs

@@ -48,6 +48,12 @@ namespace osu.Game.IO
             var source = new DirectoryInfo(GetFullPath("."));
             var destination = new DirectoryInfo(newLocation);
 
+            if (source.FullName == destination.FullName)
+                throw new ArgumentException("Destination provided is already the current location", nameof(newLocation));
+
+            if (destination.FullName.Contains(source.FullName))
+                throw new ArgumentException("Destination provided is inside the source", nameof(newLocation));
+
             // ensure the new location has no files present, else hard abort
             if (destination.Exists)
             {