Author: AlexIonescu

# Views: 7170

I’ve been a bit slow updating the blog, and so today, I want to take the time to explain what’s been keeping me busy by shareing some exciting news. As this post’s title suggests, I am indeed co-authoring Windows Internals 5th Edition, the latest update to Mark Russinovich and David Solomon’s Windows Internals 4th Edition book.

The book will mark a return to the previous format of the series — unlike the last edition which covered all supported Windows NT operating systems (2000, XP, 2003), this edition will only cover the Vista and Server 2008 operating systems. This is a great change, because it means less time is explaining minute differences between the 4 different algorithms used in a lookup in each version, and more time is spent talking about what really matters — the behavior and design decisions of the OS.

I’m happy to say this new edition will have a least 250 pages of new information, not only updating various chapters with new Vista/Server 2008 changes, but also adding entirely new sections which previous editions had never touched on, such as the user mode loader in Ntdll.dll, the user mode debugging framework, and the hypervisor. It isn’t only the internals information that will benefit from the update; as a matter of fact, all references to tools, resources and books have also been updated, including up-to-date information on the latest Sysinternals tools, as well as exciting, helpful new experiments that demonstrate the behaviors explained in the text. For those of you who have read Mark’s TechNet Magazine “Inside Vista Kernel Changes” series and its Server 2008 counterpart (for those who haven’t, I strongly suggest you do!), you’ll be glad to know that the book includes all that information and expands on it as well.

The book work is still ongoing, but planned to end soon, after which it should go to print in October and be on shelves in January 2009. My book work is about to reach its one-year anniversary, and I must say that working with Mark and David has been a pleasurable learning experience, as well as a great chance to continue my reverse engineering work and hone my skills. What made ReactOS fun was being able to share my discoveries with the world as code — the book work has allowed me to share that information as text, part of the best internals book available, to a much wider audience. I’m thankful for that, and I can’t wait for everyone to have a chance to read it!

Look for the book hitting your nearest bookstore just after New Year’s. As a sneak peak, here’s a high-quality copy of the book’s cover while you wait.

Author: AlexIonescu

# Views: 6968

This year I had the chance to present some security-related findings that I had made earlier during the year inside Win32k.sys, the Windows GUI Subsystem and Window Manager. I presented a total of four bugs, all local Denial of Service (DoS) attacks. Two of these attacks could be done on any system up to Vista SP0 (one up to Server 2008) from any account, while the other two were NULL-pointer dereferences that required administrative access to exploit (however, since they are NULL-pointer dereferences, the risk for user->kernel elevation exists in one of them).

Because the first two attacks (from any guest account) relied on highly internal behavior of the Windows NT Kernel (used in all of Microsoft’s modern operating systems today), I thought it was interesting to talk a bit about the internals themselves, and then present the actual bug, as well as some developer guidance on how to avoid hitting this bug.

I’d like to thank everyone who came to see the talk, and I really appreciated the questions and comments I received. I’ve also finally found out why Win32k functions are called “xxx” and “yyy”. Now I just need to find out about “zzz”. I’ll probably share this information in a later post, when I can talk more about Win32k internals.

As promised, you’ll find below a link to the slides. As for proof of concept code, it’s currently sitting on my home machine, so it’ll take another week. On the other hand, the slides make three of the bugs very clear in pseudo-code. I’ll re-iterate them at the bottom of the post.

One final matter — it seems I have been unable to make the [email protected] email work due to issues with my host. For the moment, please use another e-mail to contact me, such as my Google Mail account. My email address is the first letter of my first name (Alex) followed by my entire last name (Ionescu).

Slides here: Keynote and PDF.

Bug 1:
1) Handle = OpenProcess(Csrss)
2) CreateRemoteThread(Handle, …, NtUserGetThreadState, 15, …)

Bug 2:
1) Handle = CreateWindowStation(…)
2) SetHandleInformation(Handle, ProtectFromClose)
3) CloseWindowStation(Handle)

Bug 3:
1) Handle = CreateWindowStation(…)
2) Loop all handles in the process with the NtQuerySystemInformation API, or, just blindly loop every handle from 4 to 16 million and mark it as protected, or, use Process Explorer to see what the current Window Station handle is, and hard-code that for this run. The point is to find the handle to the current window station. Usually this will be a low number, typically under 0×50.
3) SetHandleInformation(CurrentWinstaHandle, ProtectFromClose)
4) SwitchProcessWindowStation(Handle);

Author: AlexIonescu

# Views: 8155

In part 1 of the series, we covered some of the changes behind Vista’s new Session 0 Isolation and showcased the UI Detection Service. Now, we’ll look at the internals behind this compatibility mechanism and describe its behavior.

First of all, let’s take a look at the service itself — although its file name suggests the name “UI 0 Detection Service”, it actually registers itself with the Service Control Manager (SCM) as the “Interactive Services Detection” service. You can use the Services.msc MMC snap-in to take a look at the details behind the service, or use Sc.exe along with the service’s name (UI0Detect) to query its configuration. In both cases, you’ll notice the service is most probably “stopped” on your machine — as we’ve seen in Part 1, the service is actually started on demand.

But if the service isn’t actually running yet seems to catch windows appearing on Session 0 and automatically activate itself, what’s making it able to do this? The secret lies in the little-known Wls0wndh.dll, which was first pointed out by Scott Field from Microsoft as a really bad name for a DLL (due to its similarity to a malicious library name) during his Blackhat 2006 presentation. The version properties for the file mention “Session0 Viewer Window Hook DLL” so it’s probably safe to assume the filename stands for “WinLogon Session 0 Window Hook”. So who loads this DLL, and what is its purpose? This is where Wininit.exe comes into play.

Wininit.exe too is a component of the session 0 isolation done in Windows Vista — because session 0 is now unreachable through the standard logon process, there’s no need to bestow upon it a fully featured Winlogon.exe. On the other hand, Winlogon.exe has, over the ages, continued to accumulate more and more responsibility as a user-mode startup bootstrapper, doing tasks such as setting up the Local Security Authority Process (LSASS), the SCM, the Registry, starting the process responsible for extracting dump files from the page file, and more. These actions still need to be performed on Vista (and even more have been added), and this is now Wininit.exe’s role. By extension, Winlogon.exe now loses much of its non-logon related work, and becomes a more agile and lean logon application, instead of a poorly modularized module for all the user-mode initialization tasks required on Windows.

One of Wininit.exe’s new tasks (that is, tasks which weren’t simply grandfathered from Winlogon) is to register the Session 0 Viewer Window Hook Dll, which it does with an aptly named RegisterSession0ViewerWindowHookDll function, called as part of Wininit’s WinMain entrypoint. If the undocumented DisableS0Viewer value isn’t present in the HKLMSystemCurrentControlSetControlWinInit registry key, it attempts to load the aforementioned Wls0wndh.dll, then proceeds to query the address of the Session0ViewerWindowProcHook inside it. If all succeeds, it switches the thread desktop to the default desktop (session 0 has a default desktop and a Winlogon desktop, just like other sessions), and registers the routine as a window hook with the SetWindowsHookEx routine. Wininit.exe then continues with other startup tasks on the system.

I’ll assume readers are somewhat familiar with window hooks, but this DLL providesjust a standard run-of-the-mill systemwide hook routine, whose callback will be notified for each new window on the desktop. We now have a mechanism to understand how it’s possible for the UI Detection Service to automatically start itself up — clearly, the hook’s callback must be responsible! Let’s look for more clues.

Inside Session0ViewerWindowProcHook, the logic is quite simple: a check is made on whether or not this is a WM_SHOWWINDOW window message, which signals the arrival of a new window on the desktop. If it is, the DLL first checks for the $$$UI0Background window name, and bails out if this window already exists. This window name is created by the service once it’s already started up, and the point of this check is to avoid attempting to start the service twice.

The second check that’s done is how much time has passed since the last attempt to start the service — because more than a window could appear on the screen during the time the UI Detection Service is still starting up, the DLL tries to avoid sending multiple startup requests.
If it’s been less than 300 seconds, the DLL will not start the service again.

Finally, if all the checks succeed, a work item is queued using the thread pool APIs with the StartUI0DetectThreadProc callback as the thread start address. This routine has a very simple job: open a handle to the SCM, open a handle to the UI0Detect service, and call StartService to instruct the SCM to start it.

This concludes all the work performed by the DLL — there’s no other code apart from these two routines, and the DLL simply acknowledges window hook callbacks as they come through in cases where it doesn’t need to start the service. Since the service is now started, we’ll turn our attention to the actual module responsible for it — UI0Detect.exe.

Because UI0Detect.exe handles both the user (client) and service part of the process, it operates in two modes. In the first mode, it behaves as a typical Windows service, and registers itself with the SCM. In the second mode, it realizes that is has been started up on a logon session, and enters client mode. Let’s start by looking at the service functionality.

If UI0Detect.exe isn’t started with a command-line, then this means the SCM has started it at the request of the Window Hook DLL. The service will proceed to call StartServiceCtrlDispatcher with the ServiceStart routine specific to it. The service first does some validation to make sure it’s running on the correct WinSta0Default windowstation and desktop and then notifies the SCM of success or failure.

Once it’s done talking to the SCM, it calls an internal function, SetupMainWindows, which registers the $$$UI0Background class and the Shell0 Window window. Because this is supposed to be the main “shell”
window that the user will be interacting with on the service desktop (other than the third-party or legacy service), this window is also registered as the current Shell and Taskbar window through the SetShellWindow and SetTaskmanWindow APIs. Because of this, when Explorer.exe is actually started up through the trick mentioned in Part 1, you’ll notice certain irregularities — such as the task bar disappearing at times. These are caused because Explorer hasn’t been able to properly register itself as the shell (the same APIs will fail when Explorer calls them). Once the window is created and a handler is setup (BackgroundWndProc), the new Vista “Task” Dialog is created and shown, after which the service enters a typical GetMessage/DispatchMessage window message handling loop.

Let’s now turn our attention to BackgroundWndProc, since this is where the main initialization and behavioral tasks for the service will occur. As soon as the WM_CREATE message comes through, UI0Detect will use the RegisterWindowMessage API with the special SHELLHOOK parameter, so that it can receive certain shell notification messages. It then initializes its list of top level windows, and registers for new session creation notifications from the Terminal Services service. Finally, it calls SetupSharedMemory to initialize a section it will use to communicate with the UI0Detect processes in “client” mode (more on this later), and calls EnumWindows to enumerate all the windows on the session 0 desktop.

Another message that this window handler receives is WM_DESTROY, which, as its name implies, unregisters the session notification, destroys the window lists, and quits.

This procedure also receives the WM_WTSSESSION_CHANGE messages that it registered for session notifications. The service listens either for remote or local session creates, or for disconnections. When a new session has been created, it requests the resolution of the new virtual screen, so that it knows where and how to display its own window. This functionality exists to detect when a switch to session 0 has actually been made.

Apart from these three messages, the main “worker” message for the window handler is WM_TIMER. All the events we’ve seen until now call SetTimer with a different parameter in order to generate some sort of notification. These notifications are then parsed by the window procedure, either to handle returning back to the user’s session, to measure whether or not there’s been any input in session 0 lately, as well as to handle dynamic window create and destroy operations.

Let’s see what happens during these two operations. The first time that window creation is being acted upon is during the afore-mentionned EnumWindows call, which generates the initial list of windows. The function only looks for windows without a parent, meaning top-level windows, and calls OnTopLevelWindowCreation to analyze the window. This analysis consists of querying the owner of the window, getting its PID, and then querying module information about this process. The version information is also extracted, in order to get the company name. Finally, the window is added to a list of tracked windows, and a global count variable is incremented.

This module and version information goes into the shared memory section we briefly described above, so let’s look into it deeper. It’s created by SetupSharedMemory, and actually generates two handles. The first handle is created with SECTION_MAP_WRITE access, and is saved internally for write access. The handle is then duplicated with SECTION_MAP_READ access, and this handle will be used by the client.

What’s in this structure? The type name is UI0_SHARED_INFO, and it contains several informational fields: some flags, the number of windows detected on the session 0 desktop, and, more importantly, the module, window, and version information for each of those windows. The function we just described earlier (OnTopLevelWindowCreation) uses this structure to save all the information it detects, and the OnTopLevelWindowDestroy does the opposite.

Now that we understand how the service works, what’s next? Well, one of the timer-related messages that the window procedure is responsible for will check whether or not the count of top level windows is greater than 0. If it is, then a call to CreateClients is made, along with the handle to the read-only memory mapped section. This routine will call the WinStationGetSessionIds API and enumerate through each ID, calling the CreateClient routine. Logically, this is so that each active user session can get the notification.

Finally, what happens in CreateClient is pretty straightforward: the WTSQueryUserToken and ImpersonateLoggedOnUser APIs are used to get an impersonation logon token corresponding to the user on the session ID given, and a command line for UI0Detect is built, which contains the handle of the read-only memory mapped section. Ultimately, CreateProcessAsUser is called, resulting in the UI0Detect process being created on that Session ID, running with the user’s credentials.

What happens next will depend on user interaction with the client, as the service will continue to behave according to the rules we’ve already described, detecting new windows as they’re being created, and detection destroyed windows as well, and waiting for a notification that someone has logged into session 0.

We’ll follow up on the client-mode behavior on Part 3, where we’ll also look at the UI0_SHARED_INFO structure and an access validation flaw which will allow us to spoof the client dialog.

Author: AlexIonescu

# Views: 8451

One of the many exciting changes in Windows Vista’s service security hardening mechanisms (which have been aptly explained and documented in multiple blogs and whitepapers , so I’ll refrain from rehashing old material) is Session 0 Isolation. I’ve thought it would be useful to talk about this change and describe the behaviour and implementation of the UI Detection Service (UI0Detect), an important part of the infrastructure in terms of providing compatible behaviour with earlier versions of Windows.

As a brief refresher or introduction to Session 0 Isolation, let’s remember how services used to work on previous versions of Windows: you could run them under various accounts (the most common being System, Local Service and Network Service), and they ran in the same session the console user, which was logged-on to session 0 as well. Services were not supposed to display GUIs, but, if they really had to, they could be marked as interactive, meaning that they could display windows on the interactive window station for session 0.

Windows implemented this by allowing such services to connect to the Winsta0 Windowstation , which is the default interactive Windowstation for the current session — unlike non-interactive services, which belonged to a special “Service-0×0-xxx$” Windowstation, where xxx was a logon session identifer (you can look at the WDK header ntifs.h for a list of the built-in account identifiers (LUIDs)). You can see the existence of these windowstations by enumerating them in the object manager namespace with a tool such as Sysinternals’ WinObj.

Essentially, this meant three things: applications could either do Denial of Service attacks against named objects that the service would expect to own and create, they could feed malicious data to objects such as sections which were incorrectly secured or trusted by the service, and , for interactive services, they could also attempt shatter attacks — sending window messages with executable payloads in their buffer, exploting service bugs and causing the payload code to execute at higher privileges.

Session 0 Isolation puts an end to all that, by first having a separate session for the console user (any user session starts at 1, thus protecting named objects), and second, by disabling support for interactive services — that is, even though services may still display a UI, it won’t appear on any user’s desktop (instead, it will appear on the default desktop of the session 0 interactive windowstation).

That’s all fine and dandy for protecting the objects, but what if the service hasn’t been recompiled not to directly show a UI (but to instead use a secondary process started with CreateProcessAsUser, or to use the WTSSendMessage API) and depends on user input before continuing? Having a dialog box on the session 0 desktop without the user’s awareness would potentially have significant application compatibility issues — this is where the UI Detection Service comes into play.

If you’re like most Vista users, you’ve actually probably never seen the default desktop on session 0’s interactive windowstation in your life (or in simpler words, never “logged-on” or “switched to” session 0)! Since you can’t log on to it, and since interactive services which displayed UIs are thankfully rare, it remains a hidden mystery of Windows Vista, unless you’ve encountered such a service. So let’s follow Alice down the rabbit hole into session 0 wonderland, with some simple Service Controller (Sc.exe) commands to create our very own interactive service.

Using an elevated command prompt, create a service called RabbitHole with the following command:

sc create RabbitHole binpath= %SYSTEMROOT%system32 otepad.exe type= interact type= own

Be careful to get the right spaces — there’s a space after each equal sign! You should expect to get a warning from Sc.exe, notifying you of changes in Windows Vista and later (the ones I’ve just described).

Now let’s start the service, and see what happens:

sc start RabbitHole

If all went well, Sc.exe should appear as if it’s waiting on the command to complete, and a new window should appear on your taskbar (it does not appear in the foreground). That window is a notification from the UI Detection Service, the main protagonist of this story.

Get ready to click on “Show me the Message” as soon as you can! Starting an essentialy fake service through Sc.exe will eventually annoy the Service Control Manager (SCM), causing it to kill notepad behind your back (don’t worry if this happens, just use the sc start RabbitHole command again).

You should now be in Session 0 (and probably unable to read the continuation of this blog, in which case the author hopes you’ve been able to find your way back!) As you can notice, Session 0 is a rather deserted place, due to the lack of any sort of shell or even the Theme service, creating a Windows 2000-like look that may bring back tears of joy (or agony) to the more nostalgic of users.

On the other hand, this desolate session it does contain our Notepad, which you should’ve seen disappear if you stayed long enough — that would be the SCM reaching its timeout of how long it’s willing to wait around hoping for Notepad to send a “service start” message back (which it never will).

Note that you can’t start any program you want on Session 0 — Cmd.exe and Explorer.exe are examples of programs that for one reason or another won’t accept to be loaded this way. However, if you’re quick enough, you can use an old trick common to getting around early 90ies “sandbox” security applications found in many libraries and elementary schools — use the common dialog control (from File, Open) to browse executable files (switch the file type to *.*, All Files), go to the System32 folder, right-click on Explorer.exe, and select Open. Notepad will now spawn the shell, and even if the SCM kills Notepad, it will remain active — feel free to browse around (try to be careful not to browse around in IE too much, you are running with System privileges!)

That’s it for this introduction to this series. In part 2, we’ll look at what makes this service tick, and in part 3, we’ll look at a technique for spoofing the dialog to lie to the user about which service is actually requesting input. For now, let’s delete the RabbitHole, unless you want to keep it around for impressing your colleagues:

sc delete RabbitHole

Author: AlexIonescu

# Views: 7958

Not all of a reverse engineer’s life has to be about undoing — sometimes it is equally as fun to build something from scratch, whether that means a new tool… or the Star Wars 30 Year Anniversary Lego Ultimate Collector’s Millennium Falcon! Over the course of the last three weeks, my best friend and myself have spent countless hours building this magnificent model, which has over 5000 pieces, 91 “major” construction steps (with each step taking up to 30 sub-steps, sometimes 2x’d or 4x’d) and a landscape, 8×14″, 310 page instruction manual.

Late last night, we completed the final pieces of the hull, the radar dish, and the commemorative plaque (itself made of Lego). We had previously built the Imperial Star Destroyer (ISD) last year, but nothing in our Lego-building lives had ever quite come close to the work we put into this set. Complete full-size pictures after the entry.

Along the way, we both learnt some important facts about the Lego manufacturing process — for example, we had already noticed that in our ISD set had some extra pieces, and that other people’s sets had different extra pieces, however, we weren’t too sure what to make of it. This year however, due to the fact we were missing exactly half the number of lever pieces and 1×2 “zit” pieces, we did some extra digging.

The first that boggles many Lego builders of such large sets, is the arrangement of pieces within the bags. These pieces are not arranged in construction order. If you break all the bags and sort the pieces, there is nothing wrong with doing so! Whether or not that will save you time however, is up for discussion. We did do a small sorting, mostly to separate hull pieces from thick pieces from greebling pieces, and that did seem to help a lot. However, I wouldn’t recommend spending 5 hours sorting the pieces as was done on Gizmodo.

By step 21 unfortunately, we noticed we were missing a piece — something that Lego says should never happen (more on this shortly). Step 22 also required that piece, and the next few steps required even more. I came up with an interesting idea — a missing bag. We counted the number of pieces we’d still need, and it came up to be exactly the number of pieces we had used. In other words, half of the pieces were missing. Those pieces were in the same bag as the level-looking pieces, and those too, after calculations, were missing half their number. By this point, we were sure that we were missing a bag, and went to check Lego’s site for help.

It turns out that Lego’s site does have a facility available for ordering missing set pieces, and for free! After putting in the appropriate information, including the set number, and piece number, I filled out my address… and received a “Thank you”. Unfortunately, we never got any further confirmation, and the missing pieces have yet to arrive. Most interesting however, was a notice on Lego’s website, claiming that missing pieces are quite rare, because each set is precision-weighted, so missing sets get flagged for extra, human QA. At first sight, this makes it very unlikely for a set to be missing a piece — so how did we end up missing nearly 60 pieces? (Note to readers: the missing pieces are all hull greeblin (decorative, not structural), and we dully marked down the steps at which they were required, so we can add them once we receive them).

The answer to that question only came to us once we completed construction of the Falcon. There it was, on our workshop table (my kitchen table): an opened bag, full of Lego pieces… which we had already used up, and which weren’t required! It then became pretty obvious to us that Lego has a major flaw in their weight-based reasoning: replacement! We couldn’t scientifically verify it, but the extra bag we had was of similar size and weight (being small pieces) to the second bag of levels and 1×2 pieces we needed. Evidently, a machine error (most probably) or human error caused an incorrect bag of pieces to be added to the set. At the QA phase, the set passed the weight tests, because this bag was of the same (or nearly the same) weight as the missing bag!

Furthermore, due to the fact both the ISD and the Falcon had pieces that were not included in the Appendix of piece counts (again, probably due to machine error while composing the set), their combined weight may have even pushed the weight of our set past the expected weight. It is unlikely that Lego would flag heavier sets for QA — at worst, the customer would get some free pieces of Lego. However, when that weight helps offset the weight of missing pieces, it can certainly become a problem. And when a bag of pieces is accidentally replaced by a similar bag, then weight measuring doesn’t do much at all.

Granted, a lot of our analysis is based on assumptions, but they certainly do check out. Lego says their primary method of checking sets is to weigh them, and we have an extra bag, and a missing bag, both of similar size and composure. The hypothesis seems valid, and perhaps a phone call to Lego will confirm it (if I don’t get the pieces soon, I certainly plan on doing that).

Ironically, this breakage of a QA test through replacement was similar to an interesting security question I received by mail recently: why is it a bad idea to use the Owner SID of an object as a way to authenticate the object, or its creator? It turns out this can leave you vulnerable to rename operations, such as on a file with write and delete access, allowing someone to impersonate the Owner SID but have their own data in the object. Breaking a security test by renaming an object, or breaking a quality assurance test by replacing a bag — the two stem from the same problem: bad design and simplistic assumptions.

And now, without further ado, here’s a link to our construction pictures.