Hiding execution of unsigned code in system threads
Anti-cheat development is, by nature, reactive; anti-cheats exist to respond to and thwart a videogame’s population of cheaters. For instance, a videogame with an exceedingly low amount of cheaters would have little need for an anti-cheat, while a videogame rife with cheaters would have a clear need for an anti-cheat. In order to catch cheaters, anti-cheats will employ as many methods as possible. Unfortunately, anti-cheats are not omniscient; they can not know of every single method or detection vector to catch cheaters. Likewise, the game hacks themselves must continue to discover new or unique methods in order to evade anti-cheats.
The Reactive Development Cycle of Game Hacking
This brings forth a reactive and continuous development cycle, for both the cheats and anti-cheats: the opposite party (cheat or anti-cheat) will employ a unique method to circumvent the adjacent party (anti-cheat or cheat) which, in response, will then do the same.
One such method employed by an increasing number of anti-cheats is to execute core anti-cheat functions from within the operating system’s kernel. A clear advantage over the alternative (i.e. usermode execution) is in the fact that, on Windows NT systems, the anti-cheat can selectively filter which processes are able to interact with the memory of the game process in which they are protecting, thus nullifying a plethora of methods used by game hacks.
In response to this, many (but not all) hack developers made (or are making) the decision to do the same; they too would, or will, execute their hack, either wholly or in part, from within the operating system’s kernel, thus nullifying what the anti-cheats had done.
Unlike with anti-cheats, however, this decision carries with it numerous concessions: namely, the fact that, for various reasons, it is most convenient (or it is only practical) to execute the hack as an unsigned kernel driver running without the kernel’s knowledge; the “driver” is typically a region of executable memory in the kernel’s address space and is never loaded or allocated by the kernel. In other words, it is a “manually-mapped” driver, loaded by a tool used by a game hack.
This ultimately provides anti-cheats with many opportunities to detect so-called “kernel-mode” or “ring 0” game hacks (noting that those terms are typically said with a marketable significance; they are literally used to market such game hacks, as if to imply robustness or security); if the anti-cheat can prove that the system is executing, or had executed, unsigned code, it can then potentially flag a user as being a cheater.
Analyzing a Thread’s Kernel Stack
One such method — the focus of this article, in fact — of detecting unsigned code execution in the kernel is to iterate each thread that is running in the system (optionally deciding to only iterate threads associated with the system process, i.e. system threads) and to initiate some kind of stack trace.
Bluntly, this allows the anti-cheat to quite effectively determine if a cheat were executing unsigned code. For example, some anti-cheats (e.g. BattlEye) will queue to each system thread an APC which will then initiate a stack trace. If the stack trace returns an instruction pointer that is not within the confines of any loaded kernel driver, the anti-cheat can then know that it may have encountered a system thread that is executing unsigned code. Furthermore, because it is a stack trace and not a direct sampling of the return instruction pointer, it would work quite reliably, even if a game hack were, for example, executing a spin-loop or continuous wait; the stack trace would always lead back to the unsigned code.
It is quite clear to any cheat developer that they can respond to this behavior by simply running their thread(s) with kernel APCs disabled, preventing delivery of such APCs and avoiding the detection vector. As is will be seen, however, this method does not entirely prevent detection of unsigned code execution.
(Copying Out, Then) Analyzing a Thread’s Kernel Stack
Certain anti-cheats — EasyAntiCheat, in particular — had a much more apt method of generating a pseudo-stacktrace: instead of generating a stack trace with a blockable APC, why not copy the contents of the thread’s kernel stack asynchronously? Continuing the reactive cheat-anti-cheat development cycle, EasyAntiCheat had opted to manually search for instances of nonpaged code pointers that may have been left behind as a result of system thread execution.
While the downsides of this method are debatable, the upside is quite clear: as long as the thread is making procedure calls (e.g. x86 call instruction) from within its own code, either to kernel routines or to its own, and regardless of its IRQL or if the thread is even running, its execution will leave behind detectable traces on its stack in the form of pointers to its own code which can be extracted and analyzed.
Callouts: Continuing The Reactive Development Cycle
Proposed is the “callout” method of system thread execution, born from the recognition that:
- A thread’s kernel stack, as identified by the kernel stack pointer in a thread’s ETHREAD object, can be analyzed asynchronously by a potential anti-cheat to detect traces of unsigned code execution; and that
- To be useful in most cases, a system thread must be able to make calls to most external NT kernel or executive procedures with little compromise.
The Life-cycle of the Callout Thread
The life-cycle of a callout thread is quite simple and can be used to demonstrate its implementation:
- Before thread creation:
- Allocate a non-paged stack to be loaded by the thread; the callout thread’s “real stack”
- Allocate shellcode (ideally in executable memory not associated with the main driver module) which disables interrupts, preserves the old/kernel stack pointer (as it was on function entry), loads the real stack, and jumps to an initialization routine (the callout thread’s “bootstrap routine”)
- Create a system thread (i.e. PsCreateSystemThread) whose start address points to the initialization shellcode
- At thread entry (i.e. the bootstrap routine):
- Preserve the stack pointer that had been given to the thread at thread entry (this must be given by the shellcode)
- (Optionally) Iterate the thread’s old/kernel stack pointer, ceasing iteration at the stack base, eliminating any references/pointers to the initialization shellcode
- (Optionally) Eliminate references to the initialization shellcode within the thread’s ETHREAD; for example, it may be worth changing the thread’s start address
- (Optionally, but recommended) Free the memory containing the initialization shellcode, if it was allocated separately from the driver module
- Proceed to thread execution
In clearer terms, the callout thread spends most of its time executing the driver’s unsigned code with interrupts disabled and with its own kernel stack — the real stack. It can also attempt to wipe any other traces of its execution which may have been present upon its creation.
The Usefulness of the Callout Thread
The callout thread must also be capable of executing most, if not all, NT kernel and executive procedures. As proposed, this is effectively impossible; the thread must run with interrupts disabled and with its own stack, thus creating an obvious problem as most procedures of interest would run at an IRQL <= DISPATCH_LEVEL. Furthermore, the NT IRQL model may be liable to ignore our setting of the interrupt flag, causing most routines to unpredictibly enter a deadlock or enable interrupts without our consent.
A mechanism to allow for a callout thread to invoke these routines of interest, the callout mechanism, is therefore used to:
- Provide a routine which can be used to conveniently invoke (“call out”) an external function; and in this routine,
- Load the thread’s original/kernel stack pointer;
- Copy function arguments on to the kernel thread’s stack from the real stack;
- Enable interrupts;
- Invoke the requested routine (within the same instruction boundary as when interrupts are enabled);
- Cleanly return from the routine without generating obvious stack traces (e.g. function pointers);
- Load the real stack pointer and disable the interrupt flag, and do so before returning to unsigned code; and
- Continue execution, preserving the function’s return value
While somewhat complicated, the callout mechanism can be achieved easily and, to a reasonable degree, portably, using two widely-available ROP gadgets from within the NT kernel.
The Usefulness of IRET(Q)
The constraint of needing to load a new stack pointer, interrupt flag, and interrupt pointer within an instruction boundary was immediately satisfied by the IRET instruction.
For those unfamiliar, the IRET (lit. “interrupt return”) instruction is intended to be used by an operating system or executive (here, the NT kernel) to return from an interrupt routine. To support the recognition of an interrupt from any mode of execution, and to generically resume to any mode of execution, the processor will need to (effectively) preserve the instruction pointer, stack pointer, CPL or privilege level (through the CS and SS selectors; and while they have a more general use-case, this is effectively what is preserved on most operating systems with a flat memory model), and RFLAGS register (as interrupts may be liable to modify certain flags).
To report this information to the OS interrupt handler, the CPU will, in a specific order:
- Push the SS (stack segment selector) register;
- Push the RSP (stack pointer) register;
- Push the RFLAGS (arithmetic/system flags) register;
- Push the CS (code segment selector) register;
- Push the RIP (instruction pointer) register; and, for some exception-class interrupts,
- Push an error code which may describe certain interrupt conditions (e.g. a page fault will know if the fault was caused by a non-present page, or if it were caused by a protection violation)
Note that the error code is not important to the CPU and must be accounted for by the interrupt handler. Each operation is an 8-byte push, meaning that, when the interrupt handler is invoked, the stack pointer will point to the preserved RIP (or error code) values.
It is hopefully obvious as to how, approximately, the IRET instruction would be implemented:
- Pop a value from the stack to retrieve the new instruction pointer (RIP)
- Pop a value from the stack to retrieve the new code segment selector (CS)
- Pop a value from the stack to retrieve the new arithmetic/system flags register (RFLAGS)
- Pop a value from the stack to retrieve the new stack pointer (RSP)
- Pop a value from the stack to retrieve the new stack segment selector (SS)
Or, as modeled as a series of pseudo-assembly instructions,
GENERIC_INTERRUPT:;note that all push and pop operations are 8 bytes (64 bits) wide!
push rip ;return instruction pointer
;optionally, push a zero-extended 4-byte error code. any interrupt which pushes an error code must have its handler add 8 bytes to their instruction pointer before executing its IRET.IRET:pop rip ;pop return instruction pointer into RIP. do not treat this as a series of regular assembly instructions; treat it instead as CPU microcode!
The callout mechanism uses the IRET instruction to accomplish its constraints, as the desired RFLAGS (which holds the interrupt flag), instruction pointer, and stack pointer can be loaded by the instruction at the same time (within an instruction boundary).
ROP: Chaining It All Together
To reiterate, the callout routine uses IRET to change the instruction pointer, stack pointer, and interrupt flag within the same instruction boundary in order to jump to external procedures with the interrupt flag enabled. This must be done within an instruction boundary to prevent unfortunately-timed external interrupts from being received just before the external procedure call.
It, however, must also be able to return from the external procedure call without leaving unsigned code pointers on the kernel stack; furthermore, it must also not rely on unlikely/unaligned ROP gadgets (e.g. a cli;ret sequence) which may not exist on future NT kernel builds. Thus also required is an IRET instruction to be executed upon the routine’s completion.
It must be recognized that the nature of the IRET instruction is such that the return instruction pointer is located on the stack. However, it is also recognized that a new stack pointer is loaded. We can therefore use IRET to load the callout thread’s real stack, with the stack pointer pointing to the actual return address.
This eliminates the problem of code pointers being present in the kernel stack; the return address back to our thread’s execution is located on another stack loaded by IRET and which isn’t obviously visible on a stack trace. To facilitate this, the stack frame loaded by the IRET gadget must be such that the return instruction pointer simply contains a RET instruction.
So, the ideal stack frame when calling an external procedure is as such:
- IRET return data, where the return address is a RET instruction within ntoskrnl.exe (or any region of signed code), and where the stack pointer to load is the thread’s real stack; which would have a return address pushed on to it; and
- The address of an IRET instruction within a region of signed code
Within most, if not all, versions of ntoskrnl.exe, this can be achieved with a simple RET instruction (0xC3 byte); along with the following gadget:
mov rsp, rbp
mov rbp, [rbp + some_offset] ;where some_offset could be liable to change
add rsp, some_other_offset
This also slightly modifies the mechanism of the ROP chain in that it must also load a pointer to the desired IRET frame in RBP when calling the function. Thankfully, the x64 calling convention specifies the RBP register as non-volatile, or unchanging across function calls, meaning that we can initialize it with our desired pointer when invoking the external procedure. It also means that the callout mechanism is permitted to allocate a non-paged region of memory to be given in RBP; preventing it from having to keep an IRET frame on the kernel stack. This notes, of course, the potential for an awful race condition where an interrupt is received in between the
mov rsp, rbp and
iretq instructions; the stack pointer value may point to memory that is insufficient to use for stack operations.
In having the external procedure return to the above IRET gadget, we can easily return to our unsigned code without ever leaking unsigned code pointers on the kernel stack.