Water wheels come in two basic designs:
This led me to further analyze how UMS worked before this processor feature was added — something which I knew a little bit about, but not enough to write on. As such, reading that paper should be considered a prerequisite to this post. Please, take into consideration that all these techniques no longer work on Anniversary Update systems or later, nor will they work on Intel Ivy Bridge processors or later, which is why I am presenting them now.
A number of videos on Channel 9 explain how this is done, as does the patent.
Because UMS would now need to allow switching the base address of this per-thread register from user-mode as involving a kernel transition would defy the whole pointtwo problems exist: On x86 systems, this could be implemented through segmentation, allowing a process to have additional FS segments.
On x64 systems, modifying the base address of the GS segment requires modifying the aforementioned MSRs — which is a Ring 0 operation. It is worth bringing up the fact that fibers never solved this problem writing a pico research question generator having all fibers share a single thread and TEB.
But the whole point of UMS is to provide true thread isolation. So, what can Windows do? This literally brought back memories of Unreal Mode. Clearly, though, Microsoft was paying attention did they request this?
As you can probably now guess, UMS leverages this particular feature which is why it is only available on x64 versions of Windows. This was my second surprise, as I had no idea LDTs were still something supported when executing native bit code i.
At this point, the process has an LDT. The next step is to fill it out. The function now reads the address of the TEB. If the TEB happens to fall in the bit portion of the address space i. Teb field set to indicate it is a special allocation. This prevents any changes to be made to this address through calls such as VirtualProtect.
Why this 4GB limitation and re-mapping? How does an LDT help here? Well, it turns out that the AMD64 manuals are pretty clear about the fact that the mov gs, XXX and pop gs instructions: However, because there is no bit data segment descriptor table entry, only a bit base address can be used, requiring this complex remapping done by the kernel.
Although Windows NT never used such call gates internally, a number of poorly written AV software did, a few emulators, as well as exploits, both on 9x and NT systems, because of the easy way they allowed someone with access to physical memory or with a Write-What-Where vulnerability in virtual memory to create a backdoor way to elevate privileges.
Additionally, on x64 systems, since Call Gates are expected to be inserted into the Global Descriptor Table GDTwhich PatchGuard is known to protect, the technique is even further degraded.
On top of that, most people myself included assumed that AMD had simply removed this oft-unused feature completely from the x64 architecture.
That means that if a call gate were to find itself into a descriptor table, the processor would still support the usage of a far call or far jmp in order to reference a call gate descriptor and change CS: RIP to a new location! This means that the ability to install a bit Call Gate is still a viable technique for getting controlled execution with Ring 0 privileges.
Well, given that the LDT is a static, 64KB allocation, from non-paged pool, this does still leave us with an option. As explained a few years ago on my post about the Big Poolsuch a large allocation will be easily enumerable from user-mode as long as its tag is known: Given that this is a fairly large size allocation, however, it means that if a controlled 64KB allocation can be made in non-paged pool and its address leaked from Low IL, one can still guess the LDT address.
By default, if this is the initial scheduler thread, we expect to find its TEB. Indeed, on this sample Windows 8. However, doing so will now create a call gate with the following CS: The second problem can be fixed in a few ways. It is also worth mentioning that since Windows 7 has all of non-paged pool marked as executable, and the LDT is itself a 64KB non-paged pool allocation, it is made up of entirely executable pages, so an arbitrary write could be used to set the Call Gate offset to somewhere within the LDT allocation itself.
Writing the Ring 0 Payload Writing x64 Ring 0 payload code is a lot harder than x For starters, the GS segment must be immediately set to its correct value, else a triple fault could occur. This is done through the swapgs instruction. These ought to be in sync, but keep in mind that a call gate does not disable interrupts automatically, unlike an interrupt gate.
A reliable exploit must take note of all these details to avoid crashing the machine. Once again, another caveat applies: Indeed, for this to work, a single bit in fact, even less arbitrary write is required, which must, at minimum, set the fields:How to conduct effective desk-top research?
This article is for any university student about to embark on writing essays or completing dissertations and projects for the first time. I have also run workshops introducing these methods and they do seem to be overwhelmingly .
Published: Mon, 5 Dec When working as a nurse, clinical questions come up regarding which is the best way to care for our patients. To help nurses make effective clinical decisions, they can use evidence-based research in their decision-making process.
Not all research questions that clinicians wish answered are feasible using this research methodology and the use of a PICOT format is also applicable to other study designs. The clinical research question being asked ideally determines the best research design for . Foreground questions are specific knowledge questions.
that affect clinical decisions and ; include a broad range of biologic, psychological, and sociologic issues. These are the questions that generally require a search of the primary medical literature and that are best suited to the PICO format. This page contains the complete set of materials for my FPGA & Verilog design course which I taught in Isfahan University of Technology, The Medical Services Advisory Committee (MSAC) is an independent non-statutory committee established by the Australian Government Minister for Health in