msrexec is a small project that can be used to elevate arbitrary MSR writes to kernel execution on 64 bit Windows-10 systems. This project is part of the VDM (vulnerable driver manipulation) namespace
and can be integrated into any prior VDM projects. Although this project falls under the VDM namespace, Voyager and bluepill can be used to provide arbitrary wrmsr writes.
SYSCALL invokes an OS system-call handler at privilege level 0. It does so by ***loading RIP from the IA32_LSTAR MSR*** (after saving the address of the instruction following SYSCALL into RCX). (The WRMSR instruction ensures that the IA32_LSTAR MSR always contain a canonical address.)
SYSCALL also saves RFLAGS into R11 and then masks RFLAGS using the IA32_FMASK MSR (MSR address C0000084H); specifically, the processor clears in RFLAGS every bit corresponding to a bit that is set in the IA32_FMASK MSR.
SYSCALL loads the CS and SS selectors with values derived from bits 47:32 of the IA32_STAR MSR. However, the CS and SS descriptor caches are not loaded from the descriptors (in GDT or LDT) referenced by those selectors. Instead, the descriptor caches are loaded with fixed values. See the Operation section for details. It is the responsibility of OS software to ensure that the descriptors (in GDT or LDT) referenced by those selector values correspond to the fixed values loaded into the descriptor caches; the SYSCALL instruction does not ensure this correspondence.
***The SYSCALL instruction does not save the stack pointer (RSP).*** If the OS system-call handler will change the stack pointer, it is the responsibility of software to save the previous value of the stack pointer. This might be done prior to executing SYSCALL, with software restoring the stack pointer with the instruction following SYSCALL (which will be executed after SYSRET). Alternatively, the OS system-call handler may save the stack pointer and restore it before executing SYSRET.
ROP or return-oriented programming, is a technique where an attacker gains control of the call stack to hijack program control flow and then executes carefully chosen machine instruction sequences that are already present in the machine's memory, called "gadgets". Note: ***"The SYSCALL instruction does not save the stack pointer (RSP)"***. This allows for an attacker to setup the stack with addresses of ROP gadgets are specific values. In this situation SMEP and SMAP are two cpu protections which prevent an attacker from setting IA32_LSTAR to a user controlled page.
### SMEP - Supervisor Mode Execution Protection
SMEP or Supervisor Mode Execution Protection, prevents a logical processor with a lower CPL from executing code mapped into virtual memory with super supervisor bit set. This is relevant to this project as one could not simply set LSTAR to a user controlled page. However, with ROP one could disable SMEP by executing the following gadgets:
However, when the syscall instruction is executed, the address of the next instruction (the one after the syscall instruction) is placed into RCX. In order to preserve RIP, it should be placed onto the stack before any addresses of gadgets are placed onto the stack.
changing IA32_LSTAR to a ROP chain as described above will work just fine on CPU's that done support SMAP. Windows 10 will use SMAP if your CPU supports it. This means RSP is unaccessable since it is a user controlled page.
SMAP or Supervisor Mode Access Prevention is a CPU protection which prevents accessing data controlled by a higher CPL. In other words, if SMAP is set in CR4, a logical
This is an issue with ROP as RSP after a syscall contains a usermode address. Interfacing with this usermode stack in any way will cause a fault. However, you can essentially disable SMAP from usermode. There is a bit in the RFLAGS register which can be set to nullify SMAP. The instruction to set this bit is called `STAC` (Set AC Flag in EFLAGS Register). However this instruction is privilaged and will throw a #UD. However as @drew pointed out, you can `POPFQ` an RFLAGS value with that bit set and the CPU will not throw any exceptions. I assumed that since `STAC` cannot be used in usermode, that `POPFQ` would also throw an exception, however this is not the case... Again thank you @drew, without this key information the project would have been a complete mess as there are no useable `mov cr4, [non rax registers] ; ret` gadgets which exist across windows versions.
