Tuesday 15 April 2014

Understanding Memory Probes - A Quick Introduction

You may notice with Stop 0x50, there is the mentioning of something called the memory probe, the memory probe is a type of function which is used to check that a buffer (chunk of virtual memory) resides within user-mode and is correctly aligned to a boundary. I've spoken about memory alignment before in a previous blog post, however, I will mention the topic again in post.

Memory Alignment is very useful for performance in processors, if data is aligned to a certain boundary, then larger chunks of data can be accessed much more efficiently rather than lots of small accesses with a large chunk of data. Data misalignment is a common problem with debugging, especially with x64 processors.

We can check for alignment issues by checking the EFLAGS register and AC flag, which when set to 1, will mean that data being accessed must be aligned to the correct boundary otherwise you'll experience access violations and potential BSODs.On the other hand, using malloc or new should always create aligned data accesses.

Supposedly, the interrupt handler is assigned the 17h vector number within the IDT table.

Now, lets move onto the concept of a try-except block, and the Probe. The Memory Probe has to reside within the try-except block to be able to raise the appropriate exception code to the operating system. The two versions of Memory Probes are the ProbeForRead and ProbeForWrite. Memory Probes can't be used within the Kernel Mode Address Range otherwise it will lead to an exception.

The try-except block is a block of code which tested to see if it will run properly, and if it doesn't then a exception handler will be invoked.

I've created a very quick template for a try-except block which will catch all exceptions regardless of their type, in real programs you'll most likely have specific handlers for exception errors. Typically, inside the catch block, the code will producing a error message to the user. The try block is the code we're attempting to execute with no problems.

The ProbeForRead function takes three parameters: the starting address of the buffer, the length of the buffer and the required alignment.

Memory Probes are used within the win32k.sys subsystem (Kernel-Mode) when dealing with System Calls from User-Mode and storing certain libraries within User-Mode. The Win32k.sys has it's own SSDT called the Shadow SSDT. This article may also be useful for information on SSDT Hooking.

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