Windows processes in kernel vs system

Question

I have a few questions related to Windows processes in kernel and usermode.

If I have a hello world application, and a hello world driver that exposes a new system call, foo(), I am curious about what I can and can't do once I am in kernel mode.

For starters, when I write my new hello world app, I am given a new process, which means I have my own user mode VM space (lets keep it simple, 32 bit windows). So I have 2GB of space that I "own", I can poke and peek until my hearts content. However, I am bound by my process. I can't (lets not bring shared memory into this yet) touch anyone elses memory.

If, I write this hello world driver, and call it from my user app, I (the driver code) is now in kernel mode.

First clarification/questions: I am STILL in the same process as the user mode app, correct? Still have the same PID?

Memory Questions: Memory is presented to my process as VM, that is even if I have 1GB of RAM, I can still access 4GB of memory (2GB user / 2GB of kernel - not minding details of switches on servers, or specifics, just a general assumption here). As a user process, I cannot peek at any kernel mode memory address, but I can do whatever I want to the user space, correct?

If I call into my hello world driver, from the driver code, do I still have the same view of the usermode memory? But now I also have access to any memory in kernel mode?

Is this kernel mode memory SHARED (unlike User mode, which is my own processes copy)? That is, writing a driver is more like writing a threaded application for a single process that is the OS (scheduling aside?)

Next question. As a driver, could I change the process that I am running. Say, I knew another app (say, a usermode webserver), and load the VM for that process, change it's instruction pointer, stack, or even load different code into the process, and then switch back to my own app? (I am not trying to do anything nefarious here, I am just curious what it really means to be in kernel mode)?

Also, once in kernel mode, can I prevent the OS from preempting me? I think (in Windows) you can set your IRQL level to do this, but I don't fully understand this, even after reading Solomons book (Inside Windows...). I will ask another question, directly related to IRQL/DPCs but, for now, I would love to know if a kernel driver has the power to set an IRQL to High and take over the system.

More to come, but answers to these questions would help.

Answer 1

Each process has a "context" that, among other things, contains the VM mappings specific to that process (<2 GB normally in 32bit mode). When thread executing in user mode enteres kernel mode (e.g. from a system call or IO request), the same thread is still executing, in the process, with the same context. PsGetCurrentProcessId will return the same thing at this point as GetCurrentProcessID would have just before in user mode (same with thread IDs).

The user memory mappings that came with the context are still in place upon entering kernel mode: you can access user memory from kernel mode directly. There are special things that need to be done for this to be safe though: Using Neither Buffered Nor Direct I/O. In particular, an invalid address access attempt in the user space range will raise a SEH exception that needs to be caught, and the contents of user memory can change at any time due to the action of another thread in that process. Accessing an invalid address in the kernel address range causes a bugcheck. A thread executing in user mode cannot access any kernel memory.

Kernel address space is not part of a process's context, so is mapped the same between all of them. However, any number of threads may be active in kernel mode at any one time, so it is not like a single threaded application. In general, threads service their own system calls upon entering kernel mode (as opposed to having dedicated kernel worker threads to handle all requests).

The underlying structures that save thread and process state is all available in kernel mode. Mapping the VM of another process is best done ahead of time from the other process by creating an MDL from that process and mapping it into system address space. If you just want to alter the context of another thread, this can be done entirely from user mode. Note that a thread must be suspended to change its context without having a race condition. Loading a module into a process from kernel mode is ill advised; all of the loader APIs are designed for use from user mode only.

Each CPU has a current IRQL that it is running at. It determines what things can interrupt what the CPU is currently doing. Only an event from a higher IRQL can preempt the CPU's current activity.

• PASSIVE_LEVEL is where all user code and most kernel code executes. Many kernel APIs require the IRQL to be PASSIVE_LEVEL
• APC_LEVEL is used for kernel APCs
• DISPATCH_LEVEL is for scheduler events (known as the dispatcher in NT terminology). Running at this level will prevent you from being preempted by the scheduler. Note that it is not safe to have any kind of page fault at this level; there would be a deadlock possibility with the memory manager trying to retrieve pages. The kernel will bugcheck immediately if it has a page fault at DISPATCH_LEVEL or higher. This means that you can't safely access paged pool, paged code segments or any user memory that hasn't been locked (i.e. by an MDL).
• Above this are levels connected to hardware device interrupt levels, known as DIRQL.
• The highest level is HIGH_LEVEL. Nothing can preempt this level. It's used by the kernel during a bugcheck to halt the system.

I recommend reading Scheduling, Thread Context, and IRQL