Windows Internals, 6th Edition from Microsoft Press says that in Windows NT, each thread has 2 stacks: one used when running in user mode, and one used in kernel mode.
Why is this so? It seems that the user-mode stack could also be used while in a system call. Is there some advantage to this design?
The main reason is that the kernel mode cannot trust user mode. If the kernel used a user-mode stack, some other user mode thread could observe the values on that stack and modify them at will. It would be trivial for malware to gain complete control of the system.
Related
I understand that an Operating System forces security policies on users when they use the system and filesystem via the System Calls supplied by stated OS.
Is it possible to circumvent this security by implementing your own hardware instructions instead of making use of the supplied System Call Interface of the OS? Even writing a single bit to a file where you normally have no access to would be enough.
First, for simplicity, I'm considering the OS and Kernel are the same thing.
A CPU can be in different modes when executing code.
Lets say a hypothetical CPU has just two modes of execution (Supervisor and User)
When in Supervisor mode, you are allowed to execute any instructions, and you have full access to the hardware resources.
When in User mode, there is subset of instructions you don't have access to, such has instructions to deal with hardware or change the CPU mode. Trying to execute one of those instructions will cause the OS to be notified your application is misbehaving, and it will be terminated. This notification is done through interrupts. Also, when in User mode, you will only have access to a portion of the memory, so your application can't even touch memory it is not supposed to.
Now, the trick for this to work is that while in Supervisor Mode, you can switch to User Mode, since it's a less privileged mode, but while in User Mode, you can't go back to Supervisor Mode, since the instructions for that are not permitted anymore.
The only way to go back to Supervisor mode is through system calls, or interrupts. That enables the OS to have full control of the hardware.
A possible example how everything fits together for this hypothetical CPU:
The CPU boots in Supervisor mode
Since the CPU starts in Supervisor Mode, the first thing to run has access to the full system. This is the OS.
The OS setups the hardware anyway it wants, memory protections, etc.
The OS launches any application you want after configuring permissions for that application. Launching the application switches to User Mode.
The application is running, and only has access to the resources the OS allowed when launching it. Any access to hardware resources need to go through System Calls.
I've only explained the flow for a single application.
As a bonus to help you understand how this fits together with several applications running, a simplified view of how preemptive multitasking works:
In a real-world situation. The OS will setup an hardware timer before launching any applications.
When this timer expires, it causes the CPU to interrupt whatever it was doing (e.g: Running an application), switch to Supervisor Mode and execute code at a predetermined location, which belongs to the OS and applications don't have access to.
Since we're back into Supervisor Mode and running OS code, the OS now picks the next application to run, setups any required permissions, switches to User Mode and resumes that application.
This timer interrupts are how you get the illusion of multitasking. The OS keeps changing between applications quickly.
The bottom line here is that unless there are bugs in the OS (or the hardware design), the only way an application can go from User Mode to Supervisor Mode is through the OS itself with a System Call.
This is the mechanism I use in my hobby project (a virtual computer) https://github.com/ruifig/G4DevKit.
HW devices are connected to CPU trough bus, and CPU does use to communicate with them in/out instructions to read/write values at I/O ports (not used with current HW too much, in early age of home computers this was the common way), or a part of device memory is "mapped" into CPU address space, and CPU controls the device by writing values at defined locations in that shared memory.
All of this should be not accessible at "user level" context, where common applications are executed by OS (so application trying to write to that shared device memory would crash on illegal memory access, actually that piece of memory is usually not even mapped into user space, ie. not existing from user application point of view). Direct in/out instructions are blocked too at CPU level.
The device is controlled by the driver code, which is either run is specially configured user-level context, which has the particular ports and memory mapped (micro-kernel model, where drivers are not part of kernel, like OS MINIX). This architecture is more robust (crash in driver can't take down kernel, kernel can isolate problematic driver and restart it, or just kill it completely), but the context switches between kernel and user level are a very costly operation, so the throughput of data is hurt a bit.
Or the device drivers code runs on kernel-level (monolithic kernel model like Linux), so any vulnerability in driver code can attack the kernel directly (still not trivial, but lot more easier than trying to get tunnel out of user context trough some kernel bug). But the overall performance of I/O is better (especially with devices like graphics cards or RAID disc clusters, where the data bandwidth goes into GiBs per second). For example this is the reason why early USB drivers are such huge security risk, as they tend to be bugged a lot, so a specially crafted USB device can execute some rogue code from device in kernel-level context.
So, as Hyd already answered, under ordinary circumstances, when everything works as it should, user-level application should be not able to emit single bit outside of it's user sandbox, and suspicious behaviour outside of system calls will be either ignored, or crash the app.
If you find a way to break this rule, it's security vulnerability and those get usually patched ASAP, when the OS vendor gets notified about it.
Although some of the current problems are difficult to patch. For example "row hammering" of current DRAM chips can't be fixed at SW (OS) or CPU (configuration/firmware flash) level at all! Most of the current PC HW is vulnerable to this kind of attack.
Or in mobile world the devices are using the radiochips which are based on legacy designs, with closed source firmware developed years ago, so if you have enough resources to pay for a research on these, it's very likely you would be able to seize any particular device by fake BTS station sending malicious radio signal to the target device.
Etc... it's constant war between vendors with security researchers to patch all vulnerabilities, and hackers to find ideally zero day exploit, or at least picking up users who don't patch their devices/SW fast enough with known bugs.
Not normally. If it is possible it is because of an operating system software error. If the software error is discovered it is fixed fast as it is considered to be a software vulnerability, which equals bad news.
"System" calls execute at a higher processor level than the application: generally kernel mode (but system systems have multiple system level modes).
What you see as a "system" call is actually just a wrapper that sets up registers then triggers a Change Mode Exception of some kind (the method is system specific). The system exception hander dispatches to the appropriate system server.
You cannot just write your own function and do bad things. True, sometimes people find bugs that allow circumventing the system protections. As a general principle, you cannot access devices unless you do it through the system services.
I know that user applications can run only in user mode, which is for system security. On the contrary most drivers run in kernel mode, to access I/O devices. Nevertheless some driver run in user mode, but are allowed to access I/O devices. So I have the following question. What is the main difference between drivers and user applications? Can't user application be allowed to access I/O devices like some drivers do?
Thanks.
First of all, some preview from this link :-
Applications run in user mode, and core operating system components
run in kernel mode. Many drivers run in kernel mode, but some drivers
run in user mode.
When you start a user-mode application, Windows(/any OS) creates a process for
the application. The process provides the application with a private
virtual address space and a private handle table. Because an
application's virtual address space is private, one application cannot
alter data that belongs to another application.
In addition to being private, the virtual address space of a user-mode
application is limited. A processor running in user mode cannot access
virtual addresses that are reserved for the operating system. Limiting
the virtual address space of a user-mode application prevents the
application from altering, and possibly damaging, critical operating
system data.
All code that runs in kernel mode shares a single virtual address
space. This means that a kernel-mode driver is not isolated from other
drivers and the operating system itself. If a kernel-mode driver
accidentally writes to the wrong virtual address, data that belongs to
the operating system or another driver could be compromised.
Also, from this link
Software drivers
Some drivers are not associated with any hardware device at all. For
example, suppose you need to write a tool that has access to core
operating system data structures, which can be accessed only by code
running in kernel mode. You can do that by splitting the tool into two
components. The first component runs in user mode and presents the
user interface. The second component runs in kernel mode and has
access to the core operating system data. The component that runs in
user mode is called an application, and the component that runs in
kernel mode is called a software driver. A software driver is not
associated with a hardware device.
Also, software drivers() always run in kernel mode. The main reason
for writing a software driver is to gain access to protected data that
is available only in kernel mode. But device drivers do not always
need access to kernel-mode data and resources. So some device drivers
run in user mode.
What is the main difference between drivers and user applications?
The difference is same as that between a sub-marine and a ship. Drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface. Hence, almost all of them run in kernel mode. Whereas, as specified in the second paragraph, to prevent applications damaging critical OS data, the user applications are bound to run in user space.
Also, not all drivers communicate directly with a device. For a given I/O request (like reading data from a device), there are often several drivers, layered in a stack, that participate in the request. The one driver in the stack that communicates directly with the device is called the function driver; the drivers that perform auxiliary processing are called filter drivers.
Can't user application be allowed to access I/O devices like some
drivers do?
The application calls a function implemented by the operating system, and the operating system calls a function implemented by the driver. The driver knows how to communicate with the device hardware to get the data. After the driver gets the data from the device, it returns the data to the operating system, which returns it to the application.
Applications connect to IO devices through APIs/interfaces presented by device drivers(rather the OS). The OS handles most hardware/software interaction. Hardware vendors write "plugins/modules/drivers" which allows the OS to control their specific hardware. So using the interfaces provided by the OS, you can write your Application to access IO devices.
So, you can't have a user application directly access the hardware without the help of drivers because it's all drivers below the hierarchy to access the device as device drivers are written in low-level languages which can communicate with hardware, whereas user-applications are written in high-level languages.
Also, check this answer to get more idea about drivers address space in various OS'.
Your question lacks any concrete operating system tags, so I'll answer generically about the subject
Yes, drivers can be very similar to a user application in a microkernel-based operating system. In a microkernel, usually a driver is just a userspace app that queries the kernel for some generic capabilities and then functions normally in userspace
Take for example a hypothetical implementation of a pagein/pageout driver in such an OS.
When a page fault happens in user mode, the transition is made to kernel as usual with synchronous exceptions, but a microkernel would queue the pagein request to a userspace driver via local message queue.
After that (still in the exception handler) the faulting process is made blocking and dequeued.
Then the userspace driver app does the necessary page-in and notifies the microkernel on the work done again via local message queue. The kernel is then ready to reschedule the faulting process as it will have its memory mappings ready.
Such functioning incurs a performance overhead but it has its advantages, mainly that the drivers can be implemented and tested just like any other user app.
In a monolithic kernel such disposition is impossible and that case is better described by other answers.
From Wikipedia it says:
A kernel thread is the "lightest" unit of kernel scheduling. At least one kernel thread exists within each process.
I've learned that a process is a container that houses memory space, file handles, device handles, system resources, etc... and the thread is the one that really gets scheduled by the kernel.
So in single-threaded applications, is that one thread(main thread i believe) a kernel thread?
I assume you are talking about this article:
http://en.wikipedia.org/wiki/Kernel_thread
According to that article, in a single threaded application, since you have only one thread by definition, it has to be a kernel thread, otherwise it will not get scheduled and will not run.
If you had more than one thread in your application, then it would depend on how user mode multi threading is implemented (kernel threads, fibers, etc ...).
It's important to note however it would be a kernel thread running in user mode, when executing the application code (unless you make a system call). Any attempt to execute a protected instruction when running in user mode would cause a fault that will eventually lead to the process being terminated.
So kernel thread here not to be confused with supervisor/privileged mode and kernel code.
You can execute kernel code, but you have to go through a system call gate first.
No. In modern operating systems applications and the kernel run at different processor protection levels (often called rings). For example, Intel CPUs have four protection levels. Kernel code runs at Ring 0 (kernel mode) and is able to execute the most privileged processor instructions, whereas application code runs at Ring 3 (user mode) and is not allowed to execute certain operations. See http://en.wikipedia.org/wiki/Ring_(computer_security)
I'm trying to figure out how does Node.JS (of its Windows version) is working behind the scenes.
I know there is user mode and kernel mode threads, and I know the processing model looks like this:
I also know that moving from a kernel mode thread to a user mode thread is consider to be a context switching.
Does Node.JS C++ Non-Blocking worker threads are kernel mode ? and where does the single event loop thread lives at kernel mode or user mode ?
As you know node.js has a single threaded architecture. The JavaScript environment and event-loop is managed by a single thread only, internally all the other threads are handled by a C++ level thread pool (like asynchronous I/O handled by libuv thread) .
To answer your question these node.js C++ non-blocking worker threads are not kernel mode. They are user mode. The event-loop thread is also user mode. The threads request kernel mode as and when needed.
When the CPU is in kernel mode, it is assumed to be executing trusted software. Kernel mode is the highest privelege level and the code has full access to all devices. In Windows, only select files written by Windows developers runs completely on kernel mode. All user mode software must request use of the kernel by means of a system call in order to perform privileged instructions, such as process creation or I/O operations.
All processes begin execution in user mode, and they switch to kernel mode only when obtaining a service provided by the kernel. This change in mode is termed mode switch, not context switch, which is the switching of the CPU from one process to another.
I hope it is clear to you that even user-mode threads can execute privileged operations (network access) via system calls, and return to user-mode when required task is finished. Node.js simply uses system calls.
Source : http://www.linfo.org/kernel_mode.html
Update
I should have mentioned that mode switch does not always mean context switch. Quoting the wiki:
When a transition between user mode and kernel mode is required in an
operating system, a context switch is not necessary; a mode transition
is not by itself a context switch. However, depending on the operating
system, a context switch may also take place at this time.
What you mention is also correct that mode switch can cause context switch. But it does not happen always. It is not desirable to have context switches (heavy performance penalty) whenever mode switch happens. What happens inside Windows is difficult to say, but most likely mode switch does not cause context switch every time.
Regarding the one-to-one thread model. Both Windows and Linux follow that. So given each user thread (like node.js event loop thread) OS provides a kernel thread, which takes care of the system calls. Node.js can only invoke mode switch through system calls. Context switch is controlled only by the kernel (thread scheduler).
Update 2
Yes, HTTP.SYS executes in kernel mode. But there is more to it. Node.js does not have many threads, so fewer context switching happens between threads unlike IIS. Context switch (mode switch) for each request is definitely less in HTTP.SYS. It is an improvement from past (which happened to be a disaster), see here. The context switching due to multiple threads is much more than reduction of context switch by using HTTP.SYS. So overall node.js has less context switches.
HTTP.SYS also has other advantages over node's own HTTP implementation that helps IIS. It may be possible (in future) to use HTTP.SYS from node itself to take those advantages. But for now, I don't think HTTP.SYS/IIS compete anywhere near node.js.
I am aware of the difference between a process running in user mode and one running in kernel mode (based on access restrictions, access to hardware etc.). But just out of curiosity, what is the difference between a process running in kernel mode and one running as root?
kernel mode and root are two separate ideas that aren't really related to each other. The concept of running a process as root is a unix/linux term that means you're logged in as the administrator of the system.
Any process you run, whether as root or a normal user, generally runs in both user mode and kernel mode. The system is continually switching between user mode (where the application code runs) and kernel mode (where the kernel code runs).
Some programs, like many device drivers, always run in kernel mode, meaning they have full access to the hardware. A normal application running with root privileges still exists in user mode and only switches to kernel mode when a kernel system call is made and then switches right back to user mode.