Q78325: Ring Protection Under Windows 3.1

Article: Q78325
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.1
Operating System(s): 
Keyword(s): 
Last Modified: 03-NOV-1999

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The information in this article applies to:

- Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1 
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SUMMARY
=======

The privilege level at which applications and dynamic-link libraries (DLLs) run
has changed from ring 1 in versions of Microsoft Windows prior to Windows 3.1 to
ring 3 in Windows 3.1. This change makes Windows more consistent with the
protection scheme used in other operating systems, and enables future versions
of Windows to take better advantage of the page protection mechanism of the 386
processor.

MORE INFORMATION
================

The architecture of the 386 processor defines four privilege levels to protect
system code and data from being overwritten inadvertently or maliciously by code
of lesser privilege. The processor's current privilege level (CPL) is contained
in the low-order two bits of the selector in the CS register. The privilege
level of 0 (zero) is the most privileged; the privilege level of 3 is the least
privileged. When code is running at a given privilege level, it is said to be
running in that "ring" [for example, code running at privilege level 0 (zero) is
said to run in ring 0].

When Windows is running in enhanced mode, the enhanced mode layer (the virtual
machine manager and all virtual devices) runs in ring 0. Code run in
virtual-8086 (V86) mode, like MS-DOS and the BIOS, runs in ring 3. The remaining
code (applications and DLLs, including kernel, user, GDI, and all Windows device
drivers) runs in ring 1 under Windows 3.0, and in ring 3 under Windows 3.1.

In discussing the implications of this change, it is important to realize that
all of the code that was running at ring 1 has been moved to ring 3. No
distinction is made between any Windows applications and DLLs, not even the
Windows kernel DLL or device drivers. This means that there is no net change in
the privilege of this software. In other words, under both Windows 3.0 and 3.1,
there are functionally only two privilege levels in use: ring 0 code, and
everything else. The fact that both rings 1 and 3 are used in Windows 3.0 is
insignificant because the code in these rings is generally in isolated address
spaces.

Therefore, for Windows 3.1, the only real implication of the change is that the
privilege level is numerically different. If an application specifically tests
to see if it is running in ring 1 (by checking the bits in the code selector),
that test will fail under Windows 3.1. Similarly, if an application directly
modifies the local descriptor table (LDT), this modification can create an
invalid selector in Windows 3.1. Other than that, this privilege change does not
affect what applications can do.

One of the main reasons for the change is to pave the way for future enhancements
to protection in Windows. One of the 386 processor's protection features is
called page protection. This mechanism allows the operating system to flag
memory pages (4K in size) as either a "User" or a "Supervisor" page. Code that
runs at ring 0, 1, or 2 may access either user or supervisor pages. However,
code running at ring 3 cannot access supervisor pages, and can write only to
pages that are marked as read/write. This powerful protection mechanism may be
used to further enhance protection under Windows. This feature is not
implemented in Windows 3.1.

Additional query words: 3.10 DDKVXD protection ring1 ring3

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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK310
Version           : :3.1

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