[release/6.0] Add info about .NET 6 compatibility with Intel CET shadow stacks and how to try it

docs/design/features/intel-cet-dotnet6.md

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+# .NET 6 compatibility with Intel CET shadow stacks (early preview on Windows)
+
+Intel�s [Control-flow Enforcement Technology (CET)](https://newsroom.intel.com/editorials/intel-cet-answers-call-protect-common-malware-threats/) is a security feature available in some newer Intel and AMD processors. It adds capabilities to the hardware that protect against some common types of attacks involving control-flow hijacking. With CET shadow stacks, the processor and operating system can track the control flow of calls and returns in a thread in the shadow stack in addition to the data stack, and detect unintended changes to the control flow. The shadow stack is protected from application code memory accesses and helps to defend against attacks involving return-oriented programming (ROP). Windows offers [Hardware-enforced Stack Protection](https://techcommunity.microsoft.com/t5/windows-kernel-internals/understanding-hardware-enforced-stack-protection/ba-p/1247815) for CET shadow stack compatibility and is available as an early preview in [Windows Insider preview builds (Dev channel)](https://insider.windows.com/en-us/understand-flighting).
+
+## Opting into CET shadow stack compatibility
+
+CET compatibility in .NET 6 is available as an early preview. In order to receive the security benefits of CET shadow stacks in .NET applications, ensure the following to opt into the feature for an executable:
+- Verify that your processor supports Intel CET. There are Intel and AMD processors currently available with the feature.
+- Get a recent build of Windows that has Hardware-enforced Stack Protection. The latest versions from [Windows Insider preview builds (Dev channel)](https://insider.windows.com/en-us/understand-flighting) have the feature.
+- Install [.NET 6](https://dotnet.microsoft.com/download/dotnet) RC1 or a more recent release
+- Build or publish your application targeting framework `net6.0` and runtime `win-x64`
+- Open the Windows Security app
+  - Inside the "App & browser control" pane, select "Explicit protection settings"
+  - Under the "Program settings" tab, select "Add program to customize" and "Add by program name"
+  - Enter the file name of your application�s executable file and select "Add"
+  - Select the following options and select "Apply":
+
+    ![Hardware-enforced Stack Protection settings](images/intel-cet-dotnet6-fig1.png)
+  - See the [Developer Guidance for Hardware-enforced Stack Protection](https://techcommunity.microsoft.com/t5/windows-kernel-internals/developer-guidance-for-hardware-enforced-stack-protection/ba-p/2163340) for more details
+- Task Manager�s Details tab has a new column "Hardware-enforced Stack Protection" that shows the compatibility level of a running process. If CET shadow stacks are enabled for the process, the column for the application should say "Compatible modules only".
+
+## Limitations
+
+CET shadow stack compatibility in .NET is under development and will be available as an early preview from .NET 6 RC1 as an opt-in for x64 applications. Aside from a notable limitation, most run-time scenarios should work as expected. There are plans for expanding the compatibility for .NET 7 and to have it be enabled by default on supported hardware for Windows x64 applications.
+
+For a .NET 6 application that is opted into CET shadow stack compatibility, managed or mixed-mode debugging through Visual Studio may not work as expected, and may cause the application to crash. Some basic features such as expression evaluation (including autos, locals, watch list expressions, quick watch, etc.) and stepping through code are currently incompatible with CET shadow stacks. Native debugging through Visual Studio and WinDbg/SOS should continue to work as expected with CET shadow stacks enabled for the application being debugged.
+
+When opted in as above, CET shadow stacks are enabled in compatibility mode. .NET 6 is not compatible with strict mode, support for strict mode is planned for .NET 7.
+
+For .NET 6 applications, CET shadow stack compatibility is available to try for Windows x64 applications. Support for the feature on Linux is pending.