What is blocking the INVD (invalidate cache) under Windows

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StrenoJr

Joined: 13 Mar 2014
Posts: 22
Location: Slovakia

StrenoJr 26 Jan 2015, 21:45

Hello.
I was looking for a list of x86 instructions mnemonics.
Accidentally, I found an "INVD" instruction, that should clear the CPU cache.
Compiles fine, but the program closes after executing the instruction.
The question is: What is blocking that instruction?
Is it the OS? Or somehow the CPU handles this in protected mode?
Thanks for reading, have a nice day.

26 Jan 2015, 21:45

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20421
Location: In your JS exploiting you and your system

revolution 27 Jan 2015, 01:29

TFM wrote:

The INVD instruction is a privileged instruction. When the processor is running in protected mode, the CPL of a
program or procedure must be 0 to execute this instruction.

27 Jan 2015, 01:29

StrenoJr

Joined: 13 Mar 2014
Posts: 22
Location: Slovakia

StrenoJr 27 Jan 2015, 06:45

revolution wrote:

TFM wrote:
The INVD instruction is a privileged instruction. When the processor is running in protected mode, the CPL of a
program or procedure must be 0 to execute this instruction.

Thanks, I found this resource too but I don't know what is the CPL of a program. Could you explain?

27 Jan 2015, 06:45

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20421
Location: In your JS exploiting you and your system

revolution 27 Jan 2015, 07:00

What is means is that you have to be running at the highest privilege level to execute INVD. So in Windows you have to have your code as part of the kernel or as a driver that runs in ring 0. No user application can execute the instruction because user applications run in ring 3.

27 Jan 2015, 07:00

DOS386

Joined: 08 Dec 2006
Posts: 1903

DOS386 30 Jan 2015, 10:35

You will soon find out that many other privileged and sensitive instructions don't work either (access to CRx & DRx , CLI , LIDT , IN , OUT , RDMSR , WRMSR , ...).

> The question is: What is blocking that instruction?

The protected mode.

30 Jan 2015, 10:35

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