Thanks a lot for the reply. I think i didn't make my point clear.

I will try to explain the problem with the help of a simple example.

Lets assume GDT starts at memory address, say 100(to make things simple) and it is already loaded into GDTR.

Lets say i am currently at ring 3 and need access to higher privilege level.  Then i should access this GDT via a call gate and do some privilege checking. If the privilege checking fails then the access is denied.

What i am wondering is we could always do a hack like the following and pass the privilege cheking -

MOV some_value, appropriate_entry_inGDT

We can modify the GDT in memory with the above instruction and hack the appropriate entry and then i can follow it with a call gate access now with the right privilege levels to access a higher privilege level.

What i am wondering about is the mechanisms that prevent this MOV instruction from being executed.


Hope i made my problem clear.

Thanks,