Re: [v6 PATCH 21/21] selftests/x86: Add tests for User-Mode Instruction Prevention

From: Andy Lutomirski
Date: Wed Mar 08 2017 - 11:26:46 EST

On Tue, Mar 7, 2017 at 4:32 PM, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> Certain user space programs that run on virtual-8086 mode may utilize
> instructions protected by the User-Mode Instruction Prevention (UMIP)
> security feature present in new Intel processors: SGDT, SIDT and SMSW. In
> such a case, a general protection fault is issued if UMIP is enabled. When
> such a fault happens, the kernel catches it and emulates the results of
> these instructions with dummy values. The purpose of this new
> test is to verify whether the impacted instructions can be executed without
> causing such #GP. If no #GP exceptions occur, we expect to exit virtual-
> 8086 mode from INT 0x80.
> The instructions protected by UMIP are executed in representative use
> cases:
> a) the memory address of the result is given in the form of a displacement
> from the base of the data segment
> b) the memory address of the result is given in a general purpose register
> c) the result is stored directly in a general purpose register.
> Unfortunately, it is not possible to check the results against a set of
> expected values because no emulation will occur in systems that do not have
> the UMIP feature. Instead, results are printed for verification.

You could pre-initialize the result buffer to a bunch of non-matching
values (1, 2, 3, ...) and then check that all the invocations of the
same instruction gave the same value.

If you do this, maybe make it a follow-up patch -- see other email.