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

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Date:   Wed, 8 Mar 2017 07:56:20 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Liang Z Li <liang.z.li@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        Alexandre Julliard <julliard@...ehq.org>,
        Stas Sergeev <stsp@...t.ru>, Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, linux-msdos@...r.kernel.org,
        wine-devel@...ehq.org
Subject: Re: [v6 PATCH 21/21] selftests/x86: Add tests for User-Mode
 Instruction Prevention

On Tue, Mar 7, 2017 at 4:32 PM, Ricardo Neri
<ricardo.neri-calderon@...ux.intel.com> 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.