linux-kernel - Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE

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Date:   Thu, 5 Sep 2019 08:54:17 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
CC:     Ingo Molnar <mingo@...nel.org>, Juergen Gross <jgross@...e.com>,
        "Stefano Stabellini" <sstabellini@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on
 XEN_EMULATE_PREFIX

On 05/09/2019 02:49, Masami Hiramatsu wrote:
> On Wed, 4 Sep 2019 12:54:55 +0100
> Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>
>> On 04/09/2019 12:45, Masami Hiramatsu wrote:
>>> Hi,
>>>
>>> These patches allow x86 instruction decoder to decode
>>> xen-cpuid which has XEN_EMULATE_PREFIX, and prohibit
>>> kprobes to probe on it.
>>>
>>> Josh reported that the objtool can not decode such special
>>> prefixed instructions, and I found that we also have to
>>> prohibit kprobes to probe on such instruction.
>>>
>>> This series can be applied on -tip master branch which
>>> has merged Josh's objtool/perf sharing common x86 insn
>>> decoder series.
>> The paravirtualised xen-cpuid is were you'll see it most in a regular
>> kernel, but be aware that it is also used for testing purposes in other
>> circumstances, and there is an equivalent KVM prefix which is used for
>> KVM testing.
> Good catch! I didn't notice that. Is that really same sequance or KVM uses
> another sequence of instructions for KVM prefix?

I don't know if you've spotted, but the prefix is a ud2a instruction
followed by 'xen' in ascii.

The KVM version was added in c/s 6c86eedc206dd1f9d37a2796faa8e6f2278215d2

>
>> It might be better to generalise the decode support to "virtualisation
>> escape prefix" or something slightly more generic.
> Agreed, it is easy to expand it, we can switch the prefix template.
> Could you tell me where I should look? I will add it.

These are the only two I'm aware of.

~Andrew