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

Masami Hiramatsu
2019-09-05T11:32:31Z

On Thu, 5 Sep 2019 08:54:17 +0100
Andrew Cooper <andrew.cooper3@citrix.com> wrote:

> On 05/09/2019 02:49, Masami Hiramatsu wrote:
> > On Wed, 4 Sep 2019 12:54:55 +0100
> > Andrew Cooper <andrew.cooper3@citrix.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
>

Ah, OK. I see it. But it seems that another ud0/ud1 can be used by
other (new) virtualization. So at this moment I will just add a sequence
as a pattern of prefix. Not use a fixed ud2 + sig.

Thank you,

> >
> >> 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

--
Masami Hiramatsu <mhiramat@kernel.org>