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

Masami Hiramatsu
2019-09-05T01:49:45Z

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?

>
> 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.

Thank you,

>
> ~Andrew

--
Masami Hiramatsu <mhiramat@kernel.org>