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

Masami Hiramatsu
2019-09-05T13:10:05Z

On Thu, 5 Sep 2019 20:32:24 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> 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

Hmm, I think I might misunderstand what the "emulate prefix"... that is not
a prefix which replace actual prefix, but just works like an escape sequence.
Thus the next instruction can have any x86 prefix, correct?

If so, this patch doesn't work. I have to add a new field in struct insn
like "insn.emulate_prefix_size" so that we can keep a room for the prefixes
for real instruction.

Thank you,

--
Masami Hiramatsu <mhiramat@kernel.org>