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From: Masami Hiramatsu <mhiramat@kernel.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	Ingo Molnar <mingo@kernel.org>, Juergen Gross <jgross@suse.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>, <x86@kernel.org>,
	<linux-kernel@vger.kernel.org>, Borislav Petkov <bp@alien8.de>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	<xen-devel@lists.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>
Subject: Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX
Date: Thu, 5 Sep 2019 22:09:58 +0900	[thread overview]
Message-ID: <20190905220958.d0189e1e253f9e553b880675@kernel.org> (raw)
In-Reply-To: <20190905203224.e41d7f3dfbf918c5031f9766@kernel.org>

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>

  reply	other threads:[~2019-09-05 13:10 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 11:45 Masami Hiramatsu
2019-09-04 11:45 ` [PATCH -tip 1/2] x86: xen: insn: Decode XEN_EMULATE_PREFIX correctly Masami Hiramatsu
2019-09-04 11:46 ` [PATCH -tip 2/2] x86: kprobes: Prohibit probing on instruction which has Xen prefix Masami Hiramatsu
2019-09-04 11:54 ` [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX Andrew Cooper
2019-09-05  1:49   ` Masami Hiramatsu
2019-09-05  7:54     ` Andrew Cooper
2019-09-05  8:26       ` Peter Zijlstra
2019-09-05  8:53         ` Andrew Cooper
2019-09-05  9:26           ` Peter Zijlstra
2019-09-05  9:38             ` Andrew Cooper
2019-09-05 12:49           ` [OT] " Masami Hiramatsu
2019-09-05 11:32       ` Masami Hiramatsu
2019-09-05 13:09         ` Masami Hiramatsu [this message]
2019-09-05 13:31           ` Andrew Cooper
2019-09-05 16:12             ` Masami Hiramatsu
2019-09-04 12:02 ` Peter Zijlstra

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