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From: Masami Hiramatsu <mhiramat@kernel.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: 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: Fri, 6 Sep 2019 01:12:26 +0900	[thread overview]
Message-ID: <20190906011226.f5e8f3d69c6cc8254f97ae7c@kernel.org> (raw)
In-Reply-To: <1d868c99-58c5-1bbd-e6a4-2003dd319b5b@citrix.com>

On Thu, 5 Sep 2019 14:31:56 +0100
Andrew Cooper <andrew.cooper3@citrix.com> wrote:

> >>> 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?
> 
> There is a bit of history here :)
> 
> Originally, 13 years ago, Xen invented the "Force Emulate Prefix", which
> was the sequence:
> 
> ud2a; .ascii 'xen'; cpuid
> 
> which hit the #UD handler and was recognised as a request for
> virtualised CPUID information.  This was for ring-deprivileged
> virtualisation, and is needed because the CPUID instruction itself
> doesn't trap to the hypervisor.
> 
> Following some security issues in our instruction emulator, I reused
> this prefix with VT-x/SVM guests for testing purposes.  It behaves in a
> similar manner - when enabled, it is recognised in #UD exception
> intercept, and causes Xen to add 5 to the instruction pointer, then
> emulate the instruction starting there.
> 
> Then various folk thought that having the same kind of ability to test
> KVM's instruction emulator would be a good idea, so they borrowed the idea.
> 
> From a behaviour point of view, it is an opaque 5 bytes which means
> "break into the hypervisor, then emulate the following instruction".
> 
> The name "prefix" is unfortunate.  It was named thusly because from the
> programmers point of view, it was something you put before the CPUID
> instruction which wanted to be emulated.  It is not related to x86
> instruction concept of a prefix.

OK, then we should not use the insn->prefixes for those escape sequences.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>

  reply	other threads:[~2019-09-05 16:12 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
2019-09-05 13:31           ` Andrew Cooper
2019-09-05 16:12             ` Masami Hiramatsu [this message]
2019-09-04 12:02 ` Peter Zijlstra

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