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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>,
	Ingo Molnar <mingo@kernel.org>, Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.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 10:38:19 +0100	[thread overview]
Message-ID: <6f69e9ac-66db-049d-1484-69fb61f686a3@citrix.com> (raw)
In-Reply-To: <20190905092627.GB2332@hirez.programming.kicks-ass.net>

On 05/09/2019 10:26, Peter Zijlstra wrote:
> On Thu, Sep 05, 2019 at 09:53:32AM +0100, Andrew Cooper wrote:
>> On 05/09/2019 09:26, Peter Zijlstra wrote:
>>> On Thu, Sep 05, 2019 at 08:54:17AM +0100, Andrew Cooper wrote:
>>>
>>>> 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
>>> While the Xen one disassebles to valid instructions, that KVM one does
>>> not:
>>>
>>> 	.text
>>> xen:
>>> 	ud2; .ascii "xen"
>>> kvm:
>>> 	ud2; .ascii "kvm"
>>>
>>> disassembles like:
>>>
>>> 0000000000000000 <xen>:
>>>    0:   0f 0b                   ud2
>>>    2:   78 65                   js     69 <kvm+0x64>
>>>    4:   6e                      outsb  %ds:(%rsi),(%dx)
>>> 0000000000000005 <kvm>:
>>>    5:   0f 0b                   ud2
>>>    7:   6b                      .byte 0x6b
>>>    8:   76 6d                   jbe    77 <kvm+0x72>
>>>
>>> Which is a bit unfortunate I suppose. At least they don't appear to
>>> consume further bytes.
>> It does when you give objdump one extra byte to look at.
>>
>> 0000000000000005 <kvm>:
>>    5:    0f 0b                    ud2   
>>    7:    6b 76 6d 00              imul   $0x0,0x6d(%rsi),%esi
>>
>> I did try to point out that this property should have been checked
>> before settling on 'kvm' as the string.
> Bah you're right; when I write:
>
> 	ud2; .ascii "kvm"; cpuid
>
> The output is gibberish :/

KVM could possibly amend this.  It is an off-by-default testing-only
interface.

Xen's is part of the ABI for ring-deprivielged guests, to force CPUID to
be the virtualised view on hardware without CPUID Faulting.

~Andrew

  reply	other threads:[~2019-09-05  9:38 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 [this message]
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
2019-09-04 12:02 ` Peter Zijlstra

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