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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 09:53:32 +0100	[thread overview]
Message-ID: <4de91a14-2051-197e-6ab0-beb2538c40f9@citrix.com> (raw)
In-Reply-To: <20190905082647.GZ2332@hirez.programming.kicks-ass.net>

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.

As for the Xen prefix, it's introduction pre-dates me substantially, and
I don't know whether the disassembly was considered, or we just got lucky.

IMO, the string 'Xen' would would have been sightly nicer

0000000000000005 <Xen>:
   5:    0f 0b                    ud2   
   7:    58                       pop    %rax
   8:    65 6e                    outsb  %gs:(%rsi),(%dx)

but we're 13 years too late to amend this.

> I know it is water under the bridge at this point; but you could've used
> UD1 with a displacement with some 'unlikely' values. That way it
> would've decoded to a single instruction.
>
> Something like:
>
> 	ud1    0x6e6578(%rax),%rax
>
> which spells out "xen\0" in the displacement:
>
> 	48 0f b9 80 78 65 6e 00

:)

I seem to recall UD0 and UD1 being very late to the documentation party.

~Andrew

  reply	other threads:[~2019-09-05  8:53 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 [this message]
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
2019-09-04 12:02 ` Peter Zijlstra

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