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Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX
- To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 5 Sep 2019 09:53:32 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Borislav Petkov <bp@xxxxxxxxx>, Randy Dunlap <rdunlap@xxxxxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Masami Hiramatsu <mhiramat@xxxxxxxxxx>
- Delivery-date: Thu, 05 Sep 2019 08:53:52 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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
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