Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX

Andrew Cooper
2019-09-05T09:38:24Z

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