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

[Andrew Cooper]

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>:
>> ÂÂ 5:ÂÂÂ 0f 0bÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ÂÂÂ ud2ÂÂÂ
>> ÂÂ 7:ÂÂÂ 6b 76 6d 00ÂÂÂÂÂÂÂÂÂ ÂÂÂ imulÂÂ $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