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

Andrew Cooper
2019-09-05T08:53:45Z

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