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

[Andrew Cooper]

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.

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>:
ÂÂ 5:ÂÂÂ 0f 0bÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ÂÂÂ ud2ÂÂÂ
ÂÂ 7:ÂÂÂ 58ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ÂÂÂ popÂÂÂ %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