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

Peter Zijlstra
2019-09-05T09:26:45Z

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 :/

> but we're 13 years too late to amend this.

Yah, I figured :/

> > 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.

They were; and the documentation for still UD0 differs between vendors :/