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

[Peter Zijlstra]

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 :/