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

[Masami Hiramatsu]

On Thu, 5 Sep 2019 09:53:32 +0100
Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 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
> 

Hmm, that consumes the first byte of the next instruction.
For example, 

  .text
xen:
  ud2; .ascii "xen"; cpuid
kvm:
  ud2; .ascii "kvm"; cpuid

0000000000000000 <xen>:
   0:	0f 0b                	ud2    
   2:	78 65                	js     69 <kvm+0x62>
   4:	6e                   	outsb  %ds:(%rsi),(%dx)
   5:	0f a2                	cpuid  

0000000000000007 <kvm>:
   7:	0f 0b                	ud2    
   9:	6b 76 6d 0f          	imul   $0xf,0x6d(%rsi),%esi
   d:	a2                   	.byte 0xa2

This will disturbe decoding bytestream. Anyway, with the next version
it will be fixed in x86 insn decoder.

Thanks,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>