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Subject[OT] Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX
On Thu, 5 Sep 2019 09:53:32 +0100
Andrew Cooper <andrew.cooper3@citrix.com> 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
>

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@kernel.org>

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