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

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Date:   Thu, 5 Sep 2019 10:26:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        xen-devel@...ts.xenproject.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on
 XEN_EMULATE_PREFIX

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.

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