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From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>,
	Ingo Molnar <mingo@kernel.org>, Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Borislav Petkov <bp@alien8.de>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	xen-devel@lists.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX
Date: Thu, 5 Sep 2019 10:26:47 +0200	[thread overview]
Message-ID: <20190905082647.GZ2332@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1372ce73-e2d8-6144-57df-a98429587826@citrix.com>

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

  reply	other threads:[~2019-09-05  8:27 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 11:45 Masami Hiramatsu
2019-09-04 11:45 ` [PATCH -tip 1/2] x86: xen: insn: Decode XEN_EMULATE_PREFIX correctly Masami Hiramatsu
2019-09-04 11:46 ` [PATCH -tip 2/2] x86: kprobes: Prohibit probing on instruction which has Xen prefix Masami Hiramatsu
2019-09-04 11:54 ` [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX Andrew Cooper
2019-09-05  1:49   ` Masami Hiramatsu
2019-09-05  7:54     ` Andrew Cooper
2019-09-05  8:26       ` Peter Zijlstra [this message]
2019-09-05  8:53         ` Andrew Cooper
2019-09-05  9:26           ` Peter Zijlstra
2019-09-05  9:38             ` Andrew Cooper
2019-09-05 12:49           ` [OT] " Masami Hiramatsu
2019-09-05 11:32       ` Masami Hiramatsu
2019-09-05 13:09         ` Masami Hiramatsu
2019-09-05 13:31           ` Andrew Cooper
2019-09-05 16:12             ` Masami Hiramatsu
2019-09-04 12:02 ` Peter Zijlstra

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