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Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX
- To: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 5 Sep 2019 08:54:17 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Borislav Petkov <bp@xxxxxxxxx>, Randy Dunlap <rdunlap@xxxxxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>
- Delivery-date: Thu, 05 Sep 2019 07:54:28 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 05/09/2019 02:49, Masami Hiramatsu wrote:
> On Wed, 4 Sep 2019 12:54:55 +0100
> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
>> On 04/09/2019 12:45, Masami Hiramatsu wrote:
>>> Hi,
>>>
>>> These patches allow x86 instruction decoder to decode
>>> xen-cpuid which has XEN_EMULATE_PREFIX, and prohibit
>>> kprobes to probe on it.
>>>
>>> Josh reported that the objtool can not decode such special
>>> prefixed instructions, and I found that we also have to
>>> prohibit kprobes to probe on such instruction.
>>>
>>> This series can be applied on -tip master branch which
>>> has merged Josh's objtool/perf sharing common x86 insn
>>> decoder series.
>> The paravirtualised xen-cpuid is were you'll see it most in a regular
>> kernel, but be aware that it is also used for testing purposes in other
>> circumstances, and there is an equivalent KVM prefix which is used for
>> KVM testing.
> Good catch! I didn't notice that. Is that really same sequance or KVM uses
> another sequence of instructions for KVM prefix?
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
>
>> It might be better to generalise the decode support to "virtualisation
>> escape prefix" or something slightly more generic.
> Agreed, it is easy to expand it, we can switch the prefix template.
> Could you tell me where I should look? I will add it.
These are the only two I'm aware of.
~Andrew
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