linux-kernel - Re: Spectre mitigation doesn't seem to work at all?!

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Date:   Mon, 4 Jun 2018 12:00:28 +0200
From:   Andreas Hartmann <andihartmann@...19freenet.de>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Spectre mitigation doesn't seem to work at all?!

Hello Mark,

On 06/04/2018 at 11:19 AM Mark Rutland wrote:
> On Mon, Jun 04, 2018 at 10:50:07AM +0200, Andreas Hartmann wrote:
>> Hello Peter,
>>
>> thanks for your answer! I appreciate it!
>>
>> On 06/04/2018 at 10:15 AM Peter Zijlstra wrote:
>>> On Fri, Jun 01, 2018 at 02:19:38PM +0200, Andreas Hartmann wrote:
>>>
>>>> I tested the spectre mitigation of different machines and kernels with
>>>> https://github.com/crozone/SpectrePoC
>>>>
>>>> You can see the results below.
>>>
>>>> My question: Did I miss something?
>>>
>>> Yes.
>>>
>>>> Build: ... INTEL_MITIGATION_DISABLED LINUX_KERNEL_MITIGATION_DISABLED
>>>> Build: ... INTEL_MITIGATION_DISABLED LINUX_KERNEL_MITIGATION_DISABLED
>>>> Build: ... INTEL_MITIGATION_DISABLED LINUX_KERNEL_MITIGATION_DISABLED
>>>
>>>                               ^^^^^^^^                         ^^^^^^^^
>>>
>>> The POC is a v1 on itself. V1 needs to be fixed for every individual
>>> executable (worse, for every individual location in the code, and we're
>>> still finding them). The kernel mitigation status for v1 only indicates
>>> the kernel itself has mitigations (for some locations).
>>>
>>> The POC is meant to test effectiveness of these mitigations, either the
>>> original LFENCE or the dependent instruction thing, but you have to
>>> enable one or the other.
>>
>> Ok, this means every program running on the machine has to care itself
>> to be spectre v1 - safe.
> 
> Correct. Primiarily this matters for things like JITs, where untrusted code may
> be run in the same address space as sensitive data.
> 
>> A malicious program most probably won't care about that. Therefore, my
>> next question is: which memory regions can be exploited by a malicious
>> program? The complete physical memory or only the memory provided to the
>> malicious program? Should be the latter if this approach should have any
>> impact.
> 
> Assuming you have a CPU which is not vulnerable to meltdown / variant-3, or you
> have mitigated this, (e.g. with KPTI), a malicious program can only access data
> within its own address space.
> 
> Spectre variant-1 alone only gives access to memory in the address space of the
> program itself.

Thanks Mark! Now I've a better understanding about the effects the
different vulnerabilities around Spectre and Meltdown do have and I'm
now hopefully able to better estimate them.

As I'm mostly using AMD-CPUs (like Ryzen 1 e.g.) for virtualization, I
should be secure by default regarding unwanted global memory access from
the VM to the host memory, because the Ryzen 1 CPU is not affected by
Meltdown at all.


Regards,
Andreas