linux-kernel - Re: [PATCH v2 17/20] arm64: bp hardening: Allow late CPUs to enable work around

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Message-ID: <77efc8b3-48c1-fbaf-ce11-15123755262a@arm.com>
Date:   Thu, 8 Feb 2018 17:59:09 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Marc Zyngier <marc.zyngier@....com>,
        Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ckadabi@...eaurora.org, ard.biesheuvel@...aro.org,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com
Subject: Re: [PATCH v2 17/20] arm64: bp hardening: Allow late CPUs to enable
 work around

On 08/02/18 16:58, Suzuki K Poulose wrote:
> On 08/02/18 12:26, Marc Zyngier wrote:
>> On 08/02/18 12:19, Suzuki K Poulose wrote:
>>> On 07/02/18 10:39, Dave Martin wrote:
>>>> On Wed, Jan 31, 2018 at 06:28:04PM +0000, Suzuki K Poulose wrote:
>>>>> We defend against branch predictor training based exploits by
>>>>> taking specific actions (based on the CPU model) to invalidate
>>>>> the Branch predictor buffer (BPB). This is implemented by per-CPU
>>>>> ptr, which installs the specific actions for the CPU model.
>>>>>
>>>>> The core code can handle the following cases where:
>>>>>    1) some CPUs doesn't need any work around
>>>>>    2) a CPU can install the work around, when it is brought up,
>>>>>       irrespective of how late that happens.
>>>
>>> With the recent patches from Marc to expose this information to KVM
>>> guests, it looks like allowing a late CPU to turn this on is not going
>>> to be a good idea. We unconditionally set the capability even
>>> when we don't need the mitigation. So I am not really sure if
>>> we should go ahead with this patch. I am open to suggestions
>>>
>>> Marc,
>>>
>>> What do you think ?
>>
>> By the time we bring in that CPU that requires some level of mitigation,
>> we may be running a guest already, and we've told that guest that no
>> mitigation was required. If we bring in that CPU, we break that promise,
>> and the guest becomes vulnerable without knowing about it.
>>
>> The same thing is valid for userspace once we expose the status of the
>> mitigation in /sys, just like x86 does. If we transition from not
>> vulnerable to vulnerable (or even mitigated), we have lied to userspace.
>>
>> In either case, I don't think breaking this contract is acceptable.
> 
> Thanks Marc, I have dropped this patch from the series.

On a second thought, should we allow this if the KVM is not configured in ?

Cheers
Suzuki