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

[Suzuki K Poulose <Suzuki.Poulose@arm.com>]

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.

Cheers
Suzuki