Messages in this thread | | | Subject | Re: [PATCH v2 17/20] arm64: bp hardening: Allow late CPUs to enable work around | From | Suzuki K Poulose <> | Date | Thu, 8 Feb 2018 17:59:55 +0000 |
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On 08/02/18 17:59, Suzuki K Poulose wrote: > 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 ?
Oops, no! Please ignore it. We are going to expose this to other userpsace anyway. Sorry about the noise.
Suzuki
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