Messages in this thread | | | Date | Fri, 31 May 2019 09:02:47 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v1] KVM: x86: PMU Whitelist |
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On 05/30/2019 01:11 AM, Eric Hankland wrote: > On Wed, May 29, 2019 at 12:49 AM Wei Wang <wei.w.wang@intel.com> wrote: >> On 05/29/2019 02:14 AM, Eric Hankland wrote: >>> On Mon, May 27, 2019 at 6:56 PM Wei Wang <wei.w.wang@intel.com> wrote: >>>> On 05/23/2019 06:23 AM, Eric Hankland wrote: >>>>> - Add a VCPU ioctl that can control which events the guest can monitor. >>>>> >>>>> Signed-off-by: ehankland <ehankland@google.com> >>>>> --- >>>>> Some events can provide a guest with information about other guests or the >>>>> host (e.g. L3 cache stats); providing the capability to restrict access >>>>> to a "safe" set of events would limit the potential for the PMU to be used >>>>> in any side channel attacks. This change introduces a new vcpu ioctl that >>>>> sets an event whitelist. If the guest attempts to program a counter for >>>>> any unwhitelisted event, the kernel counter won't be created, so any >>>>> RDPMC/RDMSR will show 0 instances of that event. >>>> The general idea sounds good to me :) >>>> >>>> For the implementation, I would have the following suggestions: >>>> >>>> 1) Instead of using a whitelist, it would be better to use a blacklist to >>>> forbid the guest from counting any core level information. So by default, >>>> kvm maintains a list of those core level events, which are not supported to >>>> the guest. >>>> >>>> The userspace ioctl removes the related events from the blacklist to >>>> make them usable by the guest. >>>> >>>> 2) Use vm ioctl, instead of vcpu ioctl. The blacklist-ed events can be >>>> VM wide >>>> (unnecessary to make each CPU to maintain the same copy). >>>> Accordingly, put the pmu event blacklist into kvm->arch. >>>> >>>> 3) Returning 1 when the guest tries to set the evetlsel msr to count an >>>> event which is on the blacklist. >>>> >>>> Best, >>>> Wei >>> Thanks for the feedback. I have a couple concerns with a KVM >>> maintained blacklist. First, I'm worried it will be difficult to keep >>> such a list up to date and accurate (both coming up with the initial >>> list since there are so many events, and updating it whenever any new >>> events are published or vulnerabilities are discovered). >> Not sure about "so many" above. I think there should be much >> fewer events that may need to be blacklisted. >> >> For example the event table 19-3 from SDM 19.2 shows hundreds of >> events, how many of them would you think that need to be blacklisted? >> >>> Second, users >>> may want to differentiate between whole-socket and sub-socket VMs >>> (some events may be fine for the whole-socket case) - keeping a single >>> blacklist wouldn't allow for this. >> Why wouldn't? >> In any case (e.g. the whole socket dedicated to the single VM) we >> want to unlock the blacklisted events, we can have the userspace >> (e.g. qemu command line options "+event1, +event2") do ioctl to >> have KVM do that. >> >> Btw, for the L3 cache stats event example, I'm not sure if that could >> be an issue if we have "AnyThread=0". I'll double confirm with >> someone. >> >> Best, >> Wei >> Not sure about "so many" above. I think there should be much >> fewer events that may need to be blacklisted. > I think you're right that there are not as many events that seem like > they could leak info as events that seem like they won't, but I think > the work to validate that they definitely don't could be expensive; > with a whitelist it's easy to start with a smaller set and > incrementally add to it without having to evaluate all the events > right away.
Before going that whitelist/blacklist direction, do you have an event example that couldn't be solved by setting "AnyThread=0"?
If no, I think we could simply gate guest's setting of "AnyThread=0".
Best, Wei
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