Messages in this thread | | | Subject | Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker | From | Jason Wang <> | Date | Mon, 5 Aug 2019 16:21:48 +0800 |
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On 2019/8/5 下午2:28, Michael S. Tsirkin wrote: > On Mon, Aug 05, 2019 at 12:33:45PM +0800, Jason Wang wrote: >> On 2019/8/2 下午10:03, Michael S. Tsirkin wrote: >>> On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: >>>> Btw, I come up another idea, that is to disable preemption when vhost thread >>>> need to access the memory. Then register preempt notifier and if vhost >>>> thread is preempted, we're sure no one will access the memory and can do the >>>> cleanup. >>> Great, more notifiers :( >>> >>> Maybe can live with >>> 1- disable preemption while using the cached pointer >>> 2- teach vhost to recover from memory access failures, >>> by switching to regular from/to user path >> >> I don't get this, I believe we want to recover from regular from/to user >> path, isn't it? > That (disable copy to/from user completely) would be a nice to have > since it would reduce the attack surface of the driver, but e.g. your > code already doesn't do that. >
Yes since it requires a lot of changes.
> >>> So if you want to try that, fine since it's a step in >>> the right direction. >>> >>> But I think fundamentally it's not what we want to do long term. >> >> Yes. >> >> >>> It's always been a fundamental problem with this patch series that only >>> metadata is accessed through a direct pointer. >>> >>> The difference in ways you handle metadata and data is what is >>> now coming and messing everything up. >> >> I do propose soemthing like this in the past: >> https://www.spinics.net/lists/linux-virtualization/msg36824.html. But looks >> like you have some concern about its locality. > Right and it doesn't go away. You'll need to come up > with a test that messes it up and triggers a worst-case > scenario, so we can measure how bad is that worst-case.
> >> But the problem still there, GUP can do page fault, so still need to >> synchronize it with MMU notifiers. > I think the idea was, if GUP would need a pagefault, don't > do a GUP and do to/from user instead.
But this still need to be synchronized with MMU notifiers (or using dedicated work for GUP).
> Hopefully that > will fault the page in and the next access will go through. > >> The solution might be something like >> moving GUP to a dedicated kind of vhost work. > Right, generally GUP. > >>> So if continuing the direct map approach, >>> what is needed is a cache of mapped VM memory, then on a cache miss >>> we'd queue work along the lines of 1-2 above. >>> >>> That's one direction to take. Another one is to give up on that and >>> write our own version of uaccess macros. Add a "high security" flag to >>> the vhost module and if not active use these for userspace memory >>> access. >> >> Or using SET_BACKEND_FEATURES? > No, I don't think it's considered best practice to allow unpriveledged > userspace control over whether kernel enables security features.
Get this.
> >> But do you mean permanent GUP as I did in >> original RFC https://lkml.org/lkml/2018/12/13/218? >> >> Thanks > Permanent GUP breaks THP and NUMA.
Yes.
Thanks
> >>>
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