Messages in this thread | | | Subject | Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page | From | Ankur Arora <> | Date | Wed, 2 Dec 2020 12:33:43 -0800 |
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On 2020-12-02 2:44 a.m., Joao Martins wrote: > [late response - was on holiday yesterday] > > On 12/2/20 12:40 AM, Ankur Arora wrote: >> On 2020-12-01 5:07 a.m., David Woodhouse wrote: >>> On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote: >>>> +static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn) >>>> +{ >>>> + struct shared_info *shared_info; >>>> + struct page *page; >>>> + >>>> + page = gfn_to_page(kvm, gfn); >>>> + if (is_error_page(page)) >>>> + return -EINVAL; >>>> + >>>> + kvm->arch.xen.shinfo_addr = gfn; >>>> + >>>> + shared_info = page_to_virt(page); >>>> + memset(shared_info, 0, sizeof(struct shared_info)); >>>> + kvm->arch.xen.shinfo = shared_info; >>>> + return 0; >>>> +} >>>> + >>> >>> Hm. >>> >>> How come we get to pin the page and directly dereference it every time, >>> while kvm_setup_pvclock_page() has to use kvm_write_guest_cached() >>> instead? >> >> So looking at my WIP trees from the time, this is something that >> we went back and forth on as well with using just a pinned page or a >> persistent kvm_vcpu_map(). >> >> I remember distinguishing shared_info/vcpu_info from kvm_setup_pvclock_page() >> as shared_info is created early and is not expected to change during the >> lifetime of the guest which didn't seem true for MSR_KVM_SYSTEM_TIME (or >> MSR_KVM_STEAL_TIME) so that would either need to do a kvm_vcpu_map() >> kvm_vcpu_unmap() dance or do some kind of synchronization. >> >> That said, I don't think this code explicitly disallows any updates >> to shared_info. >> >>> >>> If that was allowed, wouldn't it have been a much simpler fix for >>> CVE-2019-3016? What am I missing? >> >> Agreed. >> >> Perhaps, Paolo can chime in with why KVM never uses pinned page >> and always prefers to do cached mappings instead? >> > Part of the CVE fix to not use cached versions. > > It's not a longterm pin of the page unlike we try to do here (partly due to the nature > of the pages we are mapping) but we still we map the gpa, RMW the steal time struct, and > then unmap the page. > > See record_steal_time() -- but more specifically commit b043138246 ("x86/KVM: Make sure > KVM_VCPU_FLUSH_TLB flag is not missed"). > > But I am not sure it's a good idea to follow the same as record_steal_time() given that > this is a fairly sensitive code path for event channels. > >>> >>> Should I rework these to use kvm_write_guest_cached()? >> >> kvm_vcpu_map() would be better. The event channel logic does RMW operations >> on shared_info->vcpu_info. >> > Indeed, yes. > > Ankur IIRC, we saw missed event channels notifications when we were using the > {write,read}_cached() version of the patch. > > But I can't remember the reason it was due to, either the evtchn_pending or the mask > word -- which would make it not inject an upcall.
If memory serves, it was the mask. Though I don't think that we had kvm_{write,read}_cached in use at that point -- given that they were definitely not RMW safe.
Ankur
> > Joao >
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