Messages in this thread | | | Date | Thu, 27 Oct 2022 13:11:01 +0200 | Subject | Re: [PATCH v2 03/16] KVM: x86: Always use non-compat vcpu_runstate_info size for gfn=>pfn cache | From | Paolo Bonzini <> |
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On 10/13/22 23:12, Sean Christopherson wrote: > Always use the size of Xen's non-compat vcpu_runstate_info struct when > checking that the GPA+size doesn't cross a page boundary. Conceptually, > using the current mode is more correct, but KVM isn't consistent with > itself as kvm_xen_vcpu_set_attr() unconditionally uses the "full" size > when activating the cache. More importantly, prior to the introduction > of the gfn_to_pfn_cache, KVM _always_ used the full size, i.e. allowing > the guest (userspace?) to use a poorly aligned GPA in 32-bit mode but not > 64-bit mode is more of a bug than a feature, and fixing the bug doesn't > break KVM's historical ABI.
I'd rather not introduce additional restrictions in KVM, mostly because it's actually easy to avoid this patch by instead enforcing that attributes are set in a sensible order:
- long mode cannot be changed after the shared info page is enabled (which makes sense because the shared info page also has a compat version)
- the caches must be activated after the shared info page (which enforces that the vCPU attributes are set after the VM attributes)
This is technically a userspace API break, but nobody is really using this API outside Amazon so... Patches coming after I finish testing.
Paolo
> Always using the non-compat size will allow for future gfn_to_pfn_cache > clenups as this is (was) the only case where KVM uses a different size at > check()+refresh() than at activate(). E.g. the length/size of the cache > can be made immutable and dropped from check()+refresh(), which yields a > cleaner set of APIs and avoids potential bugs that could occur if check() > where invoked with a different size than refresh(). > > Fixes: a795cd43c5b5 ("KVM: x86/xen: Use gfn_to_pfn_cache for runstate area") > Cc: David Woodhouse <dwmw2@infradead.org> > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > arch/x86/kvm/xen.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c > index b2be60c6efa4..9e79ef2cca99 100644 > --- a/arch/x86/kvm/xen.c > +++ b/arch/x86/kvm/xen.c > @@ -212,10 +212,7 @@ void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state) > if (!vx->runstate_cache.active) > return; > > - if (IS_ENABLED(CONFIG_64BIT) && v->kvm->arch.xen.long_mode) > - user_len = sizeof(struct vcpu_runstate_info); > - else > - user_len = sizeof(struct compat_vcpu_runstate_info); > + user_len = sizeof(struct vcpu_runstate_info); > > read_lock_irqsave(&gpc->lock, flags); > while (!kvm_gfn_to_pfn_cache_check(v->kvm, gpc, gpc->gpa,
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