Re: [Xen-devel] [PATCH v5 1/4] nvmx: implement support for MSR bitmaps

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Sent: Monday, February 17, 2020 7:46 PM
> 
> Current implementation of nested VMX has a half baked handling of MSR
> bitmaps for the L1 VMM: it maps the L1 VMM provided MSR bitmap, but
> doesn't actually load it into the nested vmcs, and thus the nested
> guest vmcs ends up using the same MSR bitmap as the L1 VMM.
> 
> This is wrong as there's no assurance that the set of features enabled
> for the L1 vmcs are the same that L1 itself is going to use in the
> nested vmcs, and thus can lead to misconfigurations.
> 
> For example L1 vmcs can use x2APIC virtualization and virtual
> interrupt delivery, and thus some x2APIC MSRs won't be trapped so that
> they can be handled directly by the hardware using virtualization
> extensions. On the other hand, the nested vmcs created by L1 VMM might
> not use any of such features, so using a MSR bitmap that doesn't trap
> accesses to the x2APIC MSRs will be leaking them to the underlying
> hardware.
> 
> Fix this by crafting a merged MSR bitmap between the one used by L1
> and the nested guest.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
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