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SubjectRe: [PATCH] KVM: VMX: Forbid userspace MSR filters for x2APIC
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On 20.10.20 11:27, Paolo Bonzini wrote:
>
> On 19/10/20 19:45, Graf (AWS), Alexander wrote:
>>> + * In principle it would be possible to trap x2apic ranges
>>> + * if !lapic_in_kernel. This however would be complicated
>>> + * because KVM_X86_SET_MSR_FILTER can be called before
>>> + * KVM_CREATE_IRQCHIP or KVM_ENABLE_CAP.
>>> + */
>>> + for (i = 0; i < ARRAY_SIZE(filter.ranges); i++)
>>> + if (range_overlaps_x2apic(&filter.ranges[i]))
>>> + return -EINVAL;
>>
>> What if the default action of the filter is to "deny"? Then only an
>> MSR filter to allow access to x2apic MSRs would make the full
>> filtering logic adhere to the constraints, no?
>
> Right; or more precisely, that is handled by Sean's patch that he had
> posted earlier. This patch only makes it impossible to set up such a
> filter.

What I'm saying is that a "filter rule" can either mean "allow" or
"deny". Here you're only checking if there is a rule. What you want to
filter for is whether there is a denying rule (including default
fallback) for x2apic after all rules are in place.

Imagine you add the following filter:

{
count: 1,
default_allow: false,
ranges: [
{
flags: KVM_MSR_FILTER_READ,
nmsrs: 1,
base: MSR_EFER,
bitmap: { 1 },
},
],
}

That filter would set all x2apic registers to "deny", but would not be
caught by the code above. Conversely, a range that explicitly allows
x2apic ranges with default_allow=0 would be rejected by this patch.


Alex



Amazon Development Center Germany GmbH
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Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
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