Re: [PATCH] KVM: x86: fix guest-initiated crash with x2apic (CVE-2013-6376)

[Gleb Natapov <gleb@kernel.org>]

On Mon, Dec 16, 2013 at 01:01:10PM +0100, Radim Krčmář wrote:
> > >  - Where does the 'only one supported cluster' come from?
> > > 
> > "only one supported cluster" comes from 8 bit cpuid limitation of KVM's x2apic
> > implementation. With 8 bit cpuid you can only address cluster 0 in logical mode.
> 
> One x2apic cluster has 16 cpus and we generate the x2apic LDR correctly,
> so 8 bit cpuid can address first 16 clusters as well.
> 
>   u32 ldr = ((id >> 4) << 16) | (1 << (id & 0xf));
> 
Interrupt from a device cannot generate such ldr, only IPI can. Only
4 cpus in cluster zero are addressable in clustering mode by a
device. Without irq remapping x2apic is a PV interface between host
and guest where guest needs to know KVM implementation's limitation to
use it. I do not see a point in fixing problems in x2apic logical mode
emulation right now since it will not make it usable, as long as
there is not security problems there.

> > >    I only see we use 'struct kvm_lapic *logical_map[16][16];', which
> > >    supports 16 clusters of 16 apics = first 256 vcpus, so if we map
> > >    everything to logical_map[0][0:15], we would not work correctly in
> > >    the cluster x2apic, with > 16 vcpus.
> > > 
> > Such config cannot work today because of 8 bit cpuid limitation. When the limitation
> > will be removed KMV_X2APIC_CID_BITS will be set to actual number of bits we want to support.
> 
> Even with KMV_X2APIC_CID_BITS = 4, which would allow us to support 8 bit
> cpuid, we would still deliver interrupts destined for cpuid > 256 to
> potentially plugged cpus.
Again, KMV_X2APIC_CID_BITS = 4 will not allow us to support 8 bit cpuids
unfortunately, not sure what you mean by the second part of the sentence.

--
			Gleb.