On Wed, 2021-08-25 at 10:21 +0200, Vitaly Kuznetsov wrote: > Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes: > > > On Tue, 2021-08-24 at 16:42 +0200, Vitaly Kuznetsov wrote: > ... > > Not a classical review but, > > I did some digital archaeology with this one, trying to understand what is going on: > > > > > > I think that 16 bit vcpu bitmap is due to the fact that IOAPIC spec states that > > it can address up to 16 cpus in physical destination mode. > > > > In logical destination mode, assuming flat addressing and that logical id = 1 << physical id > > which KVM hardcodes, it is also only possible to address 8 CPUs. > > > > However(!) in flat cluster mode, the logical apic id is split in two. > > We have 16 clusters and each have 4 CPUs, so it is possible to address 64 CPUs, > > and unlike the logical ID, the KVM does honour cluster ID, > > thus one can stick say cluster ID 0 to any vCPU. > > > > > > Let's look at ioapic_write_indirect. > > It does: > > > > -> bitmap_zero(&vcpu_bitmap, 16); > > -> kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq, &vcpu_bitmap); > > -> kvm_make_scan_ioapic_request_mask(ioapic->kvm, &vcpu_bitmap); // use of the above bitmap > > > > > > When we call kvm_bitmap_or_dest_vcpus, we can already overflow the bitmap, > > since we pass all 8 bit of the destination even when it is physical. > > > > > > Lets examine the kvm_bitmap_or_dest_vcpus: > > > > -> It calls the kvm_apic_map_get_dest_lapic which > > > > -> for physical destinations, it just sets the bitmap, which can overflow > > if we pass it 8 bit destination (which basically includes reserved bits + 4 bit destination). > > > > > > -> For logical apic ID, it seems to truncate the result to 16 bit, which isn't correct as I explained > > above, but should not overflow the result. > > > > > > -> If call to kvm_apic_map_get_dest_lapic fails, it goes over all vcpus and tries to match the destination > > This can overflow as well. > > > > > > I also don't like that ioapic_write_indirect calls the kvm_bitmap_or_dest_vcpus twice, > > and second time with 'old_dest_id' > > > > I am not 100% sure why old_dest_id/old_dest_mode are needed as I don't see anything in the > > function changing them. > > I think only the guest can change them, so maybe the code deals with the guest changing them > > while the code is running from a different vcpu? > > > > The commit that introduced this code is 7ee30bc132c683d06a6d9e360e39e483e3990708 > > Nitesh Narayan Lal, maybe you remember something about it? > > > > Before posting this patch I've contacted Nitesh privately, he's > currently on vacation but will take a look when he gets back. > > > Also I worry a lot about other callers of kvm_apic_map_get_dest_lapic > > > > It is also called from kvm_irq_delivery_to_apic_fast, and from kvm_intr_is_single_vcpu_fast > > and both seem to also use 'unsigned long' for bitmap, and then only use 16 bits of it. > > > > I haven't dug into them, but these don't seem to be IOAPIC related and I think > > can overwrite the stack as well. > > I'm no expert in this code but when writing the patch I somehow > convinced myself that a single unsigned long is always enough. I think > that for cluster mode 'bitmap' needs 64-bits (and it is *not* a > vcpu_bitmap, we need to convert). I may be completely wrong of course > but in any case this is a different issue. In ioapic_write_indirect() we > have 'vcpu_bitmap' which should certainly be longer than 64 bits. This code which I mentioned in 'other callers' as far as I see is not IOAPIC related. For regular local APIC all bets are off, any vCPU and apic ID are possible (xapic I think limits apic id to 255 but x2apic doesn't). I strongly suspect that this code can overflow as well. Best regards, Maxim Levitsky >
