Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()
From: Vitaly Kuznetsov
Date: Wed Aug 25 2021 - 05:43:48 EST
Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:
> 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.
I've probably missed something but I don't see how
kvm_apic_map_get_dest_lapic() can set bits above 64 in 'bitmap'. If it
can, then we have a problem indeed.