Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Aug 25, 2021 at 5:43 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Maxim Levitsky <mlevitsk@redhat.com> writes:
>
> > On Wed, 2021-08-25 at 10:21 +0200, Vitaly Kuznetsov wrote:
> >> Maxim Levitsky <mlevitsk@redhat.com> 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.

It would be nice if the compiler took care of validating bitmap sizes
for us. Shouldn't we make the function prototypes explicit about the
bitmap sizes they expect?

I believe some `typedef DECLARE_BITMAP(...)` or `typedef struct {
DECLARE_BITMAP(...) } ...` declarations would be very useful here.

-- 
Eduardo