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Subject: Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access
 from ioapic_write_indirect()
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Nitesh Narayan Lal <nitesh@redhat.com>,
        linux-kernel@vger.kernel.org, Eduardo Habkost <ehabkost@redhat.com>
Date:   Wed, 25 Aug 2021 12:11:26 +0300
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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.

Best regards,
	Maxim Levitsky

>