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Date:   Thu, 26 Aug 2021 15:27:18 -0400
From:   Eduardo Habkost <ehabkost@redhat.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>, 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
Subject: Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access
 from ioapic_write_indirect()
Message-ID: <20210826192718.h6ct6pe2njaazhyh@habkost.net>
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I'm re-reading this, and:

On Tue, Aug 24, 2021 at 07:07:58PM +0300, Maxim Levitsky wrote:
[...]
> Hi,
> 
> 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).

How exactly do you think kvm_apic_map_get_dest_lapic() can
overflow?  It never writes beyond `bitmap[0]`, as far as I can
see.

>  
>  
>        -> 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.
>  
>  [...]

-- 
Eduardo