linux-kernel - Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic

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Date:	Mon, 11 Jul 2016 09:43:24 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	"Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic_map



On 11/07/2016 08:07, Yang Zhang wrote:
>>
>>      mutex_lock(&kvm->arch.apic_map_lock);
>>
>> +    kvm_for_each_vcpu(i, vcpu, kvm)
>> +        if (kvm_apic_present(vcpu))
>> +            max_id = max(max_id, kvm_apic_id(vcpu->arch.apic));
>> +
>> +    new = kzalloc(sizeof(struct kvm_apic_map) +
>> +                  sizeof(struct kvm_lapic *) * (max_id + 1),
>> GFP_KERNEL);
>> +
> 
> I think this may cause the host runs out of memory if a malicious guest
> did follow thing:
> 1. vcpu a is doing apic map recalculation.
> 2. vcpu b write the apic id with 0xff
> 3. then vcpu b enable the x2apic: in kvm_lapic_set_base(), we will set
> apic_base to new value before reset the apic id.
> 4. vcpu a may see the x2apic enabled in vcpu b plus an old apic
> id(0xff), and max_id will become (0xff >> 24).

The bug is not really here but in patch 6---but you're right nevertheless!

I guess the easiest solution is to replace kvm_apic_id with a field in
struct kvm_lapic, which is already shifted right by 24 in xAPIC mode.

It can be added easily in patch 6 itself, it's like 3 new lines of code
because all reads and writes go through kvm_apic_id and kvm_apic_set_id;
the kvm_apic_id wrapper can be kept for simplicity.

Thanks again!

Paolo