linux-kernel - Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic

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Date:	Fri, 1 Jul 2016 14:44:21 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Andrew Honig <ahonig@...gle.com>, linux-kernel@...r.kernel.org,
	kvm <kvm@...r.kernel.org>, "Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic_map

2016-07-01 10:42+0200, Paolo Bonzini:
> On 01/07/2016 00:15, Andrew Honig wrote:
>>> > +       /* kvm_apic_map_get_logical_dest() expects multiples of 16 */
>>> > +       size = round_up(max_id + 1, 16);
>> Now that you're using the full range of apic_id values, could this
>> calculation overflow?  Perhaps max_id could be u64?
> 
> Good point, but I wonder if it's a good idea to let userspace allocate
> 32 GB of memory. :)

Yes, both could happen.  I'll change it to u64 to make it future proof.

> Let's put a limit on the maximum supported APIC ID, and report it
> through KVM_CHECK_EXTENSION on the new KVM_CAP_X2APIC_API capability.
> If 767 is enough for Knights Landing, the allocation below fits in two
> pages.  If you need to make it higher, please change the allocation to
> use kvm_kvzalloc and kvfree.

We sort of have a capability for maximum APIC ID, KVM_MAX_VCPU_ID,
because VCPU ID is initial APIC ID and x2APIC ID should always be the
initial APIC ID.

Userspace is able to change x2APIC with LAPIC_GET/SET ioctl -- what
about forbidding that?

> Last but not least...
> 
> >> > +       new = kzalloc(sizeof(struct kvm_apic_map) +
> >> > +                     sizeof(struct kvm_lapic) * size, GFP_KERNEL);
>                            ^^^^^^^^^^^^^^^^^^^^^^^^
> ... the sizeof here must be sizeof(struct kvm_lapic *).

Oops, thanks.