linux-kernel - Re: [PATCH 1/6] x86/kvm: fix vcpu-id indexed array sizes

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Date:   Fri, 3 Sep 2021 11:28:26 -0400
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/6] x86/kvm: fix vcpu-id indexed array sizes

On Thu, Jul 01, 2021 at 05:41:00PM +0200, Juergen Gross wrote:
> KVM_MAX_VCPU_ID is the maximum vcpu-id of a guest, and not the number
> of vcpu-ids. Fix array indexed by vcpu-id to have KVM_MAX_VCPU_ID+1
> elements.

I don't think that's true.  kvm_vm_ioctl_create_vcpu() refuses to
create a VCPU with id==KVM_MAX_VCPU_ID.
Documentation/virt/kvm/api.rst also states that
"The vcpu id is an integer in the range [0, max_vcpu_id)."

> 
> Note that this is currently no real problem, as KVM_MAX_VCPU_ID is
> an odd number, resulting in always enough padding being available at
> the end of those arrays.
> 
> Nevertheless this should be fixed in order to avoid rare problems in
> case someone is using an even number for KVM_MAX_VCPU_ID.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/kvm/ioapic.c | 2 +-
>  arch/x86/kvm/ioapic.h | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> index 698969e18fe3..ff005fe738a4 100644
> --- a/arch/x86/kvm/ioapic.c
> +++ b/arch/x86/kvm/ioapic.c
> @@ -96,7 +96,7 @@ static unsigned long ioapic_read_indirect(struct kvm_ioapic *ioapic,
>  static void rtc_irq_eoi_tracking_reset(struct kvm_ioapic *ioapic)
>  {
>  	ioapic->rtc_status.pending_eoi = 0;
> -	bitmap_zero(ioapic->rtc_status.dest_map.map, KVM_MAX_VCPU_ID);
> +	bitmap_zero(ioapic->rtc_status.dest_map.map, KVM_MAX_VCPU_ID + 1);
>  }
>  
>  static void kvm_rtc_eoi_tracking_restore_all(struct kvm_ioapic *ioapic);
> diff --git a/arch/x86/kvm/ioapic.h b/arch/x86/kvm/ioapic.h
> index 660401700075..11e4065e1617 100644
> --- a/arch/x86/kvm/ioapic.h
> +++ b/arch/x86/kvm/ioapic.h
> @@ -43,13 +43,13 @@ struct kvm_vcpu;
>  
>  struct dest_map {
>  	/* vcpu bitmap where IRQ has been sent */
> -	DECLARE_BITMAP(map, KVM_MAX_VCPU_ID);
> +	DECLARE_BITMAP(map, KVM_MAX_VCPU_ID + 1);
>  
>  	/*
>  	 * Vector sent to a given vcpu, only valid when
>  	 * the vcpu's bit in map is set
>  	 */
> -	u8 vectors[KVM_MAX_VCPU_ID];
> +	u8 vectors[KVM_MAX_VCPU_ID + 1];
>  };
>  
>  
> -- 
> 2.26.2
> 

-- 
Eduardo