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

From: Eduardo Habkost <ehabkost@redhat.com>
To: Juergen Gross <jgross@suse.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 1/6] x86/kvm: fix vcpu-id indexed array sizes
Date: Fri, 3 Sep 2021 11:28:26 -0400	[thread overview]
Message-ID: <20210903152826.75rbaedvlud3potn@habkost.net> (raw)
In-Reply-To: <20210701154105.23215-2-jgross@suse.com>

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@suse.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