linux-kernel - Re: [PATCH] KVM: arm/arm64: vgic: fix off-by-one bug in vgic_get

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Message-ID: <d01370b0-5b80-b537-3168-88c166a17e7e@arm.com>
Date:   Thu, 13 Dec 2018 10:52:52 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Mark Rutland <mark.rutland@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Will Deacon <will.deacon@....com>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm/arm64: vgic: fix off-by-one bug in
 vgic_get_irq()

Hi Gustavo,

On 12/12/2018 20:11, Gustavo A. R. Silva wrote:
> When using the nospec API, it should be taken into account that:
> 
> "...if the CPU speculates past the bounds check then
>  * array_index_nospec() will clamp the index within the range of [0,
>  * size)."
> 
> The above is part of the header for macro array_index_nospec() in
> linux/nospec.h
> 
> Now, in this particular case, if intid evaluates to exactly VGIC_MAX_SPI
> or to exaclty VGIC_MAX_PRIVATE, the array_index_nospec() macro ends up
> returning VGIC_MAX_SPI - 1 or VGIC_MAX_PRIVATE - 1 respectively, instead
> of VGIC_MAX_SPI or VGIC_MAX_PRIVATE, which, based on the original logic:
> 
> 	/* SGIs and PPIs */
> 	if (intid <= VGIC_MAX_PRIVATE)
>  		return &vcpu->arch.vgic_cpu.private_irqs[intid];
> 
>  	/* SPIs */
> 	if (intid <= VGIC_MAX_SPI)
>  		return &kvm->arch.vgic.spis[intid - VGIC_NR_PRIVATE_IRQS];
> 
> are valid values for intid.
> 
> Fix this by calling array_index_nospec() macro with VGIC_MAX_PRIVATE + 1
> and VGIC_MAX_SPI + 1 as arguments for its parameter size.
> 
> Fixes: 41b87599c743 ("KVM: arm/arm64: vgic: fix possible spectre-v1 in vgic_get_irq()")
> Cc: stable@...r.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  virt/kvm/arm/vgic/vgic.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index 50e25438fb3c..5db40696055a 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -103,13 +103,13 @@ struct vgic_irq *vgic_get_irq(struct kvm *kvm, struct kvm_vcpu *vcpu,
>  {
>  	/* SGIs and PPIs */
>  	if (intid <= VGIC_MAX_PRIVATE) {
> -		intid = array_index_nospec(intid, VGIC_MAX_PRIVATE);
> +		intid = array_index_nospec(intid, VGIC_MAX_PRIVATE + 1);
>  		return &vcpu->arch.vgic_cpu.private_irqs[intid];
>  	}
>  
>  	/* SPIs */
>  	if (intid <= VGIC_MAX_SPI) {
> -		intid = array_index_nospec(intid, VGIC_MAX_SPI);
> +		intid = array_index_nospec(intid, VGIC_MAX_SPI + 1);
>  		return &kvm->arch.vgic.spis[intid - VGIC_NR_PRIVATE_IRQS];
>  	}
>  
> 

That's a good one. Out of curiosity, how did you find it?

Thanks,
	
	M.
-- 
Jazz is not dead. It just smells funny...