Re: [PATCH v2 2/4] KVM: arm64: vgic: Prevent speculative SPI array underflow

[Joey Gouly <joey.gouly@arm.com>]

On Tue, Aug 11, 2026 at 03:10:47PM +0000, Sascha Bischoff wrote:
> For a non-GICv5 VM, SPI interrupt IDs include the private-interrupt
> offset, while KVM's SPI array is indexed from zero. The lookup applies
> array_index_nospec() to the absolute interrupt ID and subtracts the
> private-interrupt offset afterwards.
> 
> On a speculative bypass of the range check for an interrupt ID below
> the private range, the clamp preserves the small absolute value and
> the subtraction underflows to an out-of-bounds SPI array index.
> 
> Convert the interrupt ID to a zero-based index into the SPI array
> before applying array_index_nospec(). This way, we ensure that we
> clamp to a reachable SPI ID, rather than an out-of-range SPI index.
> 
> Fixes: 41b87599c743 ("KVM: arm/arm64: vgic: fix possible spectre-v1 in vgic_get_irq()")
> Link: https://sashiko.dev/#/patchset/20260724104819.1296803-1-sascha.bischoff@arm.com?part=27
> Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
> ---
>  arch/arm64/kvm/vgic/vgic.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> index 74bace10a22ed..352d52bd6315c 100644
> --- a/arch/arm64/kvm/vgic/vgic.c
> +++ b/arch/arm64/kvm/vgic/vgic.c
> @@ -93,8 +93,9 @@ struct vgic_irq *vgic_get_irq(struct kvm *kvm, u32 intid)
>  	/* SPIs */
>  	if (intid >= VGIC_NR_PRIVATE_IRQS &&
>  	    intid < (kvm->arch.vgic.nr_spis + VGIC_NR_PRIVATE_IRQS)) {
> -		intid = array_index_nospec(intid, kvm->arch.vgic.nr_spis + VGIC_NR_PRIVATE_IRQS);
> -		return &kvm->arch.vgic.spis[intid - VGIC_NR_PRIVATE_IRQS];
> +		intid -= VGIC_NR_PRIVATE_IRQS;
> +		intid = array_index_nospec(intid, kvm->arch.vgic.nr_spis);
> +		return &kvm->arch.vgic.spis[intid];
>  	}
>  
>  	/* LPIs */

Reviewed-by: Joey Gouly <joey.gouly@arm.com>