[PATCH 4/5] KVM: arm64: vgic: Prevent speculative SPI array underflow

[Sascha Bischoff <Sascha.Bischoff@arm.com>]

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 */
-- 
2.34.1