[PATCH] KVM: arm64: vgic: Use list_del_rcu() when flushing pending LPIs

[PATCH] KVM: arm64: vgic: Use list_del_rcu() when flushing pending LPIs

[Hyunwoo Kim]

vgic_v3_fold_lr_state() walks the ap_list from last_lr_irq without holding
the ap_list_lock, relying on vgic_irq being freed via kfree_rcu() and on
interrupts being disabled. vgic_flush_pending_lpis() removes entries with
list_del(), which clobbers a node's next pointer, so when another vCPU
disables LPIs via GICR_CTLR the walk can follow the clobbered next pointer
from a removed node, or from the node that last_lr_irq points to.

Remove entries with list_del_rcu() so that the next pointer stays valid
until the walk completes.

Fixes: 3cfd59f81e0f ("KVM: arm64: GICv3: Handle LR overflow when EOImode==0")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 arch/arm64/kvm/vgic/vgic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 1e9fe8764584d..73efc0f95bfb1 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -204,7 +204,7 @@ void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
 	list_for_each_entry_safe(irq, tmp, &vgic_cpu->ap_list_head, ap_list) {
 		if (irq_is_lpi(vcpu->kvm, irq->intid)) {
 			raw_spin_lock(&irq->irq_lock);
-			list_del(&irq->ap_list);
+			list_del_rcu(&irq->ap_list);
 			irq->vcpu = NULL;
 			raw_spin_unlock(&irq->irq_lock);
 			deleted |= vgic_put_irq_norelease(vcpu->kvm, irq);
-- 
2.43.0

Re: [PATCH] KVM: arm64: vgic: Use list_del_rcu() when flushing pending LPIs

[Oliver Upton]

Hi Hyunwoo,

On Fri, Jun 05, 2026 at 06:16:08AM +0900, Hyunwoo Kim wrote:
> vgic_v3_fold_lr_state() walks the ap_list from last_lr_irq without holding
> the ap_list_lock, relying on vgic_irq being freed via kfree_rcu() and on
> interrupts being disabled. vgic_flush_pending_lpis() removes entries with
> list_del(), which clobbers a node's next pointer, so when another vCPU
> disables LPIs via GICR_CTLR the walk can follow the clobbered next pointer
> from a removed node, or from the node that last_lr_irq points to.
> 
> Remove entries with list_del_rcu() so that the next pointer stays valid
> until the walk completes.
> 
> Fixes: 3cfd59f81e0f ("KVM: arm64: GICv3: Handle LR overflow when EOImode==0")
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>

Changing only one of the writer paths to use the rculist helpers does
not make the ap_list an rculist. Insertions are not RCU-safe, nor are
deleations from vgic_prune_ap_list().

And TBH, the real bug here is the fact that vgic_v3_fold_lr_state() isn't
taking the ap_list_lock.

Thanks,
Oliver

> ---
>  arch/arm64/kvm/vgic/vgic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> index 1e9fe8764584d..73efc0f95bfb1 100644
> --- a/arch/arm64/kvm/vgic/vgic.c
> +++ b/arch/arm64/kvm/vgic/vgic.c
> @@ -204,7 +204,7 @@ void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
>  	list_for_each_entry_safe(irq, tmp, &vgic_cpu->ap_list_head, ap_list) {
>  		if (irq_is_lpi(vcpu->kvm, irq->intid)) {
>  			raw_spin_lock(&irq->irq_lock);
> -			list_del(&irq->ap_list);
> +			list_del_rcu(&irq->ap_list);
>  			irq->vcpu = NULL;
>  			raw_spin_unlock(&irq->irq_lock);
>  			deleted |= vgic_put_irq_norelease(vcpu->kvm, irq);
> -- 
> 2.43.0
> 

Re: [PATCH] KVM: arm64: vgic: Use list_del_rcu() when flushing pending LPIs

[Marc Zyngier]

On Fri, 05 Jun 2026 06:47:17 +0100,
Oliver Upton <oupton@kernel.org> wrote:
> 
> Hi Hyunwoo,
> 
> On Fri, Jun 05, 2026 at 06:16:08AM +0900, Hyunwoo Kim wrote:
> > vgic_v3_fold_lr_state() walks the ap_list from last_lr_irq without holding
> > the ap_list_lock, relying on vgic_irq being freed via kfree_rcu() and on
> > interrupts being disabled. vgic_flush_pending_lpis() removes entries with
> > list_del(), which clobbers a node's next pointer, so when another vCPU
> > disables LPIs via GICR_CTLR the walk can follow the clobbered next pointer
> > from a removed node, or from the node that last_lr_irq points to.
> > 
> > Remove entries with list_del_rcu() so that the next pointer stays valid
> > until the walk completes.
> > 
> > Fixes: 3cfd59f81e0f ("KVM: arm64: GICv3: Handle LR overflow when EOImode==0")
> > Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> 
> Changing only one of the writer paths to use the rculist helpers does
> not make the ap_list an rculist. Insertions are not RCU-safe, nor are
> deleations from vgic_prune_ap_list().
> 
> And TBH, the real bug here is the fact that vgic_v3_fold_lr_state() isn't
> taking the ap_list_lock.

Yup, that'd be more sensible. I need to convince myself that there is
no possible path from vgic_v*_fold_lr() to vgic_irq_queue_unlock(),
because that one does actually acquire that lock.

	M.

-- 
Jazz isn't dead. It just smells funny.