[PATCH v2 2/3] KVM: arm64: vgic-its: Plug race in vgic_put_irq

[Andre Przywara]

Hi,

On 09/08/16 12:16, Christoffer Dall wrote:
> Right now the following sequence of events can happen:
> 
>   1. Thread X calls vgic_put_irq
>   2. Thread Y calls vgic_add_lpi
>   3. Thread Y gets lpi_list_lock
>   4. Thread X drops the ref count to 0 and blocks on lpi_list_lock
>   5. Thread Y finds the irq via the lpi_list_lock, raises the ref
>      count to 1, and release the lpi_list_lock.
>   6. Thread X proceeds and frees the irq.
> 
> Avoid this by holding the spinlock around the kref_put.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>

Reviewed-by: Andre Przywara <andre.przywara at arm.com>

Thanks!
Andre

> ---
> 
> Notes:
>     Changes since v1:
>      - Stylistic change: Don't use else branch
> 
>  virt/kvm/arm/vgic/vgic.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index e7aeac7..e83b7fe 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -117,17 +117,17 @@ static void vgic_irq_release(struct kref *ref)
>  
>  void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
>  {
> -	struct vgic_dist *dist;
> +	struct vgic_dist *dist = &kvm->arch.vgic;
>  
>  	if (irq->intid < VGIC_MIN_LPI)
>  		return;
>  
> -	if (!kref_put(&irq->refcount, vgic_irq_release))
> +	spin_lock(&dist->lpi_list_lock);
> +	if (!kref_put(&irq->refcount, vgic_irq_release)) {
> +		spin_unlock(&dist->lpi_list_lock);
>  		return;
> +	};
>  
> -	dist = &kvm->arch.vgic;
> -
> -	spin_lock(&dist->lpi_list_lock);
>  	list_del(&irq->lpi_list);
>  	dist->lpi_list_count--;
>  	spin_unlock(&dist->lpi_list_lock);
>