Re: [PATCH rc 2/2] iommufd/fault: Use a separate spinlock to protect fault->deliver list

[Baolu Lu <baolu.lu@linux.intel.com>]

On 1/15/25 07:28, Nicolin Chen wrote:
> The fault->mutex was to serialize the fault read()/write() fops and the
> iommufd_fault_auto_response_faults(). And it was also conveniently used
> to protect fault->deliver in poll() and iommufd_fault_iopf_handler().
> 
> However, copy_from/to_user() may sleep if pagefaults are enabled. Thus,
> they could take a long time to wait for user pages to swap in, blocking
> iommufd_fault_iopf_handler() and its caller that is typically a shared
> IRQ handler of an IOMMU driver, resulting in a potential global DOS.
> 
> Instead of resuing the mutex to protect the fault->deliver list, add a
> separate spinlock to do the job, so iommufd_fault_iopf_handler() would
> no longer be blocked by copy_from/to_user().
> 
> Provide two list manipulation helpers for fault->deliver:
>   - Extract the first iopf_group out of the fault->deliver list
>   - Restore an iopf_group back to the head of the fault->deliver list
> 
> Replace list_first_entry and list_for_each accordingly.
> 
> Fixes: 07838f7fd529 ("iommufd: Add iommufd fault object")
> Cc:stable@vger.kernel.org
> Suggested-by: Jason Gunthorpe<jgg@nvidia.com>
> Signed-off-by: Nicolin Chen<nicolinc@nvidia.com>
> ---
>   drivers/iommu/iommufd/iommufd_private.h | 26 +++++++++++++++
>   drivers/iommu/iommufd/fault.c           | 43 ++++++++++++++-----------
>   2 files changed, 50 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
> index b6d706cf2c66..d3097c857abf 100644
> --- a/drivers/iommu/iommufd/iommufd_private.h
> +++ b/drivers/iommu/iommufd/iommufd_private.h
> @@ -445,12 +445,38 @@ struct iommufd_fault {
>   
>   	/* The lists of outstanding faults protected by below mutex. */

It's better to update above comment as well.

>   	struct mutex mutex;
> +	spinlock_t lock; /* protects the deliver list */
>   	struct list_head deliver;
>   	struct xarray response;
>   
>   	struct wait_queue_head wait_queue;
>   };
>   
> +/* Extract the first node out of the fault->deliver list */
> +static inline struct iopf_group *
> +iommufd_fault_deliver_extract(struct iommufd_fault *fault)
> +{
> +	struct list_head *list = &fault->deliver;
> +	struct iopf_group *group = NULL;
> +
> +	spin_lock(&fault->lock);
> +	if (!list_empty(list)) {
> +		group = list_first_entry(list, struct iopf_group, node);
> +		list_del(&group->node);
> +	}
> +	spin_unlock(&fault->lock);
> +	return group;
> +}
> +
> +/* Restore a node back to the head in fault->deliver */
> +static inline void iommufd_fault_deliver_restore(struct iommufd_fault *fault,
> +						 struct iopf_group *group)
> +{
> +	spin_lock(&fault->lock);
> +	list_add(&fault->deliver, &group->node);

This is not right. It should be

	list_add(&group->node, &fault->deliver);

> +	spin_unlock(&fault->lock);
> +}

Others look good to me. With above addressed,

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>