On Mon, Dec 15, 2025 at 1:42 PM Nicolin Chen <nicolinc@nvidia.com> wrote:
>
> The iommu_deferred_attach() function invokes __iommu_attach_device(), but
> doesn't hold the group->mutex like other __iommu_attach_device() callers.
>
> Though there is no pratical bug being triggered so far, it would be better
> to apply the same locking to this __iommu_attach_device(), since the IOMMU
> drivers nowaday are more aware of the group->mutex -- some of them use the
> iommu_group_mutex_assert() function that could be potentially in the path
> of an attach_dev callback function invoked by the __iommu_attach_device().
>
> Worth mentioning that the iommu_deferred_attach() will soon need to check
> group->resetting_domain that must be locked also.
>
> Thus, grab the mutex to guard __iommu_attach_device() like other callers.
>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
> Tested-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
> drivers/iommu/iommu.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 2ca990dfbb88..170e522b5bda 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2185,10 +2185,17 @@ EXPORT_SYMBOL_GPL(iommu_attach_device);
>
> int iommu_deferred_attach(struct device *dev, struct iommu_domain *domain)
> {
> - if (dev->iommu && dev->iommu->attach_deferred)
> - return __iommu_attach_device(domain, dev, NULL);
> + /*
> + * This is called on the dma mapping fast path so avoid locking. This is
> + * racy, but we have an expectation that the driver will setup its DMAs
> + * inside probe while being single threaded to avoid racing.
> + */
> + if (!dev->iommu || !dev->iommu->attach_deferred)
> + return 0;
>
> - return 0;
> + guard(mutex)(&dev->iommu_group->mutex);
> +
> + return __iommu_attach_device(domain, dev, NULL);
> }
>
> void iommu_detach_device(struct iommu_domain *domain, struct device *dev)
> --
> 2.43.0
>
>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>