Re: [PATCH] iommu/iommufd: Require write access for writable MAP_FILE mappings

[Jason Gunthorpe]

On Sun, Jun 07, 2026 at 08:53:18AM +0000, Yiming Qian wrote:
> IOMMU_IOAS_MAP_FILE pins folios from a shmem/tmpfs or hugetlb file and
> uses them as the backing storage for an IOAS mapping.  When userspace sets
> IOMMU_IOAS_MAP_WRITEABLE, the resulting IOMMU PTEs allow DMA writes to the
> file-backed folios.

This looks like an issue with the API design in memfd_pin_folios(),
all users would have a similar bug I think.

I don't know much about memfd but this seems like a legitimate issue.

Add those involved with gup.c and the patch adding memfd_pin_folios()

>  {
>  	struct iopt_pages *pages;
> +	int rc;
> +
> +	if (writable) {
> +		if (!(file->f_mode & FMODE_WRITE))
> +			return ERR_PTR(-EPERM);
> +
> +		rc = mapping_map_writable(file->f_mapping);
> +		if (rc)
> +			return ERR_PTR(rc);
> +	}

We probably need some kind of companion API for memfd_pin_folios(), a
start/pin/destroy kind of thing to manage this?

It should not be open coded like this.

Jason