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

[Jason Gunthorpe]

On Mon, Jun 08, 2026 at 03:38:00PM +0200, David Hildenbrand (Arm) wrote:
> On 6/7/26 14:09, Jason Gunthorpe wrote:
> > 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.
> 
> Agreed.
> 
> Not sure if it should be part of memfd_pin_folios() itself.

I think it should, drivers should not be open coding this.

If there is such a thing as a read-only memfd then memfd_pin_folios()
should accept the usual FOLL_WRITE and deal with it internally.

> > start/pin/destroy kind of thing to manage this?
> > 
> > It should not be open coded like this.
> 
> The permission check is one thing that's clearly missing.
> 
> Not sure about the mapping_map_writable() handling ... it's weird to rely on
> that when we are not actually mmaping.

I don't know anything about sealing, but it shouldn't something check
if it is sealed read-only ?

> Assume we GUP a page and then munmap, mapping_unmap_writable() would be called
> while we still have a writable GUP reference. Hm.

I suspect the user doing the sealing has to ensure there are no pins
to the memfd before it seals it. If it already let the unsealed memfd
out of its control then it probably cannot be reliably sealed read
only?

Jason