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

["David Hildenbrand (Arm)" <david@kernel.org>]

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 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.

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.

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

-- 
Cheers,

David