Re: [PATCH 0/2] mm/memfd: add F_SEAL_PUNCH_HOLE

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Oct 30, 2026 at 11:20:30AM +0100, David Hildenbrand (Arm) wrote:
> On 10/26/26 23:05, Samiullah Khawaja wrote:
> > This series introduces F_SEAL_PUNCH_HOLE, a new memfd seal that prevents
> > holes from being punched into a shmem or hugetlbfs memfd while leaving
> > the file fully writable.
> > 
> > Background
> > ==========
> > 
> > Drivers such as udmabuf and iommufd (via IOMMU_IOAS_MAP_FILE) use
> > memfd_pin_folios() to pin the folios backing a memfd and hand them to a
> > device for DMA. Pinning keeps those folios allocated, but it does not
> > keep them in the file. If userspace later calls
> > fallocate(FALLOC_FL_PUNCH_HOLE) or madvise(MADV_REMOVE) on the range,
> > the folios are dropped from the page cache while the device continues
> > to use them. The pin holder is never notified, and from that point on
> > the device and the file silently diverge.
> 
> We have the exact same situation when userspace maps anonymous folios into vfio
> and then desides to MADV_DONTNEED the pages. Or any other memory backend that is
> used for such purposes.
> 
> User space must not zap/punch things that might be pinned elsewhere.
> 
> That's not specific to memfd, udmabuf, iommufd or anything else.
> 
> I don't see a reason to make userspace (that should not shoot itself into the
> foot) a kernel feature it must activate itself to not shoot itself into the foot.

The issue is live update doesn't preserve those background punched
folios and now we have created an UAF attack on the kernel across
kexec if you don't do something about it.

There was some notion here that with the right sealing flags we could
tell if everything was 'live update compatible' and then know that
these attacks are impossible.

Jason