Re: [PATCH] vfio: fix VFIO_IOMMU_UNMAP_DMA when end of range would overflow u64

[Alex Mastro <amastro@fb.com>]

On Mon, Oct 06, 2025 at 07:50:39PM -0300, Jason Gunthorpe wrote:
> Could we block right at the ioctl inputs that end at ULONG_MAX? Maybe
> that is a good enough fix?

That would be a simpler, and perhaps less risky fix than surgically fixing every
affected place.

If we were to do that, I think VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE should
report coherent limits -- i.e. it should "lie" and say the last page (or
whatever) of u64 addressable space is inaccessible so that the UAPI is coherent
with itself. It should be legal to map/unmap iova ranges up to the limits it
claims to support.

I have doubts that anyone actually relies on MAP_DMA-ing such
end-of-u64-mappings in practice, so perhaps it's OK?