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

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Message-ID: <aORhMMOU5p3j69ld@devgpu015.cco6.facebook.com>
Date: Mon, 6 Oct 2025 17:39:12 -0700
From: Alex Mastro <amastro@...com>
To: Jason Gunthorpe <jgg@...pe.ca>
CC: Alex Williamson <alex.williamson@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio: fix VFIO_IOMMU_UNMAP_DMA when end of range would
 overflow u64

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?