vfio: handle DMA map/unmap up to the addressable limit

This patch series aims to fix vfio_iommu_type.c to support 
VFIO_IOMMU_MAP_DMA and VFIO_IOMMU_UNMAP_DMA operations targeting IOVA
ranges which lie against the addressable limit. i.e. ranges where
iova_start + iova_size would overflow to exactly zero.

Today, the VFIO UAPI has an inconsistency: The
VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE capability of VFIO_IOMMU_GET_INFO
reports that ranges up to the end of the address space are available
for use, but are not really due to bugs in handling boundary conditions.

For example:

vfio_find_dma_first_node is called to find the first dma node to unmap
given an unmap range of [iova..iova+size). The check at the end of the
function intends to test if the dma result lies beyond the end of the
unmap range. The condition is incorrectly satisfied when iova+size
overflows to zero, causing the function to return NULL.

The same issue happens inside vfio_dma_do_unmap's while loop.

This bug was also reported by Alejandro Jimenez in [1][2].

Of primary concern are locations in the current code which perform
comparisons against (iova + size) expressions, where overflow to zero
is possible.

The initial list of candidate locations to audit was taken from the
following:

$ rg 'iova.*\+.*size' -n drivers/vfio/vfio_iommu_type1.c | rg -v '\- 1'
173:            else if (start >= dma->iova + dma->size)
192:            if (start < dma->iova + dma->size) {
216:            if (new->iova + new->size <= dma->iova)
1060:   dma_addr_t iova = dma->iova, end = dma->iova + dma->size;
1233:   if (dma && dma->iova + dma->size != iova + size)
1380:           if (dma && dma->iova + dma->size != iova + size)
1501:           ret = vfio_iommu_map(iommu, iova + dma->size, pfn, npage,
1504:                   vfio_unpin_pages_remote(dma, iova + dma->size, pfn,
1721:           while (iova < dma->iova + dma->size) {
1743:                           i = iova + size;
1744:                           while (i < dma->iova + dma->size &&
1754:                           size_t n = dma->iova + dma->size - iova;
1785:                   iova += size;
1810:           while (iova < dma->iova + dma->size) {
1823:                   i = iova + size;
1824:                   while (i < dma->iova + dma->size &&
2919:           if (range.iova + range.size < range.iova)

This series spend the first couple commits making mechanical preparations
before the fix lands in the last commit.

[1] https://lore.kernel.org/qemu-devel/20250919213515.917111-...
[2] https://lore.kernel.org/all/68e18f2c-79ad-45ec-99b9-99ff6...

Signed-off-by: Alex Mastro <amastro@fb.com>
---
Changes in v3:
- Fix handling of unmap_all in vfio_dma_do_unmap
- Fix !range.size to return -EINVAL for VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP
  - Dedup !range.size checking
- Return -EOVERFLOW on check_*_overflow
- Link to v2: https://lore.kernel.org/r/20251007-fix-unmap-v2-0-759bceb...

Changes in v2:
- Change to patch series rather than single commit
- Expand scope to fix more than just the unmap discovery path
- Link to v1: https://lore.kernel.org/r/20251005-fix-unmap-v1-1-6687732...

---
Alex Mastro (3):
      vfio/type1: sanitize for overflow using check_*_overflow
      vfio/type1: move iova increment to unmap_unpin_* caller
      vfio/type1: handle DMA map/unmap up to the addressable limit

 drivers/vfio/vfio_iommu_type1.c | 162 ++++++++++++++++++++++++++--------------
 1 file changed, 104 insertions(+), 58 deletions(-)
---
base-commit: 407aa63018d15c35a34938633868e61174d2ef6e
change-id: 20251005-fix-unmap-c3f3e87dabfa

Best regards,
-- 
Alex Mastro <amastro@fb.com>