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

[Alex Mastro <amastro@fb.com>]

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.

Fix by comparing to the inclusive range end, which can be expressed
by u64.

This bug was discovered after querying for vfio_iova_range's via
VFIO_IOMMU_GET_INFO, making a VFIO_IOMMU_MAP_DMA inside the last range,
and then attempting to unmap the entirety of the last range i.e.
VFIO_IOMMU_UNMAP_DMA(iova=r.start, size=r.end-r.start+1).

---
I don't think iommufd is susceptible to the same issue since
iopt_unmap_iova computes the inclusive end using checked addition, and
iopt_unmap_iova_range acts on an inclusive range.

Signed-off-by: Alex Mastro <amastro@fb.com>
---
 drivers/vfio/vfio_iommu_type1.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index f8d68fe77b41..08242d8ce2ca 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -199,7 +199,7 @@ static struct rb_node *vfio_find_dma_first_node(struct vfio_iommu *iommu,
 			node = node->rb_right;
 		}
 	}
-	if (res && size && dma_res->iova >= start + size)
+	if (res && size && dma_res->iova > start + size - 1)
 		res = NULL;
 	return res;
 }
@@ -1386,7 +1386,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 
 	while (n) {
 		dma = rb_entry(n, struct vfio_dma, node);
-		if (dma->iova >= iova + size)
+		if (dma->iova > iova + size - 1)
 			break;
 
 		if (!iommu->v2 && iova > dma->iova)

---
base-commit: 407aa63018d15c35a34938633868e61174d2ef6e
change-id: 20251005-fix-unmap-c3f3e87dabfa

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