[PATCH 7/7] vfio/cdx: reject non-shared MMIO mmaps

[Abdifatah Suruur <suruurism@gmail.com>]

vfio_cdx_mmap() accepts MAP_PRIVATE mappings.  A non-shared mapping
with VM_MAYWRITE is a COW mapping, and remap_pfn_range()'s COW special
case in get_remap_pgoff() then overwrites vma->vm_pgoff with the
physical frame number:

	if (is_cow) {
		if (addr != vm_start || end != vm_end)
			return -EINVAL;
		*vm_pgoff_p = pfn;
	}

The VMA is inserted into the device file's mapping->i_mmap interval
tree keyed by vm_pgoff, which VFIO expects to be the logical file
offset: the core links every device mmap to the device inode's
i_mapping precisely so that unmap_mapping_range() can revoke all
mappings associated with a device (see vfio_device_cdev_open()).

A raw PFN in vm_pgoff lands the interval tree entry in the wrong
coordinate space, so unmap_mapping_range() cannot find the VMA and a
stale MMIO mapping survives any revocation attempt.  Without this
check, a MAP_PRIVATE mapping reintroduces that corruption even with
the logical-vm_pgoff fix in place.

Require VM_SHARED as vfio-pci, vfio/fsl-mc and vfio/platform already
do.  This is defensive hygiene: vfio-cdx does not call
unmap_mapping_range() today, so no reachable stale-mapping issue
exists.

Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
---
diff --git a/drivers/vfio/cdx/main.c b/drivers/vfio/cdx/main.c
index b31ed4be7bdc1..6d208db0896d6 100644
--- a/drivers/vfio/cdx/main.c
+++ b/drivers/vfio/cdx/main.c
@@ -271,6 +271,9 @@ static int vfio_cdx_mmap(struct vfio_device *core_vdev,

 	index = vma->vm_pgoff >> (VFIO_CDX_OFFSET_SHIFT - PAGE_SHIFT);

+	if (!(vma->vm_flags & VM_SHARED))
+		return -EINVAL;
+
 	if (index >= cdx_dev->res_count)
 		return -EINVAL;