[PATCH v2] vfio/platform: prevent read-only region mappings from becoming writable

[Abdifatah Suruur <suruurism@gmail.com>]

vfio_platform_mmap() rejects writable mappings of regions without the
WRITE flag, but leaves VM_MAYWRITE set.  Userspace can map such a region
read-only and then upgrade the mapping to writable with mprotect().

Clear VM_MAYWRITE for regions without the WRITE flag, as i915 does for
its read-only objects and as fixed in drm/vc4 (CVE-2026-68445),
drm/panthor (CVE-2024-53071) and ptp: vmclock (commit
a5edadbae57e2298a56cf7a4e774a027905a331f).

Note this is defensive hardening: no in-tree platform driver currently
publishes a region without the WRITE flag.  The guard costs nothing and
keeps the mmap() interface honest if a read-only region ever appears.

Fixes: de49fc0d99e6a ("vfio/platform: initial skeleton of VFIO support for platform devices")
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
---
v2: commit log now states plainly that the read-only path is
unreachable in practice, per Alex's review; dropped Cc: stable as
there is no reachable issue to backport.

--- a/drivers/vfio/platform/vfio_platform_common.c
+++ b/drivers/vfio/platform/vfio_platform_common.c
@@ -594,7 +594,11 @@
 	if (!(vdev->regions[index].flags & VFIO_REGION_INFO_FLAG_WRITE)
 			&& (vma->vm_flags & VM_WRITE))
 		return -EINVAL;

+	/* Prevent read-only region mappings from being upgraded with mprotect() */
+	if (!(vdev->regions[index].flags & VFIO_REGION_INFO_FLAG_WRITE))
+		vm_flags_clear(vma, VM_MAYWRITE);
+
 	vma->vm_private_data = vdev;

 	if (vdev->regions[index].type & VFIO_PLATFORM_REGION_TYPE_MMIO)
 		return vfio_platform_mmap_mmio(vdev->regions[index], vma);