[PATCH v4] 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 commit a5edadbae57e ("ptp: vmclock:
prevent read-only mappings from becoming writable").

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: fad4d5b1f042 ("vfio/platform: support MMAP of MMIO regions")
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
---
v4:
- consolidate the duplicated !WRITE condition into one block, per
  Pranjal's review; no semantic change
v3:
- use 'commit <sha> ("<title>")' style for the vmclock reference and
  point Fixes at the MMAP support commit, per Mostafa's review
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
---
 drivers/vfio/platform/vfio_platform_common.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/vfio/platform/vfio_platform_common.c b/drivers/vfio/platform/vfio_platform_common.c
index c72db5a99ebda..ab531318f170a 100644
--- a/drivers/vfio/platform/vfio_platform_common.c
+++ b/drivers/vfio/platform/vfio_platform_common.c
@@ -591,9 +591,13 @@ int vfio_platform_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma
 			&& (vma->vm_flags & VM_READ))
 		return -EINVAL;
 
-	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)) {
+		if (vma->vm_flags & VM_WRITE)
+			return -EINVAL;
+
+		vm_flags_clear(vma, VM_MAYWRITE);
+	}
 
 	vma->vm_private_data = vdev;
 
-- 
2.53.0