[PATCH v2 2/7] vfio/fsl-mc: prevent read-only region mappings from becoming writable

[Abdifatah Suruur <suruurism@gmail.com>]

vfio_fsl_mc_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: the fsl-mc bus publishes all device
regions with the WRITE flag set, so no device can currently reach the
read-only path.  The guard costs nothing and keeps the mmap() interface
honest if a read-only region ever appears.

Fixes: 67247289688d4 ("vfio/fsl-mc: Allow userspace to MMAP fsl-mc device MMIO regions")
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
---
 drivers/vfio/fsl-mc/vfio_fsl_mc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/vfio/fsl-mc/vfio_fsl_mc.c b/drivers/vfio/fsl-mc/vfio_fsl_mc.c
index b4c3958201b25..5a6cd78dcc06a 100644
--- a/drivers/vfio/fsl-mc/vfio_fsl_mc.c
+++ b/drivers/vfio/fsl-mc/vfio_fsl_mc.c
@@ -403,9 +403,13 @@ static int vfio_fsl_mc_mmap(struct vfio_device *core_vdev,
 			&& (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 = mc_dev;
 
-- 
2.53.0