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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: 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: fb1ff4c194157 ("vfio/fsl-mc: Add VFIO framework skeleton for fsl-mc devices") Signed-off-by: Abdifatah Suruur --- 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/fsl-mc/vfio_fsl_mc.c +++ b/drivers/vfio/fsl-mc/vfio_fsl_mc.c @@ -406,7 +406,11 @@ 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 = mc_dev; return vfio_fsl_mc_mmap_mmio(vdev->regions[index], vma); }