diff --git a/drivers/ptp/ptp_vmclock.c b/drivers/ptp/ptp_vmclock.c
index eebdcd5ebc08..bb0e14bac9f2 100644
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -372,6 +372,12 @@ static int vmclock_miscdev_mmap(struct file *fp, struct vm_area_struct *vma)
 	if ((vma->vm_flags & (VM_READ|VM_WRITE)) != VM_READ)
 		return -EROFS;
 
+	/*
+	 * Restrict the read-only mapping so it cannot be upgraded to
+	 * writable later with mprotect().
+	 */
+	vm_flags_clear(vma, VM_MAYWRITE);
+
 	if (vma->vm_end - vma->vm_start != PAGE_SIZE || vma->vm_pgoff)
 		return -EINVAL;