device-dax: fail all private mapping attempts

The device-dax implementation originally tried to be tricky and allow
private read-only mappings, but in the process allowed writable
MAP_PRIVATE + MAP_NORESERVE mappings.  For simplicity and predictability
just fail all private mapping attempts since device-dax memory is
statically allocated and will never support overcommit.

Cc: <stable@vger.kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Fixes: dee4107924 ("/dev/dax, core: file operations and dax-mmap")
Reported-by: Pawel Lebioda <pawel.lebioda@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/dax.c

@@ -270,8 +270,8 @@ static int check_vma(struct dax_dev *dax_dev, struct vm_area_struct *vma,
 	if (!dax_dev->alive)
 		return -ENXIO;
 
-	/* prevent private / writable mappings from being established */
-	if ((vma->vm_flags & (VM_NORESERVE|VM_SHARED|VM_WRITE)) == VM_WRITE) {
+	/* prevent private mappings from being established */
+	if ((vma->vm_flags & VM_SHARED) != VM_SHARED) {
 		dev_info(dev, "%s: %s: fail, attempted private mapping\n",
 				current->comm, func);
 		return -EINVAL;