device-dax: fix private mapping restriction, permit read-only

Hugh notes in response to commit 4cb19355ea19 "device-dax: fail all
private mapping attempts":

  "I think that is more restrictive than you intended: haven't tried, but I
  believe it rejects a PROT_READ, MAP_SHARED, O_RDONLY fd mmap, leaving no
  way to mmap /dev/dax without write permission to it."

Indeed it does restrict read-only mappings, switch to checking
VM_MAYSHARE, not VM_SHARED.

Cc: <stable@vger.kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Pawel Lebioda <pawel.lebioda@intel.com>
Fixes: 4cb19355ea19 ("device-dax: fail all private mapping attempts")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/dax/dax.c b/drivers/dax/dax.c
index 3d94ff2..286447a 100644 (file)
--- a/drivers/dax/dax.c
+++ b/drivers/dax/dax.c
@@ -271,7 +271,7 @@ static int check_vma(struct dax_dev *dax_dev, struct vm_area_struct *vma,
                return -ENXIO;
 
        /* prevent private mappings from being established */
-       if ((vma->vm_flags & VM_SHARED) != VM_SHARED) {
+       if ((vma->vm_flags & VM_MAYSHARE) != VM_MAYSHARE) {
                dev_info(dev, "%s: %s: fail, attempted private mapping\n",
                                current->comm, func);
                return -EINVAL;