ovl: relax permission checking on underlying layers

Make permission checking more consistent:

 - special files don't need any access check on underling fs

 - exec permission check doesn't need to be performed on underlying fs

Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 3b7ed5d2279c..6bcc9dedc342 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -286,13 +286,22 @@ int ovl_permission(struct inode *inode, int mask)
         if (err)
                 return err;
 
-        old_cred = ovl_override_creds(inode->i_sb);
-        if (!upperinode &&
-            !special_file(realinode->i_mode) && mask & MAY_WRITE) {
+        /* No need to do any access on underlying for special files */
+        if (special_file(realinode->i_mode))
+                return 0;
+
+        /* No need to access underlying for execute */
+        mask &= ~MAY_EXEC;
+        if ((mask & (MAY_READ | MAY_WRITE)) == 0)
+                return 0;
+
+        /* Lower files get copied up, so turn write access into read */
+        if (!upperinode && mask & MAY_WRITE) {
                 mask &= ~(MAY_WRITE | MAY_APPEND);
-                /* Make sure mounter can read file for copy up later */
                 mask |= MAY_READ;
         }
+
+        old_cred = ovl_override_creds(inode->i_sb);
         err = inode_permission(realinode, mask);
         revert_creds(old_cred);