ovl: verify permissions in ovl_path_open()

Check permission before opening a real file.

ovl_path_open() is used by readdir and copy-up routines.

ovl_permission() theoretically already checked copy up permissions, but it
doesn't hurt to re-do these checks during the actual copy-up.

For directory reading ovl_permission() only checks access to topmost
underlying layer.  Readdir on a merged directory accesses layers below the
topmost one as well.  Permission wasn't checked for these layers.

Note: modifying ovl_permission() to perform this check would be far more
complex and hence more bug prone.  The result is less precise permissions
returned in access(2).  If this turns out to be an issue, we can revisit
this bug.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/util.c

@@ -459,7 +459,32 @@ bool ovl_is_whiteout(struct dentry *dentry)
 
 struct file *ovl_path_open(struct path *path, int flags)
 {
-	return dentry_open(path, flags | O_NOATIME, current_cred());
+	struct inode *inode = d_inode(path->dentry);
+	int err, acc_mode;
+
+	if (flags & ~(O_ACCMODE | O_LARGEFILE))
+		BUG();
+
+	switch (flags & O_ACCMODE) {
+	case O_RDONLY:
+		acc_mode = MAY_READ;
+		break;
+	case O_WRONLY:
+		acc_mode = MAY_WRITE;
+		break;
+	default:
+		BUG();
+	}
+
+	err = inode_permission(inode, acc_mode | MAY_OPEN);
+	if (err)
+		return ERR_PTR(err);
+
+	/* O_NOATIME is an optimization, don't fail if not permitted */
+	if (inode_owner_or_capable(inode))
+		flags |= O_NOATIME;
+
+	return dentry_open(path, flags, current_cred());
 }
 
 /* Caller should hold ovl_inode->lock */