bug#6953: 24.0.50; serious security bug in create backup files

[Glenn Morris]

Mark Diekhans wrote:

> Emacs, should create the last ditch backup file as access only by the
> user (no group or other access) before any data is written to the file
>
> Also, ~/%backup%~ should be configurable in a variable rather than hard
> coded in lisp files.el.

I don't think it is necessary for this to be configurable because it
is just a fallback in case of error. Eg you can customize
backup-directory-alist to control where backups normally go.

A partial solution for the first problem is simple (below).
Perhaps it would be better to use a private directory inside
user-emacs-directory. But that is less visible, and maybe these files
are supposed to be noticed?

*** lisp/files.el       2010-08-18 08:07:58 +0000
--- lisp/files.el       2010-08-31 18:33:34 +0000
***************
*** 3681,3687 ****
                     (message "Cannot write backup file; backing up in %s"
                              backupname)
                     (sleep-for 1)
!                    (backup-buffer-copy real-file-name backupname modes)))
                  (setq buffer-backed-up t)
                  ;; Now delete the old versions, if desired.
                  (if delete-old-versions
--- 3681,3691 ----
                     (message "Cannot write backup file; backing up in %s"
                              backupname)
                     (sleep-for 1)
!                    ;; The original file may have been in a private
!                    ;; directory, home might not be private.  (Bug#6953)
!                    ;; Not a perfect solution since the file is only
!                    ;; made private after being written.
!                    (backup-buffer-copy real-file-name backupname #o0600)))
                  (setq buffer-backed-up t)
                  ;; Now delete the old versions, if desired.
                  (if delete-old-versions