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Re: Curious permission problem

From: Michael Albinus
Subject: Re: Curious permission problem
Date: Wed, 17 Feb 2010 15:32:52 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1.92 (gnu/linux)

Ole Laursen <address@hidden> writes:

> (defun tramp-default-file-modes (filename)
>   (tramp-octal-to-decimal "0666"))

That's a security threat. You might open root owned files from a remote
server. Everybody can read those file on your local server, from the
tmp directory.

>> And yes, I believe Emacs shall follow file system conventions.
>> Otherwise, it would bypass the given security level, which might result
>> in undesired weaknesses.
> As I see it, the problem is that you can't just map the permissions
> directly from one system to another system because the users aren't
> the same on the two? Actually, from a security point of view, the only
> thing I care about is that nobody else on my local machine can
> accidentally read the file, unless they can also read it on the remote
> machine. Are there any other goals?

That's the major point, yes.

Using the same file permissions on different machines, for different
users, is a weakness in Tramp. OTOH, it makes sense, if you use your own
identity on different machines, even if the user names are

Hmm. Other programs, like "scp -p", do the same.

Maybe we shall add an option to Tramp, that all group and world
permissions are nuked when copying. So you can decide how to do it.

>> PS: does the patch works for you, reading that file?
> I don't have a development environment handy so I haven't figured out
> how to test it yet. If you can attach the whole function, then I can
> just override it from my .emacs, that would make it a bit easier?

--8<---------------cut here---------------start------------->8---
(defun tramp-handle-insert-file-contents
  (filename &optional visit beg end replace)
  "Like `insert-file-contents' for Tramp files."
  (setq filename (expand-file-name filename))
  (let (coding-system-used result local-copy remote-copy)
    (with-parsed-tramp-file-name filename nil
          (if (not (file-exists-p filename))
              ;; We don't raise a Tramp error, because it might be
              ;; suppressed, like in `find-file-noselect-1'.
              (signal 'file-error
                      (list "File not found on remote host" filename))

            (if (and (tramp-local-host-p v)
                     (let (file-name-handler-alist)
                       (file-readable-p localname)))
                ;; Short track: if we are on the local host, we can
                ;; run directly.
                (setq result
                       (list localname visit beg end replace)))

              ;; When we shall insert only a part of the file, we copy
              ;; this part.
              (when (or beg end)
                (setq remote-copy (tramp-make-tramp-temp-file v))
                  ((and beg end)
                   (format "tail -c +%d %s | head -c +%d >%s"
                           (1+ beg) (tramp-shell-quote-argument localname)
                           (- end beg) remote-copy))
                   (format "tail -c +%d %s >%s"
                           (1+ beg) (tramp-shell-quote-argument localname)
                   (format "head -c +%d %s >%s"
                           (1+ end) (tramp-shell-quote-argument localname)

              ;; `insert-file-contents-literally' takes care to avoid
              ;; calling jka-compr.  By let-binding
              ;; `inhibit-file-name-operation', we propagate that care
              ;; to the `file-local-copy' operation.
              (setq local-copy
                    (let ((inhibit-file-name-operation
                           (when (eq inhibit-file-name-operation
                       ((stringp remote-copy)
                          method user host remote-copy)))
                       ((stringp tramp-temp-buffer-file-name)
                        (copy-file filename tramp-temp-buffer-file-name 'ok)
                       (t (file-local-copy filename)))))

              ;; When the file is not readable for the owner, it
              ;; cannot be inserted, even it is redable for the group
              ;; or for everybody.
              (set-file-modes local-copy (tramp-octal-to-decimal "0600"))

              (when (and (null remote-copy)
                          method 'tramp-copy-keep-tmpfile))
                ;; We keep the local file for performance reasons,
                ;; useful for "rsync".
                (setq tramp-temp-buffer-file-name local-copy)
                (put 'tramp-temp-buffer-file-name 'permanent-local t))

               v 4 "Inserting local temp file `%s'..." local-copy)

              ;; We must ensure that `file-coding-system-alist'
              ;; matches `local-copy'.
              (let ((file-coding-system-alist
                      filename local-copy)))
                (setq result
                       local-copy nil nil nil replace))
                ;; Now `last-coding-system-used' has right value.
                ;; Remember it.
                (when (boundp 'last-coding-system-used)
                  (setq coding-system-used
                        (symbol-value 'last-coding-system-used))))

               v 4 "Inserting local temp file `%s'...done" local-copy)
              (when (boundp 'last-coding-system-used)
                (set 'last-coding-system-used coding-system-used))))

        ;; Save exit.
          (when visit
            (setq buffer-file-name filename)
            (setq buffer-read-only (not (file-writable-p filename)))
            (set-buffer-modified-p nil))
          (when (and (stringp local-copy)
                     (or remote-copy (null tramp-temp-buffer-file-name)))
            (delete-file local-copy))
          (when (stringp remote-copy)
             (tramp-make-tramp-file-name method user host remote-copy))))))

    ;; Result.
    (list (expand-file-name filename)
          (cadr result))))
--8<---------------cut here---------------end--------------->8---

> Ole

Best regards, Michael.

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