Re: [elpa] externals-release/org e2366ac283: * lisp/org-persist.el: Do not litter /tmp when native compiling

Re: [elpa] externals-release/org e2366ac283: * lisp/org-persist.el: Do not litter /tmp when native compiling

[Stefan Monnier]

> +(when (and org-persist--disable-when-emacs-Q
> +           ;; FIXME: This is relying on undocumented fact that
> +           ;; Emacs sets `user-init-file' to nil when loaded with
> +           ;; "-Q" argument.
> +           (not user-init-file))
> +  (setq org-persist-directory
> +        (make-temp-file "org-persist-" 'dir))
> +  ;; We don't need the temp directory to exist.
> +  ;; `org-persist-write-all' will refrain from creating and writing to the dir if
> +  ;; none exists yet.
> +  (delete-directory org-persist-directory))

Could we skip the "create dir" so we can skip the `delete-directory`
part as well?

Also, this smells like a security hole: after `delete-directory`, with
the /tmp setup as mode #o1777, some other user can come and create
a directory at that location, so that our `org-persist-directory` will
then point to their directory (where they may be able to read the
things we write) or they can replace it with a link to a directory of
our own tricking us into overwriting some of our own files.

IOW, I suggest we use something like

    (setq org-persist-directory "/non-existing")
or
    (setq org-persist-directory (make-temp-name "/non-existing"))

[ Not sure if there is a standard such name that's "guaranteed" not to
  exist.  ]


        Stefan

Re: [elpa] externals-release/org e2366ac283: * lisp/org-persist.el: Do not litter /tmp when native compiling

[Ihor Radchenko]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> +  (setq org-persist-directory
>> +        (make-temp-file "org-persist-" 'dir))
>> +  ;; We don't need the temp directory to exist.
>> +  ;; `org-persist-write-all' will refrain from creating and writing to the dir if
>> +  ;; none exists yet.
>> +  (delete-directory org-persist-directory))
>
> Could we skip the "create dir" so we can skip the `delete-directory`
> part as well?

It would be wonderful, but

    There is a race condition between calling make-temp-name and
    later creating the file, which opens all kinds of security holes.
    For that reason, you should normally use make-temp-file instead.

which scared me.

> Also, this smells like a security hole: after `delete-directory`, with
> the /tmp setup as mode #o1777, some other user can come and create
> a directory at that location, so that our `org-persist-directory` will
> then point to their directory (where they may be able to read the
> things we write) or they can replace it with a link to a directory of
> our own tricking us into overwriting some of our own files.

This also means that other user has access to Elisp machine state (the
value of org-persist-directory). If we are in that situation, your
scenario is the least of our concerns. Also, do note that this branch of
code is only executed with emacs -Q.

> IOW, I suggest we use something like
>
>     (setq org-persist-directory "/non-existing")
> or
>     (setq org-persist-directory (make-temp-name "/non-existing"))
>
> [ Not sure if there is a standard such name that's "guaranteed" not to
>   exist.  ]

If you confirm that it is safe to use `make-temp-name' here, I will go
with your suggestion.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>

Re: [elpa] externals-release/org e2366ac283: * lisp/org-persist.el: Do not litter /tmp when native compiling

[Stefan Monnier]

>>> +  (setq org-persist-directory
>>> +        (make-temp-file "org-persist-" 'dir))
>>> +  ;; We don't need the temp directory to exist.
>>> +  ;; `org-persist-write-all' will refrain from creating and writing to the dir if
>>> +  ;; none exists yet.
>>> +  (delete-directory org-persist-directory))
>>
>> Could we skip the "create dir" so we can skip the `delete-directory`
>> part as well?
>
> It would be wonderful, but
>
>     There is a race condition between calling make-temp-name and
>     later creating the file, which opens all kinds of security holes.
>     For that reason, you should normally use make-temp-file instead.
>
> which scared me.

By keeping `org-persist-directory` but deleting the dir, you basically
turn the narrow window of the race condition into an arbitrarily
long window, so your code is worse than using `make-temp-name` :-(

>> Also, this smells like a security hole: after `delete-directory`, with
>> the /tmp setup as mode #o1777, some other user can come and create
>> a directory at that location, so that our `org-persist-directory` will
>> then point to their directory (where they may be able to read the
>> things we write) or they can replace it with a link to a directory of
>> our own tricking us into overwriting some of our own files.
>
> This also means that other user has access to Elisp machine state (the
> value of org-persist-directory).

Not really.  The usual way to exploit the race condition in
`make-temp-name` is to predict the name that will be chosen (or trick
the code into choosing a particular value, e.g. by creating lost of
entries with names /tmp/org-persist-NNNNN so your Emacs's choice of free
spot is correspondingly reduced).

But, with your code it's even easier, it can just monitor the /tmp
directory to see which dir was created and then deleted and it will then
know the value of your `org-persist-directory`.

>> IOW, I suggest we use something like
>>
>>     (setq org-persist-directory "/non-existing")
>> or
>>     (setq org-persist-directory (make-temp-name "/non-existing"))
>>
>> [ Not sure if there is a standard such name that's "guaranteed" not to
>>   exist.  ]
>
> If you confirm that it is safe to use `make-temp-name' here, I will go
> with your suggestion.

The safety of this code relies on the fact that / is presumed to be
read-only (except for root), contrary to /tmp which is world-writable.


        Stefan

Re: [elpa] externals-release/org e2366ac283: * lisp/org-persist.el: Do not litter /tmp when native compiling

[Ihor Radchenko]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> It would be wonderful, but
>>
>>     There is a race condition between calling make-temp-name and
>>     later creating the file, which opens all kinds of security holes.
>>     For that reason, you should normally use make-temp-file instead.
>>
>> which scared me.
>
> By keeping `org-persist-directory` but deleting the dir, you basically
> turn the narrow window of the race condition into an arbitrarily
> long window, so your code is worse than using `make-temp-name` :-(

Fair point.

I now fixed this by creating the directory at the beginning and only
removing it before exiting Emacs.

https://git.savannah.gnu.org/cgit/emacs/org-mode.git/commit/?h=bugfix&id=987fe173acf6c6fdb4bec5c5c07796d98c6e1983

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>