Re: [PATCH] Bindings to *at functions & allowing more functions to operate on ports

[Maxime Devos]

Hi,

[CC'ing some Guile and Guix maintainers because this is
important for the security of Guix System.]

I want to explain why these patches (and the O_FLAGS (*)
patch) should be included in Guile.  Functions like "openat"
are important to avoid TOCTTOU (time-of-check to time-of-use)
vulnerabilities involving symbolic links.

For example, suppose we have a web server implemented in
Guile.  Suppose the address is https://web.gnu.  It allows
a local user U (and some others) to define their own web
pages to host at http://web.gnu/~U, by writing files to
/home/U/www.  As there are multiple users, the server has
to run as root.

Now suppose U is the malicious kind of user.  Then $U
could create a symlink at /home/U/www/maliciousity pointing
to /home/other-user/.gnupg/private-keys-v1.d/FINGERPRINT.key.

Now U could download other-user's gpg key, for example
with "wget http://web.gnu/~U/maliciousity";.  Oops!

How can this vulnerability be avoided?

* Use O_NOFOLLOW to *not* follow the symbolic link.
  Patch for adding O_NOFOLLOW to guile:
  <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=46220>.

And why do we need openat?  Well, suppose the web server
is not read-only, and supports (say) WebDAV or FTP for
modifying files remotely (I mean U can remotely modify
http://web.gnu/~U).  Then U could create a symlink
at /home/U/www/maliciousity pointing to /home/other-user.
Now U can peek into other-user's home directory and overwrite
files.  Oops!

How can the web server avoid this?

* First open "/home/U" as usual, resulting in a port $1.
  Then use (openat $1 "maliciousity" O_NOFOLLOW), resulting
  in a port $2.  Use (stat $2) to see if $2 is a directory
  or a regular file **and** to see if $2 is owned by $2!
  If necessary, recurse, etc.  Display a directory listing
  or display the file, etc.

How does this matter for Guix?

Guix has a TOCTTOU race:
<https://lists.gnu.org/archive/html/guix-devel/2021-01/msg00388.html>.
It has been partially fixed:
<https://git.savannah.gnu.org/cgit/guix.git/commit/?id=520bac7ed00a949a0391ad680de65a1498105c2b>.
However, a complete fix requires bindings to "openat".

I found another similar issue in Guix lately (not yet disclosed publicly).
While I think the conditions for this other potential security issue
to be exploitable don't ever happen in practice, I would still like
to fix this issue, and to be able to prevent similar issues from appearing
in the future.

Greetings,
Maxime.

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