CVE-2018-6954: systemd-tmpfiles root privilege escalation by following non-terminal symlinks

[Michael Orlitzky <michael () orlitzky com>]

Product: systemd (tmpfiles)
Versions-affected: 239 and earlier
Author: Michael Orlitzky
Fixed-in: v240
Bug-report: https://github.com/systemd/systemd/issues/7986
Acknowledgments:
  Franck Bui of SUSE put forth a massive amount of effort to fix this,
  and Lennart Poettering consistently provided timely reviews over the
  course of a few months.


== Summary ==

Before version 240, the systemd-tmpfiles program will follow symlinks 
present in a non-terminal path component while adjusting permissions and 
ownership. Often -- and particularly with "Z" type entries -- an 
attacker can introduce such a symlink and take control of arbitrary 
files on the system to gain root. The "fs.protected_symlinks" sysctl 
does not prevent this attack. Version 239 contained a partial fix, but 
only for the easy-to-exploit recursive "Z" type entries.


== Details ==

The systemd-tmpfiles program tries to avoid following symlinks in the 
last component of a path. To that end, the following trick is used in 
src/tmpfiles/tmpfiles.c:

  fd = open(path, O_NOFOLLOW|O_CLOEXEC|O_PATH);
  ...
  xsprintf(fn, "/proc/self/fd/%i", fd);
  ...
  if (chown(fn, ...

The call to chown will follow the "/proc/self/fd/%i" symlink, but only 
once; it will then operate on the real file described by fd.

However, there is another way to exploit the code above. The call to 
open() will follow symlinks if they appear in a non-terminal component 
of path, even with the O_NOFOLLOW flag set. Citing the open(2) man page,

  O_NOFOLLOW

  If pathname is a symbolic link, then the open fails, with the error
  ELOOP. Symbolic links in earlier components of the pathname will still
  be followed.

So, for example, if the path variable contains "/run/foo/a/b" and if "a" 
is a symlink, then open() will follow it. If systemd-tmpfiles will be 
changing ownership of "/run/foo/a/b" after that of "/run/foo", then the 
owner of "/run/foo" can exploit that fact to gain root by replacing 
"/run/foo/a" with a symlink. With a Z-type tmpfiles.d entry, the 
attacker can create this situation himself.

The "fs.protected_symlinks" sysctl does not protect against these sorts 
of attacks. Due to the widespread and legitimate use of symlinks in 
situations like these, the symlink protection is much weaker than the 
corresponding hardlink protection.


== Exploitation ==

Consider the following entry in /etc/tmpfiles.d/exploit-recursive.conf:

  d /var/lib/systemd-exploit-recursive 0755 mjo mjo
  Z /var/lib/systemd-exploit-recursive 0755 mjo mjo

Once systemd-tmpfiles has been started once, my "mjo" user will own that 
directory:

  mjo $ sudo ./build/systemd-tmpfiles --create
  mjo $ ls -ld /var/lib/systemd-exploit-recursive
  drwxr-xr-x  2 mjo mjo 4.0K 2018-02-13 09:38 /var/lib/systemd...

At this point, I am able to create a directory "foo" and a file 
"foo/passwd" under /var/lib/systemd-exploit-recursive. The next time 
that systemd-tmpfiles is run (perhaps after a reboot), the tmpfiles.c 
function item_do_children() will be called on "foo". Within that 
function, there is a macro FOREACH_DIRENT_ALL that loops through the 
entries of "foo".

The FOREACH_DIRENT_ALL macro defers to readdir(3), and thus requires the 
real directory stream pointer for "foo", because we want it to see 
"foo/passwd". However, while the macro is iterating, the "q = action(i, 
p)" will be performed on "p" which consists of the path "foo" and some 
filename "d", but without reference to its file descriptor. So, between 
the time that item_do_children() is called on "foo" and the time that "q 
= action(i, p)" is run on "foo/passwd", I have the opportunity to 
replace "foo" with a symlink to "/etc", causing "/etc/passwd" to be 
affected by the change of ownership and permissions.

But there's more: the FOREACH_DIRENT_ALL macro processes the contents of 
"foo" in whatever order readdir() returns them. Since mjo owns "foo", I 
can fill it with junk to buy myself as much time as I like before 
"foo/passwd" is reached:

  mjo $ cd /var/lib/systemd-exploit-recursive
  mjo $ mkdir foo
  mjo $ cd foo
  mjo $ echo $(seq 1 500000) | xargs touch
  mjo $ touch passwd

Now, restarting systemd-tmpfiles will change ownership of all of those 
files...

  mjo $ sudo ./build/systemd-tmpfiles --create

and it takes some time for it to process the 500,000 dummy files before 
reaching "foo/passwd". At my leisure, I can replace foo with a symlink:

  mjo $ cd /var/lib/systemd-exploit-recursive
  mjo $ mv foo bar && ln -s /etc ./foo

After some time, systemd-tmpfiles will eventually reach the path 
"foo/passwd", which now points to "/etc/passwd", and grant me root access.

A similar, but more difficult attack works against non-recursive entry 
types. Consider the following tmpfiles.d entry:

  d /var/lib/systemd-exploit 0755 mjo mjo
  d /var/lib/systemd-exploit/foo 0755 mjo mjo
  f /var/lib/systemd-exploit/foo/bar 0755 mjo mjo

After "/var/lib/systemd-exploit/foo" is created but before the 
permissions are adjusted on "/var/lib/systemd-exploit/foo/bar", there is 
a short window of opportunity for me to replace "foo" with a symlink to 
(for example) "/etc/env.d". If I'm fast enough, tmpfiles will open 
"foo/bar", following the "foo" symlink, and give me ownership of 
something sensitive in the "/etc/env.d" directory. However, this attack 
is more difficult because I can't arbitrary widen my own window of 
opportunity with junk files, as was possible with the "Z" type entries.


== Resolution ==

Commit 936f6bdb, which is present in systemd v239, changes the recursive 
loop in two important ways. First, it passes file descriptors -- rather 
than parent paths -- to each successive iteration. That allows the next 
iteration to use the openat() system call, eliminating the non-terminal 
path components from the equation. Second, it ensures that each "open" 
call has the O_NOFOLLOW and O_PATH flags to prevent symlinks from being 
followed at the current depth. Note: only the recursive loop was made 
safe; the call to open() the top-level path will still follow 
non-terminal symlinks and is vulnerable to the second attack above.

The commits in pull request 8822 aim to make everything safe from this 
type of symlink attack. As far as tmpfiles is concerned, the main idea 
is to use the chase_symlinks() function in place of the open() system 
call. Since chase_symlinks() calls openat() recursively from the root 
up, it will never follow a non-terminal symlink. Commit 1f56e4ce then 
introduces the CHASE_NOFOLLOW flag for that function, preventing it from 
following terminal symlinks. In subsequent commits (e.g. addc3e30), the 
consumers of chase_symlinks() were updated to pass CHASE_NOFOLLOW to 
chase_symlinks(), preventing them from following any symlinks.

The complete fix is available in systemd v240.