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Message-ID: <30c90215-d7f0-9540-1455-cd6fed6095b6@orlitzky.com>
Date: Fri, 21 Dec 2018 22:14:24 -0500
From: Michael Orlitzky <michael@...itzky.com>
To: oss-security@...ts.openwall.com
Subject: CVE-2018-6954: systemd-tmpfiles root privilege escalation by
following non-terminal symlinks
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.
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