Message-ID: <20150723172414.GC17931@localhost.localdomain>
Date: Thu, 23 Jul 2015 10:24:14 -0700
From: Qualys Security Advisory <qsa@...lys.com>
To: fulldisclosure@...lists.org
Subject: [FD] Qualys Security Advisory - CVE-2015-3245 userhelper -
 CVE-2015-3246 libuser


Hello, it is July 23, 2015, 17:00 UTC, the Coordinated Release Date for
CVE-2015-3245 and CVE-2015-3246.  Please find our advisory below, and
our exploit attached.


Qualys Security Advisory

CVE-2015-3245 userhelper chfn() newline filtering

CVE-2015-3246 libuser passwd file handling


--[ Summary ]-----------------------------------------------------------------

The libuser library implements a standardized interface for manipulating
and administering user and group accounts, and is installed by default
on Linux distributions derived from Red Hat's codebase. During an
internal code audit at Qualys, we discovered multiple libuser-related
vulnerabilities that allow local users to perform denial-of-service and
privilege-escalation attacks. As a proof of concept, we developed an
unusual local root exploit against one of libuser's applications.


----[ Vulnerability #1 (CVE-2015-3245 userhelper chfn() newline filtering)

We discovered a bug in userhelper, a setuid-root program from the
usermode package that provides a basic interface to change a user's
password, gecos information, and shell; its -f (Full Name), -o (Office),
-p (Office Phone) and -h (Home Phone) command-line options are
equivalent to those of the traditional chfn program.

userhelper's chfn() function verifies that the fields it was given on
the command-line are sane (i.e., contain no forbidden characters).
Unfortunately, these forbidden characters (":,=") do not include '\n'
and allow local attackers to inject newline characters into /etc/passwd
and alter this file in unexpected ways.

To the best of our knowledge, this bug is a local denial-of-service
only: we were not able to turn it into a local root exploit, but maybe
some creative minds will.

There is another, secondary aspect of this bug: userhelper depends on
libuser to modify /etc/passwd, and libuser's format_generic() and
generic_setpass() functions reject fields containing a ':' that would be
interpreted as a field separator. Vulnerability #1 could have been
prevented if libuser had also rejected '\n' characters.


----[ Vulnerability #2 (CVE-2015-3246 libuser passwd file handling)

We discovered a bug in libuser itself: even though traditional programs
like passwd, chfn, and chsh work on a temporary copy of /etc/passwd and
eventually rename() it, libuser modifies /etc/passwd directly.
Unfortunately, if anything goes wrong during these modifications,
libuser may leave /etc/passwd in an inconsistent state.

This bug is not just another local denial-of-service: we were able to
turn it into a local root exploit against userhelper and chfn (if linked
with libuser).

There is also another, secondary aspect of this bug: glibc modules like
nss and nscd do not expect /etc/passwd to be directly modified while
they parse its contents, and programs from packages like shadow-utils
and util-linux use lckpwdf() locks that are incompatible with libuser's
fcntl() locks.


--[ Exploitation Overview ]---------------------------------------------------

In this section, we outline our userhelper exploit against libuser's
Vulnerability #2; later in this advisory, we explain how it can be
easily adapted to chfn (if linked with libuser).

Our ultimate goal is to inject an arbitrary line into /etc/passwd (for
example, the a-line "\na::0:0::/:\n") but we first need to understand
how libuser's generic_mod() function modifies our own user's line in
/etc/passwd:

- open() /etc/passwd for reading and writing (O_RDWR, but not O_APPEND
  nor O_TRUNC);

- acquire the file's fcntl() write-lock (an exclusive, but advisory
  lock);

- read() the file's contents (into a g_malloc()ated buffer);

- lseek() the file to the beginning of our user's line (and skip the
  unmodified lines that precede);

- write() our user's new, modified line (and the rest of the unmodified
  lines that follow) to the file;

- ftruncate() the file (if our user's new, modified line is shorter than
  the old one);

- release the file's fcntl() write-lock;

- close() the file.

Surprisingly, we only need two things in our toolbox in order to exploit
this function and inject the a-line into /etc/passwd:

- a pencil and eraser that allows us to repeatedly write() and
  re-write() our own GECOS field (its length and last character in
  particular) in /etc/passwd: the userhelper program itself;

- a pair of scissors that allows us to interrupt write() with byte
  precision and avoid ftruncate(): the resource limit RLIMIT_FSIZE, "The
  maximum size of files that the process may create. Attempts to extend
  a file beyond this limit result in delivery of a SIGXFSZ signal. By
  default, this signal terminates a process, but a process can catch
  this signal instead, in which case the relevant system call (e.g.,
  write(2), truncate(2)) fails with the error EFBIG."

For each character in the a-line (beginning with its last character and
ending with its first character), we fork() a new process and execve()
userhelper with:

- a GECOS field that allows us to write() the character to its target
  offset in /etc/passwd;

- an RLIMIT_FSIZE that allows us to terminate the process before it
  write()s or ftruncate()s the characters that follow.

In this example, the newline character '\n' is represented by |, and the
last character written (before write() is interrupted by RLIMIT_FSIZE)
is marked with ^:

...|...|user:x:1000:1000::/home/user:/bin/bash|...|...|
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAA:/home/user:/bin/bash|...|...|
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAA:/home/user:|...|...|
                                                                ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA:/home/:|...|...|
                                                               ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA:/:|...|...|
                                                              ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA::/:|...|...|
                                                             ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA0::/:|...|...|
                                                            ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA:0::/:|...|...|
                                                           ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA0:0::/:|...|...|
                                                          ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA:0:0::/:|...|...|
                                                         ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA::0:0::/:|...|...|
                                                        ^
...|...|user:x:1000:1000:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAa::0:0::/:|...|...|
                                                       ^
...|...|user:x:1000:1000:AAAAAAAA:/home/user:/bin/bash|a::0:0::/:|...|...|
                                                      ^
...|...|user:x:1000:1000::/home/user:/bin/bash|a::0:0::/:|...|...|


--[ Exploitation Details ]----------------------------------------------------

In this section, we discuss the problems we encountered while developing
our userhelper exploit, and how we solved them.


----[ Problem #1 (missing fields)

At the end of our "Exploitation Overview" example, our home-directory
and shell-program fields seem to magically reappear in /etc/passwd,
although they were previously cut out by RLIMIT_FSIZE.

This magic trick introduces Problem #1: we cannot simply fork() a new
process for each character in the a-line, execve() userhelper, and let
it run until the character is written to its target offset in
/etc/passwd, because libuser refuses to modify our user's line if some
of its fields are missing.

In order to solve this Problem #1, we fork() a new process for each
character in the a-line, execve() userhelper, and let it load our user's
original, uncut line from /etc/passwd, but we SIGSTOP the process before
it open()s /etc/passwd for writing. Only after we have started and
stopped all userhelper processes can we safely SIGCONT them, one at a
time.


----[ Problem #2 (backup file)

Before libuser open()s /etc/passwd for writing, it creates a backup file
named /etc/passwd- and if this backup fails, libuser refuses to modify
/etc/passwd. Unfortunately, our RLIMIT_FSIZE also applies to the backup,
which will fail if the RLIMIT_FSIZE is less than the size of
/etc/passwd.

This introduces Problem #2: in apparent contradiction to what we just
said, our exploit needs to decrease RLIMIT_FSIZE after each character it
injects into /etc/passwd (as shown in the "Exploitation Overview"
example).

In order to solve this Problem #2, we refine Problem #1's
SIGSTOP/SIGCONT solution: we let each userhelper process load our user's
original, uncut line from /etc/passwd, and SIGSTOP the process after it
creates the backup file but before it modifies /etc/passwd. In other
words, we have to win a race against generic_mod()'s system calls, which
create the backup file and modify /etc/passwd:

- open() the passwd file /etc/passwd for reading;
- acquire the passwd file's fcntl() read-lock;

- open() the backup file /etc/passwd- for writing;
- acquire the backup file's fcntl() write-lock;

- read() from the passwd file;
- write() to the backup file;
- ftruncate() the backup file;

- release the backup file's fcntl() write-lock;
- close() the backup file;

- release the passwd file's fcntl() read-lock;
- close() the passwd file;

- open() /etc/passwd for reading and writing;
[RACE WINDOW BEGINS]
- acquire the file's fcntl() write-lock: failure, sleep for a few microseconds;
- acquire the file's fcntl() write-lock: failure, sleep for a few microseconds;
- acquire the file's fcntl() write-lock: failure, sleep for a few microseconds;
[RACE WINDOW ENDS]
- acquire the file's fcntl() write-lock: success;
- read() the file's contents;
- etc.

In order to reliably win this race against all userhelper processes (one
for each character in the a-line), we:

- widen the race window. We acquire a read-lock on /etc/passwd before we
  execve() userhelper, which prevents libuser from acquiring the
  write-lock on /etc/passwd, and forces it to sleep for a few
  microseconds (LU_LOCK_TIMEOUT is 2, LU_MAX_LOCK_ATTEMPTS is 6).

- pinpoint the race window. We monitor the filesystem for the following
  sequence of inotify events:

  . IN_CREATE on /etc if the backup file does not exist;
  . IN_CLOSE_WRITE on the backup file;
  . IN_CLOSE_NOWRITE on the passwd file;
  . IN_OPEN on the passwd file.

- preempt the userhelper processes. We setpriority() them to the lowest
  priority, sched_setscheduler() them to SCHED_IDLE, and
  sched_setaffinity() them to the same CPU as our exploit.


----[ Problem #3 (last user)

If our user's line is the last one in /etc/passwd, then the last
character we inject into the file (the '\n' that ends our user's line
and begins the a-line) is also the very last character of write()'s
buffer, which introduces Problem #3: this last write() will not exceed
our RLIMIT_FSIZE, and the consequent ftruncate() will delete the a-line
from the end of /etc/passwd.

In order to solve this Problem #3:

- either we SIGKILL the last userhelper process after write() but before
  ftruncate(). We reliably win this race with an IN_MODIFY event on
  /etc/passwd and the "same CPU, different priorities" preemption of
  userhelper.

- or we exploit Vulnerability #1 and inject a '\n' into our own GECOS
  field. As far as libuser is concerned, this '\n' ends our user's line
  and begins a new one (with our leftover home-directory and
  shell-program fields): our user's line is no longer the last one in
  /etc/passwd.


----[ Problem #4 (maximum GECOS_LENGTH)

As shown in our "Exploitation Overview" example, we only have two
options for arbitrary character injection into /etc/passwd:

- either we use a character that we artificially inject through our own
  GECOS field (not an option for characters like ':' and '\n');

- or we reuse a character that is naturally present in /etc/passwd (our
  only option for characters like ':' and '\n').

Unfortunately, both of these options might fail to inject a character
after the end of /etc/passwd (a consequence of Problem #2):

- if our own GECOS field is too far away from the end of /etc/passwd
  (farther than userhelper's maximum GECOS_LENGTH, 127 characters);

- if the character is not already one of the last GECOS_LENGTH
  characters in /etc/passwd.

If faced with both of these problems, we solve the first one (and
Problem #4) by repeatedly deleting lines from the end of /etc/passwd,
until our own user's line is the last one in the file: we enlarge our
own GECOS field, delete characters from the end of /etc/passwd with our
RLIMIT_FSIZE scissors, shrink our GECOS field again, repeat.


----[ Problem #5 (time complexity)

For each character in the a-line, we usually have to choose one of
several (GECOS, RLIMIT_FSIZE) pairs that allow us to write the character
to its target offset in /etc/passwd.

These pairs represent the nodes of a search tree that grows
exponentially (with the number of characters in the a-line) but may
contain few or no solutions. In order to avoid this tree's worst-case
time complexity, we:

- inject the shortest a-line possible, "\na::0:0::/:\n";

- perform a recursive depth-first search on the tree, and return the
  first solution we find (instead of, for example, the solution that
  minimizes /etc/passwd's alterations);

- replace the a-line's username with a wildcard, and accept any
  lowercase character that is not already a username (the a-line's
  username was a major problem, because it is the last character we
  inject, and therefore occurs deep down the tree's branches; the
  a-line's '0' characters are only a minor problem, because they occur
  in the middle of the tree's branches, whence we can backtrack
  quickly).


----[ chfn

util-linux's chfn from Red Hat's codebase is linked with libuser, and
can be exploited by our public roothelper.c with just a few changes
(left as an exercise for the interested reader):

- userhelper uses a simple Userhelper/Consolehelper request/response
  protocol in order to prompt for and read the user's password, but chfn
  uses traditional terminal interaction;

- if our user's line is the last one in /etc/passwd, we can exploit
  Vulnerability #1 against userhelper, but we have to win Problem #3's
  write/ftruncate race against chfn;

- userhelper returns 0/255 on success/failure, but chfn returns 0/1.


--[ Acknowledgments ]---------------------------------------------------------

We would like to thank Red Hat's Security Response Team and developers
for promptly addressing these issues.


View attachment "roothelper.c" of type "text/plain" (28947 bytes)


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