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tmpwatch.txt

tmpwatch.txt
Posted Dec 21, 2002
Authored by Michal Zalewski | Site lcamtuf.coredump.cx

Common use of 'tmpwatch' utility and its counterparts triggers race conditions in many applications, sometimes allowing privilege escalation. Includes information on races, file removal, fixes, and more.

tags | paper
systems | unix
SHA-256 | b15d4299f68a0564b2dbf1976f2695381bb7cba4b78e5f66221c135ce941492e

tmpwatch.txt

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Common use of 'tmpwatch' utility and its counterparts triggers race
conditions in many applications

Michal Zalewski <lcamtuf@razor.bindview.com>, 12/05/2002
Copyright (C) 2002 by Bindview Corporation


1) Scope and exposure info
--------------------------

A common practice of installing 'tmpwatch' utility or similar software
configured to sweep the /tmp directory on Linux and unix systems can
compromise secure temporary file creation mechanisms in certain applications,
creating a potential privilege escalation scenario. This document briefly
discusses the exposure, providing some examples, and suggesting possible
workarounds.

It is believed that many unix operating systems using 'tmpwatch' or an
equivalent are affected. Numerous Linux systems, such as Red Hat, that ship
with cron daemon running and 'tmpwatch' configured to sweep /tmp are
susceptible to the attack.


2) Application details
----------------------

'Tmpwatch' is a handy utility that removes files which haven't been
accessed for a period of time. It was developed by Erik Troan and
Preston Brown of Red Hat Software, and, with time, has become a
component of many Linux distributions, also ported to platforms
such as Solaris, *BSD or HP/UX. By default, it is installed with a
crontab entry that sweeps /tmp directory on a daily basis, deleting
files that have not been accessed for the past few days.

An alternative program, called 'stmpclean' and authored by Stanislav
Shalunov, is shipped with *BSD systems and some Linux distributions
to perform the same task, and some administrators deploy other tools or
scripts for this purpose.


3) Vulnerability details
------------------------

Numerous applications rely either on mkstemp() or custom O_EXCL file
creation mechanisms to store temporary data in the /tmp directory
in a secure manner. Of those, certain programs run with elevated
privileges, or simply at a different privilege level than the caller.

The exposure is a result of a common misconception, promoted by almost
all secure programming tutorials and manpages, that /tmp files created
with mkstemp(), granted that umask() settings were proper, are
safe against hijacking and common races. The file, since it is created
in a sticky-bit directory, indeed cannot be removed or replaced by
the attacker running with different non-root privileges, but since
many operating systems feature 'tmpwatch'-alike solutions, the only
thing that can and should be considered safe in /tmp is the descriptor
returned by mkstemp() - the filename should not be relied upon. There
are two major reasons for this:

1) unlink() races

It is very difficult to remove a file without risking a potential
race (see section 4). 'Tmpwatch' does not take any extra measures to
prevent races, and probes file creation time using lstat(). Based on this
data, it calls unlink() as root. Problem is, on a multitasking system, it
is possible for the attacker to get some CPU time between those two system
calls, remove the old "decoy" file that has been probed with lstat(), and
let the application of his choice create its own temporary file under this
name. While mkstemp() names are guaranteed to be unique, they shouldn't be
expected to be unpredictable - in most implementations, the name is a
function of process ID and time - so it is possible for the attacker to
guess it and create a decoy in advance. Once the tmpwatch process is
resumed, the file is immediately removed, based on the result of
earlier lstat() on the old, no longer existing file.

While this three-component race requires very precise timing, it
is possible to try a number of times in a single 'tmpwatch' run if
enough decoy files are created by the attacker. Additionally, since
each step of the attack would result in a corresponding filesystem
change, it is fairly easy to carefully measure timings and
coordinate the attack.

If the attacker cannot make the application run at the same time
as 'tmpwatch' - for example, if the application is executed by
hand by the administrator, or is running from cron - 'tmpwatch'
itself can be artificially delayed for almost an arbitrary amount
of time by creating and continuously expending an elaborate directory
structure in /tmp using hard links (to preserve access times of
files) and running other processes that demand disk access and
cache space to slow down the process.

'Stmpclean' offers additional protection against races by not removing
root-owned files and temporarily dropping privileges when removing
the file to match the owner of lstat()ed resource. Unfortunately,
not removing root files is a considerable drawback, and there is still
a potential for a race using carefully crafted hard links to a file
owned by the victim and two concurrent 'stmpclean' processes:

- the attacker links /tmp/foo to ~victim/.bash_profile
- tmpwatch #1 does lstat() on /tmp/foo and setuid victim
- tmpwatch #2 does lstat() on /tmp/foo and setuid victim
- tmpwatch #1 does unlink("/tmp/foo")
- victim application creates /tmp/foo at uid==victim
- tmpwatch #2 does unlink("/tmp/foo") and succeeds
- the attacker creates /tmp/foo
- victim application proceeds

On certain systems such as Owl Linux, the attack will be not possible
due to hardlink limits imposed on sticky-bit directories.

2) suspended processes and 'legitimate' file removal

Here, all conventional measures that could be exercised by /tmp cleaners
fail miserably. A vulnerable application can be often delayed or suspended
after mkstemp() / open() - for example, a setuid program can be
stopped with SIGSTOP and resumed with SIGCONT. If the application is
suspended for long enough, its temporary files are likely to be
removed. This method requires much less precision, but is also
more time-consuming and has a more limited scope (interactive
applications only).

Note that it is sometimes possible to delay the execution of
a daemon - client wait, considerable I/O or CPU loads, and subsequent
mkstemp() calls can be all used to achieve the effect. The
feasibility and efficiency is low, but the potential issue
exists. Some client applications that are often left unattended
and create temporary files - such as mail/news clients, web
browsers, irc clients, etc - can also be used to compromise
other accounts on the machine.

Not all applications are prone to the problem just because mkstemp()
is used to create files in /tmp; if the file name is not used to perform
any sensitive operations with some extra privileges afterward (read,
write, chown, chmod, link/rename, etc), and only the descriptor is
being used, the application is safe. This practice is often exercised by
programmers who want to avoid leaving dangling temporary files in case
the program is aborted or crashes. Similarly, if the application uses
temporary files improperly, but does not rely on their contents and does
not attempt to access them with higher privileges, the application is
secure in that regard.

Applications that run with higher privileges and reopen their
/tmp temporary files for reading or writing, call chown(), chmod() on
them, rename or link the file to replace some sensitive information, and
so on, are exposed. It is worth mentioning that a popular 'mktemp'
utility coming from OpenBSD passes only the filename to the
caller shell script, thus rendering almost all scripts using it
fundamentally flawed. If the script is being run as a cron job or
other administrative task, and mktemp is used, the system can be likely
compromised by replacing the file after mktemp and prior to any write
to the file. In the example quoted in the documentation for mktemp(1):

TMPFILE=`mktemp /tmp/$0.XXXXXX` || exit 1
echo "program output" >> $TMPFILE

...the attacker would want to replace temporary file right before
'echo', causing the text "program output" to be appended to a target
file of his choice using symlinks or hardlinks; or, if it is more
desirable, he'd spoof file contents to cause the program to misbehave.

Another example of the problem is a popular logrotate utility,
coded - ironically - by Erik Troan, one of co-authors of 'tmpwatch'
itself. The program suffered /tmp races in the past, but later
switched to mkstemp(). The following sequence is used to handle
post-rotation shell commands specified in config files:

open("/tmp/logrotate.wvpNmP", O_WRONLY|O_CREAT|O_EXCL, 0700) = 6
...
write(6, "#!/bin/sh\n\n", 11) = 11
write(6, "\n\t/bin/kill -HUP `cat /var/lock/"..., 79) = 79
close(6) = 0
... fork, etc ...
execve("/bin/sh", ["sh", "-c", "/bin/sh /tmp/logrotate.wvpNmP" ...

Obviously, if the attacker can have /tmp/logrotate.* replaced in
between mkstemp() (represented as open() syscall above) and the
point where another process is spawned, a shell interpreter is invoked,
then executes another copy of the shell interpreter (apparent
programmer's mistake) and finally reads the input file - which is
a considerable chunk of time - the shell will be called with
attacker-supplied commands to be executed with root privileges.

On Red Hat, logrotate is executed from crontab on a daily basis, in
a sequence before 'tmpwatch', and the easiest option for the attacker
is to maintain a still-running tmpwatch process from the previous day
to exploit the condition. On systems where those programs are not
executed sequentially - for example, when both programs are listed
directly in /etc/crontab - the attack requires less precision.


4) Workarounds and fixes:
-------------------------

Recommended immediate workaround is to discontinue the use of 'tmpwatch'
or equivalent to sweep /tmp directory if this service is not necessary.

For applications that rely on TMPDIR or a similar environment
variable, setting it to a separate, not publicly writable directory
is often a viable solution. Note that not all applications honor
this setting.

In terms of a permanent solution, two different attack vectors have
to be addressed, as discussed in section 3:

1) unlink() race

The proper way to remove files in sticky-bit directories while
minimizing the risk is as follows:

a) lstat() the file to be removed
b) if owned by root, do not remove
c) if st_nlink > 1, do not remove
d) if owned by user, temporarily change privileges to this user
e) attempt unlink()
f) if failed, warn about a possible race condition
g) switch privileges back to root

With the exception of step c, this is implemented in 'stmpclean'.
Unfortunately, step c is crucial on systems that do not have
restricted /tmp kernel patches from Openwall (http://www.openwall.com),
otherwise, there is a potential for fooling the algorithm by supplying
a hard link to a file owned by the victim, as discussed in section 3.

This approach has several drawbacks - such as the fact root-owned files
will not be removed. Other solution is to modify applications that
generate filenames on their own, and to modify mkstemp(), to generate
names that are not only unique, but not feasible to predict.

Another suggestion is to implement a funlink() capability in the kernel
of the operating system in question, to allow race-free file removal,
thus removing the non-root ownership requirement for the method described
above, and simplifying the approach. A skeleton patch to implement
funlink() semantics and make sure the file being removed is the file
opened and fstat()ed previously is available at:
http://lcamtuf.coredump.cx/soft/linux-2.4-funlink.diff (this and
other patches are not endorsed by RAZOR in any way).

2) suspended process and 'legitimate' file removal

This issue is fairly difficult to address. The most basic idea is
to use a special naming scheme for temporary files to avoid deletion -
unfortunately, this seems to defeat the purpose of using tmpwatch-alike
solutions in the first place.

An alternative approach, which is to enforce separate temporary
directories for certain applications, either process-, session- or uid-
based, is generally fairly controversial, and raises some concerns.
Advisory separation is generally acceptable, but there are a number of
applications that do not accept TMPDIR setting, and a widespread practice
of using /tmp in in-house applications. Mandatory separation (kernel
modification) raises compatibility concerns and is generally approached
with skepticism - no implementation has become particularly popular.

Ideally, implementators should carefully audit their sources. It is
recommended for privileged applications to use private temporary
directories for sensitive files, if possible; if using /tmp is necessary,
extra caution has to be exercised to avoid referencing the file by name.
Note that comparing the descriptor and a reopened file to verify inode
numbers, creation times or file ownership is not sufficient - please refer
to "Symlinks and Cryogenic Sleep" by Olaf Kirch, available at
http://www.opennet.ru/base/audit/17.txt.html .

It's worth noticing that 'tmpwatch' offers a -s option, which causes the
program to run the 'fuser' command to prevent removal of files that are
currently open. At first sight, this could be an effective way to solve the
problem. Unfortunately, this is not true, since many applications close the
file for a period of time before reopening (including logrotate and
mktemp(1)).


5) Credits and thanks
---------------------

Thanks to Solar Designer for interesting discussions on the subject,
to Matt Power for useful feedback, and to RAZOR team in general for making
this publication possible.





















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