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TUCoPS :: Unix :: General :: unix5895.htm

'tmpwatch' triggers race conditions in many applications



23th Dec 2002 [SBWID-5895]
COMMAND

	'tmpwatch' triggers race conditions in many applications

SYSTEMS AFFECTED

	

PROBLEM

	In  an  advisory  by   Michal   Zalewski   [lcamtuf@razor.bindview.com],
	Copyright (C) 2002 by Bindview Corporation :
	
	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.
	
	
	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.

SOLUTION

	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)).


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