Edit report at https://bugs.php.net/bug.php?id=52312&edit=1
ID: 52312
Updated by: rasmus@php.net
Reported by: v dot damore at gmail dot com
Summary: PHP safe_mode/open_basedir - lstat performance
problem
Status: Analyzed
Type: Bug
Package: Safe Mode/open_basedir
Operating System: Linux
PHP Version: 5.2.13
Block user comment: N
Private report: N
New Comment:
I don't know what your dd2.inc is doing, but I went through and optimized syscalls a
couple of years ago, and in normal no-openbasedir mode things are pretty clean. Here
are a few scenarios:
a.php does a require on ./b.php and a require_once on ./c.php
PHP 5.4/APC-HEAD/apc.stat=1/apc.include_once_override=0/open_basedir=off
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
stat("/var/www/./b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
open("/var/www/c.php", O_RDONLY) = 29
fstat(29, {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
fstat(29, {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
fstat(29, {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
stat("/var/www/c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
The initial stat on a.php is actually done by Apache and we ask Apache for the stat
struct to avoid an extra stat there. You can see the effect of the open and then the
(quick) fstat calls on fd=29. This is where APC tries to optimize things a bit using
the include_once_override. If we turn that on we get:
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
stat("/var/www/./b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
stat("/var/www/./c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
stat("/var/www/./c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
Not ideal since there is still an extra stat for the require_once case, but the open()
is gone. And you can eliminate those stats by turning off apc.stat which means only
the initial top-page stat from Apache on a.php will be done.
Now, of course, if we turn on open_basedir we get:
www-data 1799 0.8 0.2 310232 22560 pts/6 S+ 19:26 0:00 /usr/sbin/apache2 -X
root 1887 0.0 0.0 10892 916 pts/12 R+ 19:26 0:00 grep apache
7:26pm x220:~> strace -o out -p 1799
Process 1799 attached - interrupt to quit
^CProcess 1799 detached
7:26pm x220:~> egrep "stat|open" out
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
lstat("/var/www/./b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/var/www/b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/var/www/./b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www/./c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/var/www/c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/var/www/./c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www/./c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/var/www/c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/var/www/./c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
which is far from ideal, of course, because we are not hitting our realpath cache at
all and there is an oddity there with ./ getting resolved and re-statted and this re-
realpathed. Not sure I see where your 7 times thing is though.
With ZO+ with this config
zend_optimizerplus.revalidate_freq=0
zend_optimizerplus.enable_file_override=0
open_basedir=
we see this:
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
stat("/var/www/b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
stat("/var/www/c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
Here we see that ZO+ doesn't have the Apache stat optimization that APC has, which is
something we should obviously add, but it handles the require_once better and the
open() call and fstats are gone.
When we turn on open_basedir we get:
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
lstat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
lstat("/var/www/./b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
lstat("/var/www", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/var/www/b.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
stat("/var/www/c.php", {st_mode=S_IFREG|0664, st_size=10, ...}) = 0
Better than APC, still not great, but there are no open() calls here, so the NFS
getattr issue should be gone.
And finally, ZO+ has a TTL version of apc.stat so we can set it to only stat every 60
seconds, so even with open_basedir enabled, with this config:
zend_optimizerplus.revalidate_freq=60
zend_optimizerplus.enable_file_override=1
we get this:
stat("/var/www/a.php", {st_mode=S_IFREG|0664, st_size=49, ...}) = 0
which is just the initial stat done by Apache which we can't do anything about. So, in
general, assuming the 5.5 integrated opcode cache is based on ZO+ and we add the one
missing Apache stat optimization, I really don't think the situation is all that dire,
even for NFS-mounted scripts.
Previous Comments:
------------------------------------------------------------------------
[2013-02-23 00:49:33] Terry at ellisons dot org dot uk
Incidentally I've just straced opening a require_once on
/home/terry/work/ext/lpc/tests/dd2.inc
and PHP 5.3.17 does the 7 x lstat walk that you indicate is necessary to meet CVE-2006-5178, but it
then repeats this another 5 times and does another 3 fstats after the file is opened 45 f/lstats all
in 2.5μS (on a local disk clearly).
PHP 5.4.6 is worse, it does 5x7 walks, opens the file then does 4 fstatsand reads the file, before
doing another 3x7 lstat walks and a last stat for luck, giving a grand total of 60 f/lstats in
4μS though this one includes mmapping the contents from the file cache.
OK, I can understand -- though question -- mandating one walk but why do it seven times ????
------------------------------------------------------------------------
[2013-02-23 00:08:51] Terry at ellisons dot org dot uk
For the purposes of this bug, let's document the advisory which triggered this change:
CVE-2006-5178 see http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-5178.
Let's be honest, even with this patch there is still a minuscule chance that the exploit
described could succeed, because we can't go into kernel mode during the recursive descent and
prevent other processes with a higher nice being scheduled in and doing the dirty. So it replaces
"very small" by "even smaller" at the same time killing NFS performance.
As I said don't remove this change, just make it elective by adding an extra variant of the
open_basedir parameter, and openly documenting the issues so that sysadmins can make their own
judgement call on security vs performance. Let's face it on a typical shared service I can
think of easier ways to do this exploit. E.g. if exec and the proc functions aren't disabled
then there is no point in having a strong open_basedir. If you are an enterprise and just want to
lock down your apps programmers from breaching good practice / infrastructure standards over a NAS
infrastructure then the weak form is perfectly appropriate.
Also IMO, PHP is trying to do a better job here than the standard Linux realpath function and
failing.
------------------------------------------------------------------------
[2013-02-22 23:26:14] rasmus@php.net
There is no such thing as a "miniscule race" when it comes to computers. Either
there is a race condition or there isn't. In this case there is. So if we remove
this check open_basedir will be much less secure. Something along the lines of
Marcin's idea of separate caches might be feasible, but this is not a small
change.
------------------------------------------------------------------------
[2013-02-22 23:17:59] Terry at ellisons dot org dot uk
This bug is extant in 5.3 and 5.4 (up to 5.4.9). The flaw is the logic in
main/main.c:php_module_startup():
/* Disable realpath cache if safe_mode or open_basedir are set */
if (PG(safe_mode) || (PG(open_basedir) && *PG(open_basedir))) {
CWDG(realpath_cache_size_limit) = 0;
}
Similar logic validly exists in code such as suEXEC and suPHP to prevent a race condition allowing
and unscrupulous script author to execute a script in another UID in certain circumstances. This
simply doesn't apply in this case. Which technically a similar race could allow a script
author to break the basedir contain with miniscule probability by exploiting such a race, the
performance impacts are significant, for no material increase in security strength. Get rid of this
rule or at least split open_basedir into two separate directives open_basedir_paranoid which
disables the cache as above and open_basedir which does the rational checks and doesn't disable
the cache.
This has been lying around for nearly 3 years. How can we progress this?
------------------------------------------------------------------------
[2012-07-31 20:00:48] marcin at 6irc dot net
How about having concurrent cache tables for each basedir setting? For instance, when open_basedir
is set to '/home/teh1234;/tmp', then the lstat populates only cache table "0",
realpath_cache_* also uses exclusively this cache, and when open_basedir is set to
'/home/klaczy;/tmp' then it populates and uses only cache table "1"? Are there
any security considerations that I don't notice here?
------------------------------------------------------------------------
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