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[2007-07-22 02:50 UTC] tstarling at wikimedia dot org
Description:
------------
This appears to be Win32 specific.
The shell execution functions, exec(), passthru(), etc., deadlock if another thread in the same server is waiting for a file lock.
The most common type of file lock is a lock on a session file, that's where I saw this first. But I could reproduce it with flock() instead of session_start().
The typical way for this to manifest itself is if a browser sends two requests to PHP concurrently, with the same session ID. One request runs first, the other one blocks waiting for a lock on the session file. Then if the running request tries to run exec(), it deadlocks and both threads wait forever.
Tested with both Apache 2.0.54 (mpm_winnt) and 1.3.28. I also had a colleague confirm it on an independent system.
Reproduce code:
---------------
<?php
if ( isset( $_REQUEST['f'] ) ) {
$f = fopen( 'blah', 'w' );
flock( $f, LOCK_EX );
sleep( 1 );
passthru( "echo Hello" );
} else {
$self = $_SERVER['SCRIPT_NAME'];
echo <<<EOT
<html>
<frameset rows="50%, 50%">
<frame src="$self?f=1"/>
<frame src="$self?f=2"/>
</frameset>
</html>
EOT;
}
?>
Expected result:
----------------
The two frames should both display "Hello", after a delay of 2 seconds.
Actual result:
--------------
The frames take forever to load. Requires a force quit of Apache.
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Last updated: Thu Nov 21 19:01:29 2024 UTC |
I don't really understand what you mean, but I'll take a few guesses: * flock alone is working perfectly, it does not fail. If you replace the passthru() with print(), then the whole thing will work as expected. * The problem doesn't appear to be that passthru() is globally synchronised. The second thread never gets to the passthru() call, you could remove it altogether for f=2 and you would still see the deadlock. * This bug provides the possibility for a DoS attack against any script that calls session_start() followed by a shell execution function. max_execution_time is ignored. Are you telling me that the expected behaviour for this simple script: <?php session_start(); sleep(1); passthru('hello');?> is to consistently deadlock on Windows whenever concurrent requests are sent?