[2011-03-08 13:58 UTC] felipe@php.net

Description:
------------
"The problem is in the shmop_read php function, in the file
ext/shmop/shmop.c. This functions reads a given number of bytes from
memory, at a given offset starting from a shared memory area.

string shmop_read (int shmid, int start, int count)

Inside the code of the function itself, there are checks in the start
parameter and in the count parameter, to avoid reading arbitrary
memory outside the shared memory object:

    if (start < 0 || start > shmop->size) {
        php_error_docref(NULL TSRMLS_CC, E_WARNING, "start is out of range");
        RETURN_FALSE;
    }

    if (start + count > shmop->size || count < 0) {
        php_error_docref(NULL TSRMLS_CC, E_WARNING, "count is out of range");
        RETURN_FALSE;
    }

The first block check if start is lower than 0 or bigger than the size
of the shared memory area.

The second block checks that the SUM (ADDITION) of "start" and "count"
is not greater than the shared memory area, and later checks if count
its not lower than 0.

The problem is that both variables are signed longs, in 32 bits
architectures this means 2^31 maximum value, after this value, the
variable becomes negative.

So, if we put exactly 2^31 as a value in count, and 1 as a value in
start, the first condition: start + count would become negative
(2^31+1) and will pass the check, and the second condition (count > 0)
will also pass the check, because count its positive (2^31) and do not
get negative until the addition of 1."

CREDITS: Jose Carlos Norte <jose at eyeos dot org>

Test script:
---------------
<?php

$shm_key = ftok(__FILE__, 't');
$shm_id = shmop_open($shm_key, "c", 0644, 100);
$shm_data = shmop_read($shm_id, 1, 2147483647);
//if there is no segmentation fault past this point, we have 2gb of memory!
echo $shm_data;

Expected result:
----------------
No SIGSEGV

Actual result:
--------------
SIGSEGV