Stack allocation fail when close to heap - c++

Question

Issue Description
C++ server got several crashes at the same stack frame as below
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Investigation
Memory was not used up, 400M-800M
Stack space used is not large, about 270K-350K
Haven't found any other code issue caused crash
The distance between stack and heap is about 1M.
Binary is built with PIE
OS is 2.6.32-696.16.1.el6.i686 #1 SMP Wed Nov 15 16:16:47 UTC 2017 i686 i686 i386 GNU/Linux
Stack space used in one crash
(gdb) f 0
#0 0xb717f616 in WDMS_Byte_Stream::write (this=Cannot access memory at
address 0xbfa16b1c) at wdms_bs.cpp:63 in wdms_bs.cpp
(gdb) set $top=$esp
(gdb) f 38
#38 0xb6a1bcbf in main (argc=1, argv=0xbfa5a8b4) at main.cpp:66
main.cpp: No such file or directory.
in main.cpp
(gdb) p $esp-$top
$1 = 277728
(gdb)
Distance between stack and heap
Memory allocated
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One heap segment close to stack space, 0xb8a8f000->0xbf917000
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Stack space 0xbfa18000->0xbfa5c000
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the distance between them
(gdb) p 0xbfa18000- 0xbf917000
$3 = 1052672
Why crash
Program need to allocate 8268 bytes (0x204c) stack size at last frame
OS only expand stack size to 0xbfa18000, actual size is
(gdb) p $esp+0x204c-0xbfa18000
$19 = (void *) 0xb4c(2892 bytes)
So, when execute next instruction
call 0xb6a19ec9 <__i686.get_pc_thunk.bx>, where need access $esp, which is out of accessible memory space.
Why crash at the same stack frame? It could be because this call sequence has most call frames and to use most stack space.
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Question-1
Is this related CVE-2017-1000364 Fix ?
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Question-2
Why does OS allocate heap segment so close to stack? how does it avoid stack and heap collision and not allocation failure?