Randomized stack base PaX

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fig. 3 stack smashing attack. target of attack keeps same address; payload moves stack.


the randomization of stack base has effect on payload delivery during shellcode , return-to-libc attacks. shellcode attacks modify return pointer field address of payload; while return-to-libc attacks modify stack frame pointer. in either case, probability of success diminished significantly; position of stack unpredictable, , missing payload causes program crash.


in case of shellcode, series of instructions called nop slide or nop sled can prepended payload. add 1 more success case per 16 bytes of nop slide. 16 bytes of nop slide increase success rate 1/16m 2/16m; 128 bytes of nop slide increase 9/16m. increase in success rate directly proportional size of nop slide; doubling length of given nop slide doubles chances of successful attack.


return-to-libc attacks not use code, rather inject fixed width stack frames. because of this, stack frames have repeat aligned 16 bytes. stack frame bigger this, giving repeated stack frame payloads of same length given nop sled less of impact on success rate of attacks.







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