By Pierre-Alain, Vincent Glaume

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Once again we are not interested in the details of these treatments. This code shows how Open Wall acts to prevent any execution of code stored in the stack. txt. 1 Overview In this section, we will summarize how PaX defines its way to make the heap and the stack non-executable. The idea behind PaX is to use the paging mechanisms, and more precisely the page table entries (PTE) and the data and instruction translation lookaside buffers (DTLB and ITLB). On IA32 architectures (Intel, AMD. . ), translation lookaside buffers play the role of a cache for the recently accessed memory pages.

We will begin with the case study of the gcc compiler. This case is the easiest because the vptr is put after the member variables; therefore if there is a buffer among the variables and that we can overflow that buffer (classical method using strcpy or other unsafe functions), then we can overwrite the VPTR and make it points to our own VTable. Usually we will provide our Vtable via the buffer we overflow. cpp): 1 #include 2 class A{ 3 private: 4 char str[11]; 5 6 7 8 9 10 11 12 13 14 public: void setBuffer(char * temp){strcpy (str, temp);} virtual void printBuffer(){cout << str << endl ;} }; void main (void){ A *a; a = new A; a->setBuffer("coucou"); a->printBuffer(); } class A contains a buffer named str [4]; the unsafe strcpy [6] is used to feed the buffer.

The man page gives two possible ways to use libsafe, so we will refer to this. The idea remains the same in both cases: the libsafe functions should be loaded before the libc functions they re-implement, so they will prevail on them. 1 LD PRELOAD The first method is based on the LD PRELOAD environment variable, and is used in the script you will find in the exploits directory. /t1 This program tries to use strcpy() to overflow the buffer. If you get a /bin/sh prompt, then the exploit has worked.

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A Buffer Overflow Study - Attacks and Defenses by Pierre-Alain, Vincent Glaume

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