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    <p>I suggest reading the excellent (if a bit dated) article/tutorial on exploiting buffer overflow vulnerabilities <strong><a href="http://insecure.org/stf/smashstack.html" rel="nofollow noreferrer">Smashing The Stack For Fun And Profit</a></strong>.</p> <p>Here's a brief excerpt:</p> <blockquote> <p>The problem is that we don't know where in the memory space of the program we are trying to exploit the code (and the string that follows it) will be placed. One way around it is to use a JMP, and a CALL instruction. The JMP and CALL instructions can use IP relative addressing, which means we can jump to an offset from the current IP without needing to know the exact address of where in memory we want to jump to. </p> </blockquote> <hr> <p>You can retrieve the current value of the stack pointer with a bit of inline assembly. All the examples in <strong><a href="http://insecure.org/stf/smashstack.html" rel="nofollow noreferrer">Smashing The Stack For Fun And Profit</a></strong> overflow a buffer in <code>main</code>, but you can just as easily use the same techniques to overflow a buffer in a function called from a pthread. The code below is built on an example from the article (<em>overflow1.c</em>) to show that the same techniques will work using pthreads. The actual technique you will use will depend on the target program you are trying to exploit.</p> <pre><code> /* get value of sp off the stack - not essential to example */ unsigned long get_sp() { __asm__("movl %esp,%eax"); /* equiv. of 'return esp;' in C */ } int foo() { char buffer[96]; /* overflow buffer to overwrite return address */ /* and place code to be executed into buffer. */ ... return 0; } void *thread(void *arg) { printf("thread stack 0x%x\n", get_sp()); foo(); return NULL; } int main(int argc, char **argv) { printf("main stack 0x%x\n", get_sp()); pthread_t t; pthread_create(&t, NULL, thread, NULL); pthread_join(t, NULL); return 0; } </code></pre>
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    1. POFor buffer overflows, what is the stack address when using pthreads?
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    1. COI was actually reading that article. I'll have to study it again, but wasn't that quote referring to obtaining the address of "/bin/sh", inside the buffer? Don't we still have to overwrite the eip so that it points to the initial jump instruction?
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    2. COThe example that the excerpt is building up to overflows a character buffer using `strcpy()`. The overflow overwrites the return address (saved IP) on the stack so that it points back to a JMP instruction inside the buffer. The JMP instruction jumps to the CALL instruction which calls the `execve()` of `/bin/sh`, which has all been copied into the buffer as shellcode. The article later suggests padding the front of the overflow buffer with NOP instructions so the return address only need to point somewhere in the NOPs.
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    3. COThanks, jschmier, for all your responses. My TA mentioned that by using pthreads, the program is effectively performing stack randomization as a prevention technique against buffer overflows. The method of JMP and CALL instructions only works once the return address has been changed to point to my code in the buffer. However, since the program is using pthreads, I don't know what memory address to use to overwrite the return address.
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