Third, the system call execve() is called when we set %al to 11, and execute "int $0x80". There are other ways to set %edx to zero (e.g., xorl %edx, %edx) the one ( cdq) used here is simply a shorter instruction: it copies the sign (bit 31) of the value in the EAX register (which is 0 at this point) into every bit position in the EDX register, basically setting %edx to 0. Line 5 stores name to %ebx Line 8 stores name to %ecx Line 9 sets %edx to zero. Second, before calling the execve() system call, we need to store name (the address of the string), name (the address of the array), and NULL to the %ebx, %ecx, and %edx registers, respectively. Fortunately, "//" is equivalent to "/", so we can get away with a double slash symbol. This is because we need a 32-bit number here, and "/sh" has only 24 bits. First, the third instruction pushes "//sh", rather than "/sh" into the stack. Gcc -z execstack -o call_shellcode call_shellcode.cĪ few places in this shellcode are worth mentioning.
Stack guard stack smashing detected how to#
The following code shows how to assign an long integer to a buffer starting at buffer: Because buffer and long are of different types, you cannot directly assign the integer to buffer instead you can cast the buffer+i into an long pointer, and then assign the integer. Since each buffer space is one byte long, the integer will actually occupy four bytes starting at buffer (i.e., buffer to buffer). In your exploit program, you might need to store an long integer (4 bytes) into an buffer starting at buffer. To improve the chance of success, we can add a number of NOPs to the beginning of the malcious code therefore, if we can jump to any of these NOPs, we can eventually get to the malicious code. Even if you cannot accurately calculate the address (for example, for remote programs), you can still guess. If you can accurately calculate the address of buffer, you should be able to accurately calcuate the starting point of the malicious code. (2) Finding the starting point of the malicious code. Therefore the range of addresses that we need to guess is actually quite small.Stack is usually not very deep: most programs do not push more than a few hundred or a few thousand bytes into the stack at any one time.Stack usually starts at the same address.The following facts make guessing a quite feasible approach: If the target program is running remotely, and you may not be able to rely on the debugger to find out the address. The address of buffer may be slightly different when you run the Set-UID copy, instead of of your copy, but you should be quite close. You can even modify the copied program, and ask the program to directly print out the address of buffer. In the debugger, you can figure out the address of buffer, and thus calculate the starting point of the malicious code. Since the vulnerable program is a Set-UID program, you can make a copy of this program, and run it with your own privilege this way you can debug the program (note that you cannot debug a Set-UID program). (1) Finding the address of the memory that stores the return address.įrom the figure, we know, if we can find out the address of buffer array, we can calculate where the return address is stored. To answer these questions, we need to understand the stack layout the execution enters a function. But we have two problems: (1) we do not know where the return address is stored, and (2) we do not know where the shellcode is stored. One thing we can do is to change the return address to point to the shellcode. We can load the shellcode into “badfile”, but it will not be executed because our instruction pointer will not be pointing to it. return addresses): an overflow in the data part can affect the control flow of the program, because an overflow can change the return address. buffers) and the storage for controls (e.g. This vulnerability arises due to the mixing of the storage for data (e.g. This vulnerability can be utilized by a malicious user to alter the flow control of the program, even execute arbitrary pieces of code. SEEDlabs: Buffer Overflow Vulnerability Lab 0x00 Lab Overviewīuffer overflow is defined as the condition in which a program attempts to write data beyond the boundaries of pre-allocated fixed length buffers.
0 kommentar(er)
