Unterschiede
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| Beide Seiten der vorigen Revision Vorhergehende Überarbeitung Nächste Überarbeitung | Vorhergehende Überarbeitung | ||
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security:memory-corruption:exploitation:nop-sled [2018/01/09 21:38] nufan |
security:memory-corruption:exploitation:nop-sled [2023/04/19 01:09] (aktuell) nufan |
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| </code> | </code> | ||
| + | It is interesting to see that the opcodes of the x86 instructions have variable lengths(([[https://www.sdsc.edu/~allans/cs141/L2.ISA.pdf|Instruction Set Architecture or "How to talk to computers if you aren't in Star Trek"]])). | ||
| - | But if only the very first byte (value ''0xb8'') of the opcode is deleted, the instructions change their meaning. | + | To show the importance of correct instruction offsets, only the very first byte (value ''0xb8'') of the opcode is deleted. |
| <code> | <code> | ||
| Zeile 50: | Zeile 51: | ||
| </code> | </code> | ||
| - | Note that even for this tiny example the resulting code is significantly different from the original one. | + | Note that even for this tiny example with a single deleted byte the resulting code is significantly different from the original one. |
| What we are trying to do now is to create some kind of memory area in front of our code where we can safely redirect execution to. By definition the bytes in this area must be valid opcodes. As seen before, only one single byte of offset at the instruction address can destroy any meaning of the code. To avoid this, we need to find an instruction that is only a single byte long. Our final requirement for the instruction is to not affect any registers (except for the instruction pointer, which is naturally incremented by one after execution). The x86 instruction set provides an instruction that fulfills all our requirements - the NOP (**N**o **OP**eration) instruction. Having an opcode of ''0x90'', it is usually implemented as an alias instruction to the following code(([[https://www-ssl.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf|Intel® 64 and IA-32 Architectures Software Developer’s Manual]])): | What we are trying to do now is to create some kind of memory area in front of our code where we can safely redirect execution to. By definition the bytes in this area must be valid opcodes. As seen before, only one single byte of offset at the instruction address can destroy any meaning of the code. To avoid this, we need to find an instruction that is only a single byte long. Our final requirement for the instruction is to not affect any registers (except for the instruction pointer, which is naturally incremented by one after execution). The x86 instruction set provides an instruction that fulfills all our requirements - the NOP (**N**o **OP**eration) instruction. Having an opcode of ''0x90'', it is usually implemented as an alias instruction to the following code(([[https://www-ssl.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf|Intel® 64 and IA-32 Architectures Software Developer’s Manual]])): | ||
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| </code> | </code> | ||
| - | Next, we will take a look at a very simple example that makes use of this technique called ''NOP sled''. | + | Next, we will take a look at a simple example and make use of this technique called ''NOP sled''(([[http://phrack.org/issues/49/14.html|.:: Phrack Magazine ::. - Smashing The Stack For Fun And Profit]])). |
| <code c nop/execve.c> | <code c nop/execve.c> | ||
| - | // gcc -g -O0 -m32 -no-pie -fno-pie -mpreferred-stack-boundary=2 execve.c | + | // gcc -g -O0 -m32 -no-pie -fno-pie -mpreferred-stack-boundary=2 -fno-stack-protector -z execstack execve.c |
| #include <stdio.h> | #include <stdio.h> | ||
| #include <string.h> | #include <string.h> | ||
| Zeile 82: | Zeile 83: | ||
| </code> | </code> | ||
| - | Inspecting the code above, you will notice that the only difference to our example from the [[.basic#arbitrary_code_execution|buffer overflow introduction]] is the size of the buffer. Back then, it was of utmost importance to correctly overwrite the return address and exactly know the address to jump to. By adding a sequence of NOPs directly before the shellcode, we can loosen the second constraint. This sequence of NOPs is commonly called a "NOP sled"((Jon Erickson (2008). Hacking: The Art of Exploitation <nowiki>(2nd edition)</nowiki>)). Returning to anywhere in this sequence is equally fine to land exactly at the beginning of the shellcode. In case the NOPs are hit, the processor spends some cycles doing nothing until it reaches the real shellcode. | + | Inspecting the code above, you will notice that the only difference to our example from the [[.basic#arbitrary_code_execution|buffer overflow introduction]] is the size of the buffer. Back then, it was of utmost importance to correctly overwrite the return address and exactly know the address to jump to. By adding a sequence of NOPs directly before the shellcode, we can loosen the second constraint. This sequence of NOPs is commonly called a "NOP sled"((Jon Erickson (2008). Hacking: The Art of Exploitation <nowiki>(2nd edition)</nowiki>)). Returning to anywhere in this sequence is equally fine as to land exactly at the beginning of the shellcode. In case the NOPs are hit, the processor spends some cycles doing nothing until it reaches the real shellcode. |
| - | In this example we have a buffer of size 128 while our shellcode takes up only 28 bytes. Thus we have 100 bytes of space left for the NOP sled. As this amount of characters is cumbersome to type and copy, we will generate the input with [[perl:start|Perl]]. Even if you are not familiar with Perl, make sure you are able to understand and generate inputs in a scripting language (e.g. [[py:start|Python]] or [[bash:start|Bash]] are perfectly fine as well). The NOP sled is followed by the actual shellcode and the approximate address we want to jump to. It is sufficient to land somewhere within the 100 byte range of the NOP sled, we do not need to know the exact address of the shellcode. Assuming a correct alignment with respect to the stack variables, we can also specify the target address multiple times with a higher chance of overwriting the return address. | + | In this example we have a buffer of size 128 while our shellcode takes up only 28 bytes. Thus we have 100 bytes of space left for the NOP sled. As this amount of characters is cumbersome to type and copy, we will generate the input with [[perl:start|Perl]]. The NOP sled is followed by the actual shellcode and the approximate address we want to jump to. It is sufficient to land somewhere within the 100 byte range of the NOP sled, we do not need to know the exact address of the shellcode. Assuming a correct alignment with respect to the stack variables, we can also specify the target address multiple times with a higher chance of overwriting the return address. |
| Our payload now contains the following: | Our payload now contains the following: | ||
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| <tr> | <tr> | ||
| <td align="left" style="width:33%"></html>[[.basic|← Back to buffer overflow basics]]<html></td> | <td align="left" style="width:33%"></html>[[.basic|← Back to buffer overflow basics]]<html></td> | ||
| - | <td align="center" style="width:34%"></html>[[..|Overview]]<html></td> | + | <td align="center" style="width:34%"></html>[[..start|Overview]]<html></td> |
| <td align="right" style="width:33%"></html>[[.external-buffers|Continue with external buffers →]]<html></td> | <td align="right" style="width:33%"></html>[[.external-buffers|Continue with external buffers →]]<html></td> | ||
| </tr> | </tr> | ||
| </table> | </table> | ||
| </html> | </html> | ||