CITS3007 lab 7 (week 8) – Injection – solutions

0. Introduction

The aim of this lab is to expose you to how C programs interact with the process environment (a set of environment variables). We’ll see how we can invoke other programs with a specific environment, and how the variables defined in the environment can alter the behaviour of our programs (sometimes in unexpected ways).

1. Environment variables

Every process has access to a set of environment variables. In C, they are represented as the variable char **environ (see man 7 environ for additional details): this variable allows us to read, write, and and delete environment variables.

Let’s see how this pointer-to-pointer-to-char can be used to display the current values of environment variables. Save the following program as print_env.c, and compile it with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" print_env.o print_env.

#include <stdio.h>
#include <stdlib.h>

extern char **environ;

void printenv() {
  for(size_t i=0; environ[i] != NULL; i++) {
    printf("%s\n", environ[i]);
  }
}

int main(void) {
  printenv();
}

The environ variables represents a “list” of char * C strings, and as the man page for the environ variable explains, the end of the list is indicated by a char * which is set to NULL.

$ python3
Python 3.8.10 (default, Mar 15 2022, 12:22:08)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> path = os.environ["PATH"]
>>> print(path)
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

$ myvar=myval

$ export somevar

$ export -n somevar

$ myvar=myval
$ echo my var is $myvar
$ sh -c 'echo my var is $myvar'

$ myvar=myval
$ echo my var is $myvar
$ export myvar
$ sh -c 'echo my var is $myvar'

$ export myvar=myval

$ declare -p myvar
declare -x myvar="myval"

1.3. Environment variables and fork

Save the following program as child_env.c, and compile it with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" child_env.o child_env.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>

extern char **environ;

void printenv() {
  for(size_t i=0; environ[i] != NULL; i++) {
    printf("%s\n", environ[i]);
  }
}

int main() {
  pid_t childPid;

  switch(childPid = fork()) {
    case 0:  // child process
      //printenv();
      exit(0);
    default:   // parent process
      printenv();
      exit(0);
  }
}

Once fork has been called, the code detects whether it is running in the child process (the result of fork() was 0) or the parent (the result of fork() id the process ID of the child process – see man 2 fork). Currently, the parent’s environment is printed, using the printenv function. If you run ./child_env, you’ll see a large amount of output – so we’ll redirect it to a file:

$ ./child_env > parent_env.txt

Now comment out the parent call to printenv(), and uncomment the child’s, re-compile, and then run again:

$ ./child_env > child_env.txt

Question

Do the two files, parent_env.txt and child_env.txt, differ in any way? How can we find out?

$ diff parent_env.txt child_env.txt

1.2. Environment variables and execve

Linux provides the execve() system call for invoking other programs (see man execve), plus a number of “convenience” functions which act as “wrapper” functions around the system call (see man execl for a list of them). They all operate by executing a specified executable in the current process, such that it replaces the currently running program (unlike fork, which spawns a new child process).

We’ve seen that the fork() system call results in child programs having a copy of their parent process’s environment. The execve() call, on the other hand, gives us precise control over what environment is available to the newly-executed program: we can pass no environment at all, a copy of the existing environment, or a totally “synthetic” environment we’ve created. We’ll write programs to try out each of those approaches.

First, we’ll write a program which uses execve() to invoke the printenv program. (You can read about the printenv command by running man printenv: by default, it simply prints out the contents of all environment variables, much as our print_env.c program above does – though it has extra functionality as well.)

Try running printenv from the command line, so you can verify what its output normally looks like.

Then save the following program as use_execve.c, and compile with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" use_execve.o use_execve:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

extern char **environ;

int main(void) {
  char *myargv[] = {
    "/usr/bin/printenv",
    NULL
  };

  execve("/usr/bin/printenv", myargv, NULL);

  return 0;
}

Read man 2 execve for details of the execve function (which we also looked at in lectures). The first argument is a program to run: when execve is called, this program “replaces” the one currently running. The second argument is a list of the arguments passed to the new program. It has the same purpose and structure as argv does in main of a C program, and is an array of strings, terminated by a NULL pointer; the first of these normally holds the name of the program being executed (though this is only a convention, and programs sometimes set argv[0] to other things). The last argument is to an array of strings representing the environment of the new program.

Question

We have set the last argument to NULL – what do you predict the output of running the program will be? Run the program and see if it matches your expectations.

Now, replace the value of NULL which we passed to to execve with environ instead:

execve("/usr/bin/env", argv, environ);

Question

What do you predict will be the output? Run the program and check – does it match your expectations?

Question

How would you amend the program so as to pass exactly one, specified environment variable – say, the variable FOO, set to value BAR? (Ask your lab facilitator for some hints if you are stuck.)

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

extern char **environ;

int main(void) {
  char *myargv[] = {
    "/usr/bin/printenv",
    NULL
  };

  char *myenv[] = {
    "FOO=BAR",
    NULL
  };

  execve("/usr/bin/printenv", myargv, myenv);

  return 0;
}

Question

If are invoking execve in a program where security is important, which of the previous approaches is the most appropriate? Which is the least appropriate? Why?

1.4. Environment variables and system

Save the following program as use_system.c, and compile with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" use_system.o use_system.

#include <stdio.h>
#include <stdlib.h>

int main() {
    system("/usr/bin/printenv");
    printf("back in use_system");

    return 0;
}

You can read about the system function using man 3 system.

Question

What do you predict will be the output? Should you see the output of printf?

1.5. setuid programs and system

Save the following program as run_cat.c, and compile with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" run_cat.o run_cat.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) {
  const size_t BUF_SIZE = 1024;
  char buf[BUF_SIZE];
  buf[0] = '\0';

  if (argc != 2) {
    printf("supply a file to read\n");
    exit(1);
  }

  strcat(buf, "cat ");
  strcat(buf, argv[1]);

  system(buf);
  return 0;
}

If we invoke this as ./run_cat SOMEFILE, we should be able to get the contents of the file using the cat command (see man 1 cat). Try running ./run_cat /etc/shadow – you should get a “permission denied” error, which we expect, because root owns /etc/shadow, and normal users do not have read access.

Now make run_cat a setuid program:

$ sudo chown root:root ./run_cat
$ sudo chmod u+s ./run_cat

Question

Try running ./run_cat /etc/shadow again – what do you see, and why? Instead of cat, you can imagine that we might instead invoke some other command which normally only root can run, but which we want to let other users run.

You can find out where the cat command is that run_cat is executing by running

$ which cat

This will look through the directories in our PATH, and report the first one which contains an executable file called “cat”. Now we will manipulate the PATH environment variable so that run_cat instead of accessing the normal system cat command, executes a command of our choosing.

Create a file cat.c in the current directory, and edit with vim, adding the following contents, then compile with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" cat.o cat.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>


int main(void) {
  uid_t euid = geteuid();

  printf("DOING SOMETHING MALICIOUS, with effective user ID %d\n", euid);
}

Type

$ export PATH=$PWD:$PATH

and run run_cat again.

$ ./run_cat /etc/shadow

Question

What do you see? Why? And how would you fix this?

2. Building libraries

Save the following code as mylib.c.

#include <stdio.h>

void useful_func(int s) {
  printf("Some very useful functionality\n");
}

We can build an object file mylib.o as follows:¹

$ gcc -g -fPIC -c mylib.c

Now that our useful_func function has been compiled into object code, it can be used in other programs. There are a few options for doing so.

We could link the mylib.o object file directly into a new program – this is what we do when we build large, multi-file C programs. When given a set of object files, gcc will know it’s being asked to invoke the linker, and will combine multiple object files together (together with the builtin C standard library). When doing so, we invoke gcc like this:

$ gcc -o myprog obj1.o obj2.o ...

We could also build a library containing our new function, and make this available to other developers. There are two options for doing so: we can build a static library, or a shared (dynamic) library.

2.1. Static libraries

On Linux, a static library is a set of object files combined into a single “ar”-format “archive” file. You can think of it as being like a .zip or .tar file containing one or more “.o” files. The ar command builds archive files in this format. (You can look up man ar for more details, but they are not essential for our purposes.)

The following command will build a static library containing our object file, located in the directory static-libs:

$ mkdir -p static-libs
$ ar rcs ./static-libs/libmylib.a mylib.o

This produces the static library file libmylib.a. To use the static library in a program, we need to tell the linker to link against our library, and also where our library is located. gcc normally looks for libraries in default locations – in the standard CITS3007 development environment, one of these locations is the directory /usr/lib/x86_64-linux-gnu/. If you list the contents of that directory, you find a number of static libraries – one for instance is /usr/lib/x86_64-linux-gnu/libcrypt.a, part of the libxcrypt library.

To make our useful_func function easy to use by other developers, we would normally also provide them with appropriate header files, but in this case we will manually insert the declarations for useful_func.

Insert the following into a file usemylib.c:

#include <stdio.h>
#include <stdlib.h>

void useful_func(int s);

int main(int argc, char ** argv) {
  printf("\ncalling useful_func routine:\n");
  useful_func(1);
}

We can compile it with gcc -c -g usemylib.c, and then link it against our static library. The -L option to gcc indicates a non standard directory where libraries can be found, and the -l option gives the name of a library to link. (gcc by default assumes it should add lib in front of this name and .a after, to get the filename to link against.)

$ gcc  usemylib.o  -L./static-libs -lmylib -o statically-linked-usemylib

Run the binary with ./statically-linked-usemylib. This executable contains a full copy of the useful_func() binary code from our mylib.o file.²

2.2. Dynamic shared libraries

We can create a shared library with the following commands:

$ mkdir -p shared-libs
$ gcc -shared mylib.o -o shared-libs/libmylib.so

To link against this shared library, we invoke gcc as follows:

$ gcc usemylib.o -L./shared-libs -lmylib -o dynamically-linked-usemylib

Try running ./dynamically-linked-usemylib. You should see an error message like the following:

error while loading shared libraries: libmylib.so: cannot open shared object file: No such file or directory

When an executable that makes use of shared libraries is run, a program called the dynamic linker is responsible for finding the necessary libraries³ and looking up the location of any requested functions in those libraries.⁴ In the present case, it doesn’t know where to find the file libmylib.so, so it reports an error.⁵

We could fix this by putting the .so file in a standard location (/usr/lib/x86_64-linux-gnu/) where the dynamic linker can find it, or we can use the LD_LIBRARY_PATH environment variable to specify the location.⁶ The LD_LIBRARY_PATH environment variable contains a list of locations where the dynamic linker should look for shared libraries. Run the following:

$ LD_LIBRARY_PATH=./shared-libs ./dynamically-linked-usemylib

You should see that the executable runs without error, and calls the useful_func function in our shared library.

Now let’s imagine some adversary has created a version of the mylib library which contains malicious code.

Create the following file, evil_lib.c, and compile it with gcc -g -fPIC -c evil_lib.c.

#include <stdio.h>

void useful_func(int s) {
    // we could now run arbitrary code and cause damage.
    printf("Malicious things -- bwahaha!\n");
}

Build a library from it using the following commands:

$ mkdir -p evil-shared-libs
$ gcc -shared evil_lib.o -o evil-shared-libs/libmylib.so

And run our existing dynamically linked binary, but with a different LD_LIBRARY_PATH:

$ LD_LIBRARY_PATH=./evil-shared-libs ./dynamically-linked-usemylib

What happens, and what are the security implications of this?

In principle, we could use this technique even to override functions in libc, the standard C library.⁷ But note that in the normal case, code will only be run with a user’s normal privileges. This is still a security issue (malicious libraries could, for instance, email copies of the user’s private files), but doesn’t give superuser access to a machine. However, what happens if the binary is a setuid executable?

2.2. LD_LIBRARY_PATH and setuid

Try making dynamically-linked-usemylib a root-owned setuid program, and then running it with a specified LD_LIBRARY_PATH:

$ sudo chown root:root ./dynamically-linked-usemylib
$ sudo chmod u+s ./dynamically-linked-usemylib
$ LD_LIBRARY_PATH=./shared-libs ./dynamically-linked-usemylib

What do you observe?

Let’s find out why this occurs. Create the following program, print_ld_env.c, and compile it with make CFLAGS="-std=c11 -pedantic-errors -Wall -Wextra -Wconversion" print_ld_env.o print_ld_env:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

extern char **environ;

int main(void) {
  printf("some environment variables:\n");
  for (char **var = environ; *var != NULL; var++) {
    if (strncmp(*var, "LD", 2) == 0) {
      printf("var %s\n", *var);
    }
  }
}

Run it with several environment variables set:

$ LD_LIBRARY_PATH=./shared-libs LD_SOME_RANDOM_VAR=xxx ./print_ld_env

What do you see? What is the program doing?

Make the print_ld_env a setuid executable, and run it again:

$ sudo chown root:root ./print_ld_env
$ sudo chmod u+s ./print_ld_env
$ LD_LIBRARY_PATH=./shared-libs LD_SOME_RANDOM_VAR=xxx ./print_ld_env

What do you observe? Why might this happen? (Hint: check the man 8 ld.so man page, and look under “secure-execution mode”.)