So... are we done? You guessed it, no.

Remember when I told that the grading system had other sanity checks? Well, one of them is to check that you only use functions allowed in the subject of the exercise. And execve is never one of them, so it fails the submission before your code even gets to run. From what we experienced, the system is a little more involved than a simple grep of the source code. My guess is that it uses clang to pre-process and parse the code and then does its checks against the resulting AST.

This is where we get to the core of the problem. We need to call execve but we are not allowed to name the symbol. So how can we do it?

Zooming back a bit, we know that our C code essentially gets compiled to a binary and calling a function boils down to a jump to some place in memory. Could we not retrieve the address of execve in memory, hardcode it in our program and call it directly?

In modern laptop software, memory addresses are often execution-dependent. This happens because modern kernels implement Address Space Layout Randomization (ASLR) which randomizes addresses at which your binary, its libraries, heap and stack are placed. Nowadays, C code compiles using the -pie flag by default, this creates a position-independent executable and means that your binary can indeed take advantage of ASLR. We could use the -no-pie C compiler flag to disable this behavior and force fixed addresses for functions in our binary. This doesn't help because we most often do not control the build process in our case. Second, the execve function is not directly part of our executable but dynamically loaded, so it still is at a random place.

Static addresses won't work. But ASLR only acts on the mapping of the segments, their content is left intact. Inside a segment, functions are still ordered in the same way. If we manage to know the static offset between the memory addresses of two functions fA and fB, obtaining the address of fA allows us to construct the address of fB and vice-versa.

The offset varies from a machine to another because of a different version of the C library, or maybe a different architecture, or maybe for some other reason. There are multiple ways to obtain the fixed offset between two functions on the target machine. One of them is to just leak the offset in an exercise where you can name symbols of both fA and fB.

Our target function is still execve which is part of libc. We assume we always have access to printf. We just have to leak the offset between the two functions on the VM. Let's test this locally first:

printf("%td", (ptrdiff_t)((size_t)execve - (size_t)printf));

Running the program a couple of times gives us different results. Didn't we just say that the offset was fixed? Let's open the binary to investigate.

$ objdump --dynamic-syms ./offset | grep -E "\s(execve|printf)"
0000000000000000      DF *UND*    0000000000000000 (GLIBC_2.2.5) execve
000000000004dd64  w   DF .text    0000000000000005  Base        printf

objdump is useful for any kind of object file analysis. The --dynamic-syms option gives information about each symbol that will be loaded at runtime. The sixth column gives us a bit of information about where the symbol comes from. execve indeed comes from glibc, which is the implementation of the C standard library available on my laptop. But printf doesn't seem to come from glibc. Actually, the raw output of objdump has a bunch of __interceptor_* symbols which gives a clue on what's going on here.

Since the beginning, I have been compiling all my code with ASan (a.k.a -fsanitize=address) to catch memory errors early. ASan works by replacing a bunch of code, including memory reads and write, by a bunch of slow function calls that check the correctness of each action. It also replaces a bunch of functions in the C standard library like malloc or printf. By doing so, these functions do not live in the glibc object anymore as opposed to functions that aren't replaced, in our case execve. The automatic grader system also happens to check for memory errors and compiles with ASan.

We could try to use a different symbol than printf that lives in the same segment as execve, but we still need a symbol that is allowed most of the time. Most of these symbols live in the ASan replaced segment, so let's commit to it. Maybe execve is not the only way. Looking for other useful symbols in this segment, mmap catches my eye.

Why is mmap special? Because of the W^X (write-xor-execute) security policy, there is no section of memory that is writable and executable at the same time. Thus we cannot just write raw x86 instructions in a buffer and jump to it as if it was a function nor can we overwrite an existing function with our own code. But mmap solves this problem because we can just tell it to give us a page that has write and execute permissions.

But let's not get ahead of ourselves. We can leak the offset between printf and mmap on the VM via another exercise where we can name both symbols. To use the offset we could use a bunch of casts but the code we hand to the grading system has to compile with -pedantic and this disallows cast from, to and between function pointers types. Because function pointers are still pointers in the end, we can try to modify it without the compiler noticing. I started to modify memory based on unoptimized stack slots2 but I was reminded that in C, unsafety is a feature, so we can actually just use unions.

union notmmap_build {
  int (*basefn)(const char *);
  size_t addr;
  void *(*fn)(void *addr, size_t length, int prot, int flags, int fd,
              long offset);
};

union notmmap_build notmmap = {.basefn = printf};
notmmap.addr += OFFSET; // whatever offset you leaked earlier

We can just make a bad x86 assembly routine that lets us make Linux syscalls as if it was a C function. This routine allows us to call any syscall with up to 3 arguments.

000000000040106a <notsyscall>:
  40106a:    48 89 f8                 mov    %rdi,%rax
  40106d:    48 89 f7                 mov    %rsi,%rdi
  401070:    48 89 d6                 mov    %rdx,%rsi
  401073:    48 89 ca                 mov    %rcx,%rdx
  401076:    0f 05                    syscall
  401078:    c3                       ret

Copy those bytes into a newly mapped writable and executable page.

unsigned char instructions[] = {
  0x48, 0x89, 0xf8, 0x48, 0x89, 0xf7, 0x48, 0x89,
  0xd6, 0x48, 0x89, 0xca, 0x0f, 0x05, 0xc3,
};

char *code =
  notmmap.fn(NULL, 4096, PROT_READ | PROT_WRITE | PROT_EXEC,
             MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

And call our routine as if nothing happened. You can check chromium syscall docs for the arguments Linux expects.

char *argv[] = {
    "/bin/sh",
    "-c",
    "echo 'Hello, World!'",
    NULL,
};

union notsyscall_build {
  char *ptr;
  // execve is syscall 59 on x86-64 Linux
  int (*notexecve)(int sys, char *path, char **argv, char **envp);
};

union notsyscall_build notsyscall = {.ptr = code};

notsyscall.notexecve(59, argv[0], argv, NULL);

Running it on the target VM successfully prints Hello, World!.

You can find the finished proof-of-concept on GitHub.

Conclusion

Because we know our code is compiled with ASan, we chose to leak the offset between printf, a function that is always allowed, and mmap, a function that gives us arbitrary assembly code execution. We can thus bypass the school's checks and call any syscall, in our case execve to get shell access.

We did not investigate further and simply reported this possible issue to the school.

I see no trivial way of patching this. As we've seen ASLR is not going to help here. One possible idea would be to relink the C standard library with a different order for each execution, randomizing the functions order each time (OpenBSD does that at boot time here). But, you have to keep in mind that ASan still replaces functions with its own (so it doesn't mitigate anything here).

The exploit is not that complicated but it was fun to understand the whole process by ourselves. It's in these kind of moment that all the rabbit holes you followed finally click together and help you move forward. Also the simplicity of the exploit resides in the fact that we already have quite liberal remote code execution (as a feature). Still, it involves ASLR bypass and requires a fair amount of understanding of what is going on.

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1

You can watch a talk about it (in French) from the research lab of the school.

2

Enjoy this snippet.