Symbolic Execution Fuzzing With KLEE

What is Symbolic Execution?

symbolic execution is a means of analyzing a program to determine what inputs cause each part of a program to execute.
An interpreter follows the program, assuming symbolic values for inputs rather than obtaining actual inputs as normal execution of the program would.

KLEE Symbolic Execution Engine

KLEE is a dynamic symbolic execution engine built on top of the LLVM compiler infrastructure

How to install KLEE with Docker

$ git clone https://github.com/klee/klee.git
$ cd klee
$ docker build -t klee/klee .
$ docker run --rm -ti --ulimit='stack=-1:-1' klee/klee

Let's me start fuzzing OSS(Open Source Software)

1- Download source code

$ git clone https://github.com/kokke/tiny-bignum-c

2- Add main to code bn.c

From 174727a8d41cf1276849e5d4524f8cd4c4955afd Mon Sep 17 00:00:00 2001
From: raminfp <[e-mail]>
Date: Sun, 18 Jul 2021 03:04:40 +0000
Subject: [PATCH] Change code for fuzzing KLEE

---
 bn.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/bn.c b/bn.c
index 6526ea7..f71474d 100644
--- a/bn.c
+++ b/bn.c
@@ -663,3 +663,45 @@ static void _rshift_one_bit(struct bn* a)
 }


+void factorial(struct bn* n, struct bn* res)
+{
+  struct bn tmp;
+
+  /* Copy n -> tmp */
+  bignum_assign(&tmp, n);
+
+  /* Decrement n by one */
+  bignum_dec(n);
+  
+  /* Begin summing products: */
+  while (!bignum_is_zero(n))
+  {
+    /* res = tmp * n */
+    bignum_mul(&tmp, n, res);
+
+    /* n -= 1 */
+    bignum_dec(n);
+    
+    /* tmp = res */
+    bignum_assign(&tmp, res);
+  }
+
+  /* res = tmp */
+  bignum_assign(res, &tmp);
+}
+
+int main()
+{
+  struct bn num;
+  struct bn result;
+  char buf[8192];
+
+  klee_make_symbolic(buf, sizeof(buf), "buf");
+  klee_assume(buf[sizeof(buf) - 1] == 0);
+  bignum_from_int(&num, 100);
+  factorial(&num, &result);
+  bignum_to_string(&result, buf, sizeof(buf));
+
+  return 0;
+}
+
-- 
2.17.1

3- Compile with Clang

$ clang -emit-llvm -O0 -c -g bn.c
$ llvm-dis ./bn.bc
$ vim bn.ll
$ klee bn.bc

4- Crashed.

5- Details

klee@500746a23210:~/tiny-bignum-c$ cat klee-out-0/test000001.ptr.err
Error: memory error: out of bound pointer
File: bn.c
Line: 157
assembly.ll line: 521
State: 2
Stack: 
        #000000521 in bignum_to_string (n=93858042189952, str=93858042462208, nbytes=8192) at bn.c:157
        #100002836 in main () at bn.c:703
Info: 
        address: 93858042470400
        next: object at 22510123060032 of size 1536
                MO40[1536] (no allocation info)