From 52e3ab594b34bf309b8669fcaa855c295a00e114 Mon Sep 17 00:00:00 2001
From: Nicholas Hall <ngh@grandcare.com>
Date: Thu, 14 Sep 2017 16:42:29 -0500
Subject: DC414 meeting presentation files

---
 notes.md            |  13 ++++++++++
 presentation        |  69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/secret.c        |  19 +++++++++++++++
 src/secret.h        |   7 ++++++
 src/strcmp.c        |  27 ++++++++++++++++++++
 what_is_hooking.png | Bin 0 -> 44030 bytes
 6 files changed, 135 insertions(+)
 create mode 100644 notes.md
 create mode 100644 presentation
 create mode 100644 src/secret.c
 create mode 100644 src/secret.h
 create mode 100644 src/strcmp.c
 create mode 100644 what_is_hooking.png

diff --git a/notes.md b/notes.md
new file mode 100644
index 0000000..b7a6116
--- /dev/null
+++ b/notes.md
@@ -0,0 +1,13 @@
+# presentation software
+sent - Simple plaintext presentation tool.
+https://tools.suckless.org/sent/
+
+# glibc version
+ldd --version
+
+# glibc source
+git://sourceware.org/git/glibc.git
+
+# compile as shared object
+gcc -fPIC -c strcmp.c -o strcmp.o
+gcc -shared -o strcmp.so strcmp.o
diff --git a/presentation b/presentation
new file mode 100644
index 0000000..0e1e009
--- /dev/null
+++ b/presentation
@@ -0,0 +1,69 @@
+Hooking on Linux Introduction
+
+What is hooking?
+
+@what_is_hooking.png
+
+Relies on dynamically linked binaries
+\
+most binaries are dynamic.
+$ file /usr/bin/* | grep dynamic | wc -l
+1500
+
+Hooking libc is an obvious choice
+- open source
+- everything uses libc
+
+Picking a libc function to hook
+\
+strcmp:
+compare two strings
+
+Why?  Because:
+\
+if strcmp("user password", saved_password) == 0
+    Access granted
+else
+    Access denied
+
+Goal:
+print out saved password
+
+Step 1:
+Find the source Luke
+
+Step 2:
+Find the implementation
+
+Step 3:
+Code your goals, you can do it!
+
+☢ Caveats ☢
+
+Caveats
+☢ retain the original implementation (or not)
+
+Caveats
+☢ retain the original implementation (or not)
+☢ segfaults
+
+Caveats
+☢ retain the original implementation (or not)
+☢ segfaults
+☢ low level C can be hard
+
+Caveats
+☢ retain the original implementation (or not)
+☢ segfaults
+☢ low level C can be hard
+☢ segfaults
+
+Step 4:
+isolate and compile as shared object
+
+Step 5:
+trick the linker
+\
+LD_PRELOAD is bae
+
+Questions?
diff --git a/src/secret.c b/src/secret.c
new file mode 100644
index 0000000..d4113b2
--- /dev/null
+++ b/src/secret.c
@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include <string.h>
+#include "secret.h"
+
+int main(int argc, char* argv[]){
+    if(argc <2){
+        printf("Usage: %s <secret_password>\n", argv[0]);
+        return 1;
+    }
+
+    UNHIDE_STR(pass);
+
+    if(! strcmp(pass, argv[1]))
+        printf("Access granted\n");
+    else
+        printf("Access denied\n");
+
+    return 0;
+}
diff --git a/src/secret.h b/src/secret.h
new file mode 100644
index 0000000..d6ee2ea
--- /dev/null
+++ b/src/secret.h
@@ -0,0 +1,7 @@
+#define A(c)            (c)-0x19
+#define UNHIDE_STR(str) do { char *p = str;  while (*p) *p++ += 0x19; } while (0)
+#define HIDE_STR(str)   do { char *p = str;  while (*p) *p++ -= 0x19; } while (0)
+
+char pass[] = {
+    A('c'), A('a'), A('t'), A('z'), A('r'), A('u'), A('l'), 0
+};
diff --git a/src/strcmp.c b/src/strcmp.c
new file mode 100644
index 0000000..d626bd3
--- /dev/null
+++ b/src/strcmp.c
@@ -0,0 +1,27 @@
+#include <stdio.h>
+/* Compare S1 and S2, returning less than, equal to or
+   greater than zero if S1 is lexicographically less than,
+   equal to or greater than S2.  */
+int strcmp (const char *p1, const char *p2)
+{
+  const unsigned char *s1 = (const unsigned char *) p1;
+  const unsigned char *s2 = (const unsigned char *) p2;
+  unsigned char c1, c2;
+
+  // print out input values
+  printf("strcmp('%s', '%s')", p1, p2);
+  do
+    {
+      c1 = (unsigned char) *s1++;
+      c2 = (unsigned char) *s2++;
+      if (c1 == '\0') {
+          // print return value
+          printf(" = %d\n", c1 - c2);
+          return c1 - c2;
+      }
+    }
+  while (c1 == c2);
+
+  printf(" = %d\n", c1 - c2);
+  return c1 - c2;
+}
diff --git a/what_is_hooking.png b/what_is_hooking.png
new file mode 100644
index 0000000..04742eb
Binary files /dev/null and b/what_is_hooking.png differ
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