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Local security bug in OpenBSD semaphore handling

Product:         OpenBSD kernel (3.3-release, -current before 10/09/2003)
Impact:          Root may bypass securelevel
Bug class:       Integer overflow
Vendor notified: Yes
Fix available:   Yes

Details:
An integer overflow condition exists in the OpenBSD 3.3-release kernel
and all previous versions.  It is possible for root to write to semi-
arbitrary kernel memory irrespective of securelevel(7). This potentially
bypasses securelevel as root may modify the running kernel, introducing
kernel level backdoors etc. The mechanism used to achieve this is an
integer overflow in the semget(2) syscall, described below:

sys_semget() allocates a buffer here:

src/sys/kern/sysv_sem.c:
sys_semget():
  semaptr_new->sem_base = malloc(nsems * sizeof(struct sem),
      M_SEM, M_WAITOK);


provided the following checks are passed:

src/sys/kern/sysv_sem.c:
sys_semget():
  if (nsems <= 0 || nsems > seminfo.semmsl) {
      DPRINTF(("nsems out of range (0<%d<=%d)\n", nsems,
        seminfo.semmsl));
      return (EINVAL);
  }
  if (nsems > seminfo.semmns - semtot) {
      DPRINTF(("not enough semaphores left (need %d, got %d)\n",
        nsems, seminfo.semmns - semtot));
      return (ENOSPC);
  }

If these checks are passed and the buffer is successfully allocated,

the nsems (number of semaphores) value associated with the semaphore

set is set here:

src/sys/kern/sysv_sem.c:
sys___semctl():
  semaptr_new->sem_nsems = nsems;

Please also note that an int is being assigned to a short here, which

is a potential source of another bug. Since root is able to raise the

values of seminfo.semmns and seminfo.semmsl to arbitrary values via sysctl,
 it is possible to mis-size the malloc'd buffer, allowing memory to be
read and written via the semctl(2) syscall.

Exploit:
This condition may be reproduced using the attached programs, allowing
root to inspect and modify kernel memory.

Workaround:
None, don't trust securelevel(7) to protect your kernel.

Fix:
Upgrade to -current or apply the following patch:

===================================================================
RCS file: /usr/OpenBSD/cvs/src/sys/kern/sysv_sem.c,v
retrieving revision 1.20
retrieving revision 1.21
diff -u -r1.20 -r1.21
- --- src/sys/kern/sysv_sem.c	2003/08/20 18:02:20	1.20
+++ src/sys/kern/sysv_sem.c	2003/09/09 18:57:36	1.21
@@ -1,4 +1,4 @@
- -/*	$OpenBSD: sysv_sem.c,v 1.20 2003/08/20 18:02:20 millert Exp $	*/
+/*	$OpenBSD: sysv_sem.c,v 1.21 2003/09/09 18:57:36 tedu Exp $	*/
 /*	$NetBSD: sysv_sem.c,v 1.26 1996/02/09 19:00:25 christos Exp $	*/

 /*
@@ -884,7 +884,7 @@
 		if ((error = sysctl_int(oldp, oldlenp, newp, newlen, &val)) ||
 		    val == seminfo.semmns)
 			return (error);
- -		if (val < seminfo.semmns)
+		if (val < seminfo.semmns || val > 0xffff)
 			return (EINVAL);	/* can't decrease semmns */
 		seminfo.semmns = val;
 		return (0);
@@ -902,7 +902,7 @@
 		if ((error = sysctl_int(oldp, oldlenp, newp, newlen, &val)) ||
 		    val == seminfo.semmsl)
 			return (error);
- -		if (val < seminfo.semmsl)
+		if (val < seminfo.semmsl || val > 0xffff)
 			return (EINVAL);	/* can't decrease semmsl */
 		seminfo.semmsl = val;
 		return (0);


Discovered by:
blexim@hush.com of isen

Thanks go to the OpenBSD team for an extremely fast response and fix.
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