Integer overflow in OpenBSD kernel

From: blexim (blexim_at_hush.com)
Date: 09/10/03

  • Next message: blexim: "Re: Integer overflow in OpenBSD kernel"
    Date: Wed, 10 Sep 2003 06:56:08 -0700
    To: bugtraq@securityfocus.com
    
    

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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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