=============================================================================

FreeBSD-SA-99:04                                            Security Advisory

                                                                FreeBSD, Inc.



Topic:          Coredumps and symbolic links



Category:       core

Module:         kernel

Announced:      1999-09-15

Affects:        FreeBSD 3.2 (and earlier)

                FreeBSD-current before the correction date.

                FreeBSD 3.2-stable before the correction date.

                FreeBSD 2.2.8-stable before the correction date.

Corrected:      FreeBSD-3.3 RELEASE

                FreeBSD-current as of 1999/08/26

                FreeBSD-3.2-stable as of 1999/08/26

                FreeBSD-2.2.8-stable as of 1999/08/29

                The FreeBSD-3.3-RC series of releases are not affected.

FreeBSD only:   NO



Patches:        ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-99:04/



I.   Background



As a diagnostic aid to help programmers find bugs in their programs,

the system creates core files when an illegal instruction or other

fatal error happens.  A flaw in the kernel allowed it to follow

symbolic links when creating core files.



II.  Problem Description



The fts library functions had a flaw in them where which would lead to

a core dump when periodic ran the security checking scripts (or other

scripts which traverse trees that can be controlled by users).

periodic(3) should limit core size to zero to disable core dumps while

it is executing commands, but does not do so.  In addition, the kernel

should not follow symbolic links.



All three of these problems caused a situation where it was possible

for an attacker could create or overwrite an arbitrary file on the

system with a moderate degree of controll of its contents to cause a

problem.



III. Impact



Local users could gain root access.



IV.  Workaround



One can workaround this problem by preventing core dumps for periodic.

This solution is less than completely satisfying, since it only plugs

the known exploit hole.  None the less, this may provide a short term

stopgap solution until a new kernel and/or userland can be installed.



    # mv /usr/sbin/periodic /usr/sbin/periodic.bin

    # cat > /usr/sbin/periodic

    #!/bin/sh

    ulimit -c 0

    /usr/sbin/periodic.bin $*

    ^D

    # chmod 555 /usr/sbin/periodic



Another alternative would be to update the fts routines to a version

newer than 1999/09/02 (for -current or 3.3-stable) or 1999/09/04 (for

2.2.8-stable).  However, this requires that you rebuild via "make

world" to take effect.



V.   Solution



Please note: there is a separate advisory describing the fts problem

and solution.  Please see FreeBSD-SA-99:05.fts.asc in the advisories

directory for additional information about the fts patch.



Apply the following patches to your kernel.  They will disallow

following symbolic links when creating core files.  This will stop

this attack, and all similar such attacks.



Here's the patch for freebsd-current:



    *** kern/imgact_elf.c       1999/07/09 19:10:14     1.61

    --- kern/imgact_elf.c       1999/08/26 17:32:48     1.62

    ***************

    *** 722,729 ****

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */



    !   NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);

    --- 722,729 ----

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */



    !   NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE | O_NOFOLLOW, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);

    *** kern/imgact_aout.c      1999/05/17 00:53:36     1.52

    --- kern/imgact_aout.c      1999/08/26 17:32:48     1.53

    ***************

    *** 264,271 ****

            name = expand_name(p->p_comm, p->p_ucred->cr_uid, p->p_pid);

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */

    !   NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);

    --- 264,271 ----

            name = expand_name(p->p_comm, p->p_ucred->cr_uid, p->p_pid);

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */

    !   NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE | O_NOFOLLOW, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);



Here's the patch for freebsd-3.2-stable:



    *** kern/imgact_elf.c       1999/07/15 13:01:54     1.44.2.4

    --- kern/imgact_elf.c       1999/08/26 17:35:03     1.44.2.5

    ***************

    *** 699,706 ****

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */



    !   NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);

    --- 699,706 ----

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */



    !   NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE | O_NOFOLLOW, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);

    *** kern/imgact_aout.c      1999/04/14 04:55:22     1.44.2.1

    --- kern/imgact_aout.c      1999/08/26 17:35:02     1.44.2.2

    ***************

    *** 259,266 ****

            name = expand_name(p->p_comm, p->p_ucred->cr_uid, p->p_pid);

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */

    !   NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);

    --- 259,266 ----

            name = expand_name(p->p_comm, p->p_ucred->cr_uid, p->p_pid);

            if (name == NULL)

                    return (EFAULT);    /* XXX -- not the best error */

    !   NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, name, p);

    !   error = vn_open(&nd, O_CREAT | FWRITE | O_NOFOLLOW, S_IRUSR | S_IWUSR);

            free(name, M_TEMP);

            if (error)

                    return (error);



Here's the patch for FreeBSD-2.2.8-stable



    *** sys/LINK/fcntl.h        Wed Dec 18 05:08:08 1996

    --- sys/fcntl.h     Fri Aug 27 14:39:26 1999

    ***************

    *** 84,89 ****

    --- 84,90 ----

      #define   O_EXLOCK        0x0020          /* open with exclusive file

lock */

      #define   O_ASYNC         0x0040          /* signal pgrp when data ready

*/

      #define   O_FSYNC         0x0080          /* synchronous writes */

    + #define   O_NOFOLLOW      0x0100          /* don't follow symlinks */

      #endif

      #define   O_CREAT         0x0200          /* create if nonexistent */

      #define   O_TRUNC         0x0400          /* truncate to zero length */

    *** kern/LINK/kern_sig.c    Sat Dec 21 10:57:24 1996

    --- kern/kern_sig.c Fri Aug 27 14:38:25 1999

    ***************

    *** 1241,1249 ****

                p->p_rlimit[RLIMIT_CORE].rlim_cur)

                    return (EFAULT);

            sprintf(name, "%s.core", p->p_comm);

    !   NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, name, p);

            if ((error = vn_open(&nd,

    !       O_CREAT | FWRITE, S_IRUSR | S_IWUSR)))

                    return (error);

            vp = nd.ni_vp;



    --- 1241,1249 ----

                p->p_rlimit[RLIMIT_CORE].rlim_cur)

                    return (EFAULT);

            sprintf(name, "%s.core", p->p_comm);

    !   NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, name, p);

            if ((error = vn_open(&nd,

    !       O_CREAT | FWRITE | O_NOFOLLOW, S_IRUSR | S_IWUSR)))

                    return (error);

            vp = nd.ni_vp;



    *** kern/LINK/vfs_vnops.c   Sat Mar  8 07:16:18 1997

    --- kern/vfs_vnops.c        Fri Aug 27 14:37:01 1999

    ***************

    *** 87,93 ****

            if (fmode & O_CREAT) {

                    ndp->ni_cnd.cn_nameiop = CREATE;

                    ndp->ni_cnd.cn_flags = LOCKPARENT | LOCKLEAF;

    !           if ((fmode & O_EXCL) == 0)

                            ndp->ni_cnd.cn_flags |= FOLLOW;

                    error = namei(ndp);

                    if (error)

    --- 87,93 ----

            if (fmode & O_CREAT) {

                    ndp->ni_cnd.cn_nameiop = CREATE;

                    ndp->ni_cnd.cn_flags = LOCKPARENT | LOCKLEAF;

    !           if ((fmode & O_EXCL) == 0 && (fmode & O_NOFOLLOW) == 0)

                            ndp->ni_cnd.cn_flags |= FOLLOW;

                    error = namei(ndp);

                    if (error)

    ***************

    *** 119,125 ****

                    }

            } else {

                    ndp->ni_cnd.cn_nameiop = LOOKUP;

    !           ndp->ni_cnd.cn_flags = FOLLOW | LOCKLEAF;

                    error = namei(ndp);

                    if (error)

                            return (error);

    --- 119,126 ----

                    }

            } else {

                    ndp->ni_cnd.cn_nameiop = LOOKUP;

    !           ndp->ni_cnd.cn_flags =

    !               ((fmode & O_NOFOLLOW) ? NOFOLLOW : FOLLOW) | LOCKLEAF;

                    error = namei(ndp);

                    if (error)

                            return (error);

    *** kern/LINK/vfs_syscalls.c        Wed Aug  4 12:44:30 1999

    --- kern/vfs_syscalls.c     Sat Aug 28 10:48:51 1999

    ***************

    *** 694,699 ****

    --- 694,701 ----

            flags = FFLAGS(uap->flags);

            if ((flags & FREAD + FWRITE) == 0)

                    return (EINVAL);

    +   if (flags & O_NOFOLLOW)

    +           flags &= ~O_NOFOLLOW;

            error = falloc(p, &nfp, &indx);

            if (error)

                    return (error);





=============================================================================

FreeBSD, Inc.



Web Site:                       http://www.freebsd.org/

Confidential contacts:          security-officer@freebsd.org

Security notifications:         security-notifications@freebsd.org

Security public discussion:     freebsd-security@freebsd.org

PGP Key:                ftp://ftp.freebsd.org/pub/FreeBSD/CERT/public_key.asc



Notice: Any patches in this document may not apply cleanly due to

        modifications caused by digital signature or mailer software.

        Please reference the URL listed at the top of this document

        for original copies of all patches if necessary.

=============================================================================