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Date: Wed, 15 Sep 1999 23:25:03 -0600 (MDT)
From: FreeBSD Security Officer <security-officer@FreeBSD.ORG.>
To: undisclosed-recipients: ;
Subject: FreeBSD Security Advisory: FreeBSD-SA-99:04.core

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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:          [email protected]
Security notifications:         [email protected]
Security public discussion:     [email protected]
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.


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