__________________________________________________________
The U.S. Department of Energy
Computer Incident Advisory Capability
___ __ __ _ ___
/ | /_\ /
\___ __|__ / \ \___
__________________________________________________________
INFORMATION BULLETIN
FreeBSD mmap Vulnerability
June 3, 1998 23:00 GMT Number I-037A
______________________________________________________________________________
PROBLEM: A vulnerability exists in the 4.4BSD VM system which allows
files to be "memory mapped".
PLATFORM: FreeBSD 2.2.*, FreeBSD-stable before 1998/05/24 and
FreeBSD-current before 1998/05/19
DAMAGE: If exploited, unauthorized users may gain root access.
SOLUTION: Apply patches listed below.
______________________________________________________________________________
VULNERABILITY FreeBSD recommends that the patches be applied on affected
ASSESSMENT: systems.
______________________________________________________________________________
[ Updated on June 3, 1998 with additional information from FreeBSD ]
[ Start FreeBSD, Inc. Advisory ]
=============================================================================
D-SA-98:04 Security Advisory
FreeBSD, Inc.
Topic: security compromise via mmap
Category: core
Module: kernel
Announced: 1998-06-02
Affects: FreeBSD 2.2.*, FreeBSD-stable before 1998/05/24
and FreeBSD-current before 1998/05/19 suffer from
this problem.
Corrected: FreeBSD-current as of 1998/05/19
FreeBSD-stable as of 1998/05/24
FreeBSD only: no (also other 4.4BSD based systems may be affected)
Patches: ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-98:04/
=============================================================================
IMPORTANT MESSAGE: The FreeBSD security officer now uses the policy
ftp://freebsd.org/pub/CERT to ftp://ftp.freebsd.org/pub/FreeBSD/POLICY
for sending out advisories.
=============================================================================
I. Background
The 4.4BSD VM system allows files to be "memory mapped", which
causes the specified contents of a file to be made available
to a process via its address space. Manipulations of that file
can then be performed simply by manipulating memory, rather
than using filesystem I/O calls. This technique is used to
simplify code, speed up access to files, and provide interprocess
communication.
In 4.4BSD, 4 new FFS flags were added that give the possibility
to mark files as append-only or immutable.
II. Problem Description
It is possible for a process to open an append-only file
according to the limitations of the flags, and then mmap the
file shared with write permission even when the file is marked
as append-only or immutable. This circumvents the concept of
the the append-only flag.
III. Impact
It is possible to change the contents of append-only files.
IV. Workaround
No workaround is known.
V. Solution
Apply one of the following patches, rebuild your kernel,
install it and reboot your system.
The patches below can be found on
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-98:04/
NOTE: Users of FreeBSD 2.2.5 or FreeBSD-current or FreeBSD-stable
dated before 1998/03/12 will need to apply the patch mentioned in
FreeBSD advisory SA-98:02.
Patch for 3.0-current systems:
Index: vm_mmap.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/vm/vm_mmap.c,v
retrieving revision 1.75
retrieving revision 1.77
diff -u -r1.75 -r1.77
--- vm_mmap.c 1998/03/12 19:36:18 1.75
+++ vm_mmap.c 1998/05/19 07:13:21 1.77
@@ -58,6 +58,7 @@
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/conf.h>
+#include <sys/stat.h>
#include <sys/vmmeter.h>
#include <miscfs/specfs/specdev.h>
@@ -295,12 +296,25 @@
* we're at securelevel < 1, to allow the XIG X server
* to continue to work.
*/
- if (((flags & MAP_SHARED) != 0 ||
- (vp->v_type == VCHR && disablexworkaround)) &&
- (fp->f_flag & FWRITE) == 0 && (prot & PROT_WRITE) != 0)
- return (EACCES);
- else
+
+ if ((flags & MAP_SHARED) != 0 ||
+ (vp->v_type == VCHR && disablexworkaround)) {
+ if ((fp->f_flag & FWRITE) != 0) {
+ struct vattr va;
+ if ((error =
+ VOP_GETATTR(vp, &va,
+ p->p_ucred, p)))
+ return (error);
+ if ((va.va_flags &
+ (IMMUTABLE|APPEND)) == 0)
+ maxprot |= VM_PROT_WRITE;
+ else if (prot & PROT_WRITE)
+ return (EPERM);
+ } else if ((prot & PROT_WRITE) != 0)
+ return (EACCES);
+ } else
maxprot |= VM_PROT_WRITE;
+
handle = (void *)vp;
}
}
Patch for 2.2 systems:
Index: vm_mmap.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/vm/vm_mmap.c,v
retrieving revision 1.53.2.3
retrieving revision 1.53.2.4
diff -u -r1.53.2.3 -r1.53.2.4
--- vm_mmap.c 1998/03/12 19:36:50 1.53.2.3
+++ vm_mmap.c 1998/05/24 19:47:02 1.53.2.4
@@ -57,6 +57,7 @@
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/conf.h>
+#include <sys/stat.h>
#include <sys/vmmeter.h>
#include <miscfs/specfs/specdev.h>
@@ -275,12 +276,26 @@
* we're at securelevel < 1, to allow the XIG X server
* to continue to work.
*/
- if (((flags & MAP_SHARED) != 0 ||
- (vp->v_type == VCHR && disablexworkaround)) &&
- (fp->f_flag & FWRITE) == 0 && (prot & PROT_WRITE) != 0)
- return (EACCES);
- else
+
+ if ((flags & MAP_SHARED) != 0 ||
+ (vp->v_type == VCHR && disablexworkaround)) {
+ if ((fp->f_flag & FWRITE) != 0) {
+ struct vattr va;
+
+ if ((error =
+ VOP_GETATTR(vp, &va,
+ p->p_ucred, p)))
+ return (error);
+ if ((va.va_flags &
+ (IMMUTABLE|APPEND)) == 0)
+ maxprot |= VM_PROT_WRITE;
+ else if (prot & PROT_WRITE)
+ return (EPERM);
+ } else if ((prot & PROT_WRITE) != 0)
+ return (EACCES);
+ } else
maxprot |= VM_PROT_WRITE;
+
handle = (caddr_t) vp;
}
}
VI. Thanks
This advisory is based on NetBSD Security Advisory 1998-003.
In porting the NetBSD patch, we accidentally mentioned that we
obtained the patch from OpenBSD, which was evidently wrong.
=============================================================================
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.
[ End FreeBSD, Inc. Advisory ]
______________________________________________________________________________
CIAC wishes to acknowledge the contributions of FreeBSD, Inc. for the
information contained in this bulletin.
______________________________________________________________________________
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