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Home » Security Advisories» FreeBSD » Security compromise via open()

Security compromise via open()

A problem exists in the open() syscall that allows processes to obtain a valid file descriptor without having read or write permissions on the file being opened. This is normally not a problem. The FreeBSD way of obtaining the right to do io instructions however, is based on the right to open a specific file (/dev/io).

  • Vendor: FreeBSD
  • Vendor ID: SA-97:05
  • Date: October 29, 1997


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=====================================================================
FreeBSD-SA-97:05 Security Advisory
FreeBSD, Inc.

Topic: security compromise via open()

Category: core
Module: kern
Announced: 1997-10-29
Affects: FreeBSD 2.1.*, FreeBSD 2.2.*,
FreeBSD-stable and FreeBSD-current
Corrected: FreeBSD-current as of 1997/10/23 (partly even on 1997/04/14)
FreeBSD-stable as of 1997/10/24
FreeBSD 2.1-stable as of 1997/10/29
FreeBSD only: yes

Patches: ftp://freebsd.org/pub/CERT/patches/SA-97:05/

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

I. Background

In FreeBSD, the open() system call is used in normal file operations.
When calling open(), the caller should specify if the file is
to be opened for reading, for writing or for both.
The right to reading from and/or writing to a file is controlled
by the file's mode bits in the filesystem.
In FreeBSD, open() is also used to obtain the right to do
privileged io instructions.


II. Problem Description

A problem exists in the open() syscall that allows processes
to obtain a valid file descriptor without having read or write
permissions on the file being opened. This is normally not a
problem. The FreeBSD way of obtaining the right to do io
instructions however, is based on the right to open a specific
file (/dev/io).

III. Impact

The problem can be used by any user on the system to do unauthorised
io instructions.


IV. Workaround

No workaround is available.

V. Solution

Apply the following patches. The first one in /usr/src/sys/kern,
and the second one in /usr/src/sys/i386/i386,
Rebuild your kernel, install it and reboot your system.

patch 1:
For FreeBSD-current before 1997/10/23:

Index: vfs_syscalls.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.76
retrieving revision 1.77
diff -u -r1.76 -r1.77
--- vfs_syscalls.c 1997/10/12 20:24:27 1.76
+++ vfs_syscalls.c 1997/10/22 07:28:51 1.77
@@ -863,11 +863,13 @@
struct flock lf;
struct nameidata nd;

+ flags = FFLAGS(SCARG(uap, flags));
+ if ((flags & FREAD + FWRITE) == 0)
+ return (EINVAL);
error = falloc(p, &nfp, &indx);
if (error)
return (error);
fp = nfp;
- flags = FFLAGS(SCARG(uap, flags));
cmode = ((SCARG(uap, mode) &~ fdp->fd_cmask) & ALLPERMS) &~ S_ISTXT;
NDINIT(&nd, LOOKUP, FOLLOW, UIO_USERSPACE, SCARG(uap, path), p);
p->p_dupfd = -indx - 1; /* XXX check for fdopen */


For FreeBSD 2.1.* and 2.2.*:

Index: vfs_syscalls.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.51.2.5
diff -u -r1.51.2.5 vfs_syscalls.c
--- vfs_syscalls.c 1997/10/01 06:23:48 1.51.2.5
+++ vfs_syscalls.c 1997/10/28 22:04:43
@@ -688,11 +688,13 @@
struct flock lf;
struct nameidata nd;

+ flags = FFLAGS(uap->flags);
+ if ((flags & FREAD + FWRITE) == 0)
+ return (EINVAL);
error = falloc(p, &nfp, &indx);
if (error)
return (error);
fp = nfp;
- flags = FFLAGS(uap->flags);
cmode = ((uap->mode &~ fdp->fd_cmask) & ALLPERMS) &~ S_ISTXT;
NDINIT(&nd, LOOKUP, FOLLOW, UIO_USERSPACE, uap->path, p);
p->p_dupfd = -indx - 1; /* XXX check for fdopen */

patch 2:
For FreeBSD 2.1.* and 2.2.* and For FreeBSD-current before 1997/04/14:

Index: mem.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/i386/i386/mem.c,v
retrieving revision 1.38
retrieving revision 1.38.2.1
diff -u -r1.38 -r1.38.2.1
--- mem.c 1996/09/27 13:25:06 1.38
+++ mem.c 1997/10/23 22:14:24 1.38.2.1
@@ -169,6 +169,7 @@
int fmt;
struct proc *p;
{
+ int error;
struct trapframe *fp;

switch (minor(dev)) {
@@ -179,6 +180,11 @@
return ENODEV;
#endif
case 14:
+ error = suser(p->p_ucred, &p->p_acflag);
+ if (error != 0)
+ return (error);
+ if (securelevel > 0)
+ return (EPERM);
fp = (struct trapframe *)curproc->p_md.md_regs;
fp->tf_eflags |= PSL_IOPL;
break;

=====================================================================
FreeBSD, Inc.

Web Site: http://www.freebsd.org/
Confidential contacts: [email protected]
PGP Key: ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications: [email protected]
Security public discussion: [email protected]

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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