=============================================================================
FreeBSD-SA-00:42 Security Advisory
FreeBSD, Inc.

Topic: Linux binary compatability mode can cause system compromise

Category: core
Module: kernel
Announced: 2000-08-28
Credits: Boris Nikolaus <[email protected]>
Affects: FreeBSD 3.x, 4.x and 5.x prior to the correction date
Corrected: 2000-07-23 (FreeBSD 5.0-CURRENT)
2000-07-29 (FreeBSD 4.1-STABLE)
2000-08-24 (FreeBSD 3.5-STABLE)
FreeBSD only: Yes

I. Background

FreeBSD is binary-compatible with the Linux operating system through a
loadable kernel module/optional kernel component.

II. Problem Description

The linux binary-compatability module implements a "shadow" filesystem
hierarchy rooted in /compat/linux, which is overlayed against the
regular filesystem hierarchy so that Linux binaries "see" files in the
shadow hierarchy which can mask the native files.

Filenames in this shadow hierarchy are treated incorrectly by the
linux kernel module under certain circumstances, and a kernel stack
overflow leading to a system compromise by an unprivileged user may be
possible when very long filenames are used. This is only possible when
the linux kernel module is loaded, or the equivalent functionality is
statically compiled into the kernel. It is not enabled by default.

This vulnerability was fixed just after the release of FreeBSD
4.1-RELEASE, and 3.5-RELEASE is also vulnerable.

III. Impact

Local users may be able to obtain root privileges on the system when
linux compatability mode is enabled.

IV. Workaround

To determine whether the linux compatability module has been loaded,
execute the following command as root and look for a 'linux.ko' entry:

kldstat

Id Refs Address Size Name
1 7 0xc0100000 270be0 kernel
2 1 0xc0371000 5540 vesa.ko
3 1 0xc0377000 10094 randomdev.ko
4 1 0xc0e17000 4e000 nfs.ko
5 1 0xc0e83000 11000 linux.ko

If present, unload the "linux" module by executing the following
command as root:

kldunload linux

For safety, remove the /modules/linux.ko file to prevent it being
reloaded accidentally, and add or change the following line in
/etc/rc.conf:

linux_enable="NO" # Linux binary compatibility loaded at startup (or NO).

If the module is not loaded, to determine whether the functionality
has been statically compiled into the kernel, check the kernel
configuration file for the following line:

options COMPAT_LINUX

If present, remove and recompile the kernel as described in
http://www.freebsd.org/handbook/kernelconfig.html and reboot the
system.

V. Solution

One of the following:

1) Upgrade your vulnerable FreeBSD system to 3.5-STABLE, 4.1-STABLE or
5.0-CURRENT after the respective correction dates.

2) Apply the patch below and recompile your kernel.

Either save this advisory to a file, or download the patch and
detached PGP signature from the following locations, and verify the
signature using your PGP utility.

ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:42/linux.patch
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:42/linux.patch.asc

cd /usr/src/sys/i386/linux

patch -p < /path/to/patch_or_advisory

[ Recompile your kernel as described in
http://www.freebsd.org/handbook/kernelconfig.html and reboot the
system ]

Index: linux_misc.c
===================================================================
RCS file: /home/ncvs/src/sys/i386/linux/linux_misc.c,v
retrieving revision 1.77.2.3
retrieving revision 1.77.2.4
diff -u -r1.77.2.3 -r1.77.2.4
--- linux_misc.c    2000/07/20 05:31:56     1.77.2.3
+++ linux_misc.c    2000/07/30 05:36:11     1.77.2.4
@@ -954,6 +954,8 @@
    tv[1].tv_usec = 0;
    /* so that utimes can copyin */
    tvp = (struct timeval *)stackgap_alloc(&sg, sizeof(tv));
+   if (tvp == NULL)
+           return (ENAMETOOLONG);
    if ((error = copyout(tv, tvp, sizeof(tv))))
        return error;
    bsdutimes.tptr = tvp;
Index: linux_util.c
===================================================================
RCS file: /home/ncvs/src/sys/i386/linux/linux_util.c,v
retrieving revision 1.9.2.1
retrieving revision 1.9.2.2
diff -u -r1.9.2.1 -r1.9.2.2
--- linux_util.c    2000/07/07 01:23:45     1.9.2.1
+++ linux_util.c    2000/07/30 05:36:11     1.9.2.2
@@ -162,7 +162,10 @@
    else {
            sz = &ptr[len] - buf;
            *pbuf = stackgap_alloc(sgp, sz + 1);
-           error = copyout(buf, *pbuf, sz);
+           if (*pbuf != NULL)
+                   error = copyout(buf, *pbuf, sz);
+           else
+                   error = ENAMETOOLONG;
            free(buf, M_TEMP);
    }
 
Index: linux_util.h
===================================================================
RCS file: /home/ncvs/src/sys/i386/linux/linux_util.h,v
retrieving revision 1.10
retrieving revision 1.10.2.1
diff -u -r1.10 -r1.10.2.1
--- linux_util.h    1999/12/04 11:10:22     1.10
+++ linux_util.h    2000/07/30 05:36:11     1.10.2.1
@@ -56,29 +56,27 @@
 static __inline caddr_t stackgap_init(void);
 static __inline void *stackgap_alloc(caddr_t *, size_t);
 
+#define szsigcode (*(curproc->p_sysent->sv_szsigcode))
+
 static __inline caddr_t
 stackgap_init()
 {
-#define szsigcode (*(curproc->p_sysent->sv_szsigcode))
    return (caddr_t)(PS_STRINGS - szsigcode - SPARE_USRSPACE);
 }
 
-
 static __inline void *
 stackgap_alloc(sgp, sz)
    caddr_t *sgp;
    size_t   sz;
 {
-   void    *p = (void *) *sgp;
-   *sgp += ALIGN(sz);
+   void *p = (void *) *sgp;
+
+   sz = ALIGN(sz);
+   if (*sgp + sz > (caddr_t)(PS_STRINGS - szsigcode))
+           return NULL;
+   *sgp += sz;
    return p;
 }
-
-#ifdef DEBUG_LINUX
-#define DPRINTF(a)      printf a;
-#else
-#define DPRINTF(a)
-#endif
 
 extern const char linux_emul_path[];