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Home : Advisories : top allows reading of kernel memory

Title: top allows reading of kernel memory
Released by: FreeBSD
Date: 1st November 2000
Printable version: Click here


FreeBSD-SA-00:62                                           Security Advisory

                                                                FreeBSD, Inc.

Topic:          top allows reading of kernel memory

Category:       core

Module:         top

Announced:      2000-11-01

Credits:        vort@wiretapped.net via OpenBSD

Affects:        FreeBSD 3.x (all releases), FreeBSD 4.x (all releases prior

                to 4.2), FreeBSD 3.5.1-STABLE and 4.1.1-STABLE prior

                to the correction date.

Corrected:      2000/10/04 (FreeBSD 4.1.1-STABLE)

                2000/10/04 (FreeBSD 3.5.1-STABLE)

FreeBSD only:   NO

I.   Background

top is a utility for displaying current system resource statistics

such as process CPU and memory use.  It is externally-maintained,

contributed software which is included in FreeBSD by default.

II.  Problem Description

A "format string vulnerability" was discovered in the top(1) utility

which allows unprivileged local users to cause the top process to

execute arbitrary code.  The top utility runs with increased

privileges as a member of the kmem group, which allows it to read from

kernel memory (but not write to it).  A process with the ability to

read from kernel memory can monitor privileged data such as network

traffic, disk buffers and terminal activity, and may be able to

leverage this to obtain further privileges on the local system or on

other systems, including root privileges.

All released versions of FreeBSD prior to the correction date

including 4.0, 4.1, 4.1.1 and 3.5.1 are vulnerable to this problem,

but it was fixed in the 4.1.1-STABLE branch prior to the release of


III. Impact

Local users can read privileged data from kernel memory which may

provide information allowing them to further increase their local or

remote system access privileges.

IV.  Workaround

Remove the setgid bit on the top utilities.  This has the side-effect

that users who are not a member of the kmem group or who are not the

superuser cannot use the top utility.

# chmod g-s /usr/bin/top

V.   Solution

One of the following:

1) Upgrade your vulnerable FreeBSD system to 4.1.1-STABLE or

3.5.1-STABLE after the respective correction dates.

2) Apply the patch below and recompile the relevant files:

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.



Execute the following commands as root:

# cd /usr/src/contrib/top

# patch -p < /path/to/patch_or_advisory

# cd /usr/src/usr.bin/top

# make depend && make all install

Patch for vulnerable systems:

    Index: display.c


    RCS file: /mnt/ncvs/src/contrib/top/display.c,v

    retrieving revision 1.4

    retrieving revision 1.5

    diff -u -r1.4 -r1.5

    --- display.c 1999/01/09 20:20:33 1.4

    +++ display.c 2000/10/04 23:34:16 1.5

    @@ -829,7 +831,7 @@

         register int i;

         /* first, format the message */

    -    (void) sprintf(next_msg, msgfmt, a1, a2, a3);

    +    (void) snprintf(next_msg, sizeof(next_msg), msgfmt, a1, a2, a3);

         if (msglen > 0)


    Index: top.c


    RCS file: /mnt/ncvs/src/contrib/top/top.c,v

    retrieving revision 1.4

    retrieving revision 1.5

    diff -u -r1.4 -r1.5

    --- top.c 1999/01/09 20:20:34 1.4

    +++ top.c 2000/10/04 23:34:16 1.5

    @@ -807,7 +809,7 @@


         if ((errmsg = kill_procs(tempbuf2)) != NULL)


    - new_message(MT_standout, errmsg);

    + new_message(MT_standout, "%s", errmsg);


     no_command = Yes;



Version: GnuPG v1.0.4 (FreeBSD)

Comment: For info see http://www.gnupg.org







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