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=============================================================================
FreeBSD-SA-02:36.nfs Security Advisory
The FreeBSD Project

Topic: Bug in NFS server code allows remote denial of service

Category: core
Module: nfs
Announced: 2002-08-05
Credits: Mike Junk <[email protected]>
Affects: All releases prior to 4.6.1-RELEASE-p7
4.6-STABLE prior to the correction date
Corrected: 2002-07-19 17:19:53 UTC (RELENG_4)
2002-08-01 19:31:55 UTC (RELENG_4_6)
2002-08-01 19:31:54 UTC (RELENG_4_5)
2002-08-01 19:31:54 UTC (RELENG_4_4)
FreeBSD only: NO

I. Background

The Network File System (NFS) allows a host to export some or all of
its filesystems, or parts of them, so that other hosts can access them
over the network and mount them as if they were on local disks. NFS is
built on top of the Sun Remote Procedure Call (RPC) framework.

II. Problem Description

A part of the NFS server code charged with handling incoming RPC
messages had an error which, when the server received a message with a
zero-length payload, would cause it to reference the payload from the
previous message, creating a loop in the message chain. This would
later cause an infinite loop in a different part of the NFS server
code which tried to traverse the chain.

III. Impact

Certain Linux implementations of NFS produce zero-length RPC messages
in some cases. A FreeBSD system running an NFS server may lock up
when such clients connect.

An attacker in a position to send RPC messages to an affected FreeBSD
system can construct a sequence of malicious RPC messages that cause
the target system to lock up.

IV. Workaround

1) Disable the NFS server: set the nfs_server_enable variable to "NO"
in /etc/rc.conf, and reboot.

Alternatively, if there are no active NFS clients (as listed by the
showmount(8) utility), just killing the mountd and nfsd processes
should suffice.

2) Add firewall rules to block RPC traffic to the NFS server from
untrusted hosts.

V. Solution

The following patch has been verified to apply to FreeBSD 4.4, 4.5, and
4.6 systems.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:36/nfs.patch

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:36/nfs.patch.asc

b) Apply the patch.

cd /usr/src

patch < /path/to/patch

c) Recompile your kernel and modules as described in
<URL:http://www.freebsd.org/handbook/kernelconfig.html&gt; and reboot the
system.

VI. Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

Path Revision
Branch

---

src/sys/nfs/nfs_socket.c
RELENG_4 1.60.2.5
RELENG_4_6 1.60.2.3.2.1
RELENG_4_5 1.60.2.1.6.1
RELENG_4_4 1.60.2.3.4.1

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