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Posted on 12 December 2023
FreeBSD security notificat=============================================================================FreeBSD-SA-23:18.nfsclient Security Advisory
The FreeBSD Project
Topic: NFS client data corruption and kernel memory disclosure
Category: core
Module: nfsclient
Announced: 2023-12-12
Credits: Hostpoint AG
Affects: FreeBSD 13.2 and 14.0
Corrected: 2023-12-12 19:13:50 UTC (stable/14, 14.0-STABLE)
2023-12-12 19:17:36 UTC (releng/14.0, 14.0-RELEASE-p3)
2023-12-12 19:14:16 UTC (stable/13, 13.2-STABLE)
2023-12-12 19:18:17 UTC (releng/13.2, 13.2-RELEASE-p8)
CVE Name: CVE-2023-6660
For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.
I. Background
The Network File System (NFS) is a distributed file system that allows remote
systems to access files and directories over a network as if they were local.
FreeBSD includes both server and client implementations of NFS.
II. Problem Description
In FreeBSD 13.2 and 14.0, the NFS client was optimized to improve the
performance of IO_APPEND writes, that is, writes which add data to the end of
a file and so extend its size. This uncovered an old bug in some routines
which copy userspace data into the kernel. The bug also affects the NFS
client's implementation of direct I/O; however, this implementation is
disabled by default by the vfs.nfs.nfs_directio_enable sysctl and is only
used to handle synchronous writes.
III. Impact
When a program running on an affected system appends data to a file via an
NFS client mount, the bug can cause the NFS client to fail to copy in the
data to be written but proceed as though the copy operation had succeeded.
This means that the data to be written is instead replaced with whatever data
had been in the packet buffer previously. Thus, an unprivileged user with
access to an affected system may abuse the bug to trigger disclosure of
sensitive information. In particular, the leak is limited to data previously
stored in mbufs, which are used for network transmission and reception, and
for certain types of inter-process communication.
The bug can also be triggered unintentionally by system applications, in
which case the data written by the application to an NFS mount may be
corrupted. Corrupted data is written over the network to the NFS server, and
thus also susceptible to being snooped by other hosts on the network.
Note that the bug exists only in the NFS client; the version and
implementation of the server has no effect on whether a given system is
affected by the problem.
IV. Workaround
No workaround is available.
V. Solution
Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date
and reboot.
Perform one of the following:
1) To update your vulnerable system via a binary patch:
Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms,
or the i386 platfrom on FreeBSD 13 and earlier, can be updated via
the freebsd-update(8) utility:
# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"
2) To update your vulnerable system via a source code patch:
The following patches have been verified to apply to the applicable
FreeBSD release branches.
a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.
# fetch https://security.FreeBSD.org/patches/SA-23:18/nfsclient.patch
# fetch https://security.FreeBSD.org/patches/SA-23:18/nfsclient.patch.asc
# gpg --verify nfsclient.patch.asc
b) Apply the patch. Execute the following commands as root:
# cd /usr/src
# patch < /path/to/patch
c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.
VI. Correction details
This issue is corrected as of the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:
Branch/path Hash Revision
- -------------------------------------------------------------------------
stable/14/ 8d42f85d9d7b stable/14-n265954
releng/14.0/ ab60666a00c9 releng/14.0-n265397
stable/13/ f1d1d50e1d08 stable/13-n256860
releng/13.2/ 3f079b3f2f33 releng/13.2-n254649
- -------------------------------------------------------------------------
Run the following command to see which files were modified by a
particular commit:
# git show --stat <commit hash>
Or visit the following URL, replacing NNNNNN with the hash:
<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>
To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:
# git rev-list --count --first-parent HEAD
VII. References
<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-6660>
The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-23:18.nfsclient.asc>
