Increase NFS Client Mount Point Security For a Web-Server noexec, nosuid, nodev Options

by on February 19, 2014 · 0 comments· LAST UPDATED February 19, 2014

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I am using NFS server version 4.x on a CentOS/RHEL based system. I'm mounting my shared /var/www/ directory on five Apache based nodes using the following syntax:

mount -t nfs4 -o rw,intr,hard,proto=tcp rocknas02:/httproot/www /var/www/

I noticed that due to bug in my app user can sometime upload executable or other device files to get out of chrooted Apache server. How can I prevent such security issues on a CentOS or RHEL based NFS client and sever setup?

Tutorial details
DifficultyIntermediate (rss)
Root privilegesYes
RequirementsNFS v4.x
Estimated completion time10m
First, you need to fix your application. Next, you can pass the following three options to mount command to increase overall security on Apache/Nginx/Lighttpd nfs based client:

  1. noexec - Prevents execution of binaries on mounted file systems. This prevents remote users from executing unwanted binaries on your system.
  2. nosuid - Disables set-user-identifier or set-group-identifier bits. This prevents remote users from gaining higher privileges by running a setuid program.
  3. nodev - Prevents character and block special devices from being defined, or if they exist, from being used as character and block special devices. This prevents remote users from getting out of chrooted server jails.

Modify your mount command as follows:
# mount -t nfs4 -o rw,intr,hard,proto=tcp,nodev,noexec,nosuid rocknas02:/httproot/www /var/www/
OR attempt to remount an already-mounted nfsv4.0 filesystem:
# mount -t nfs4 -o remount,rw,intr,hard,proto=tcp,nodev,noexec,nosuid rocknas02:/httproot/www /var/www/

Test it

To verify new settings, enter:
# mount
# mount | grep rocknas02

Sample outputs:

rocknas02:/httproot/www on /var/www type nfs4 (rw,noexec,nosuid,nodev,sync,intr,hard,proto=tcp,addr=192.168.1.10,clientaddr=192.168.1.100)

Copy /bin/ls to rocknas02:/httproot/www i.e. type the following on your nfsv4.0 server called rocknas02
# cp /bin/ls /httproot/www
On client, type:

cd /var/www
 
## run /bin/ls
ls -l
 
# Run uploaded ls
./ls
 

Sample outputs:

Fig. 01: Running ls command on nfs client

Fig. 01: Running ls command on nfs client


Updating /etc/fstab is left as an exercise for the reader.

Mount the filesystem read-only

If possible mount the filesystem in read-only mode. Modify your mount command as follows:
# mount -t nfs4 -o ro,intr,hard,proto=tcp,nodev,noexec,nosuid rocknas02:/httproot/www /var/www/
OR attempt to remount an already-mounted nfsv4.0 filesystem:
# mount -t nfs4 -o remount,ro,intr,hard,proto=tcp,nodev,noexec,nosuid rocknas02:/httproot/www /var/www/

Recommend file/directory permission for Apache

I suggest the following schema:

Run Apache as apache user and group

You must run httpd as root initially and it will switch to apache user and group:
# egrep -i '^(User|Group)' /etc/httpd/conf/httpd.conf
Sample outputs:

User apache
Group apache

NFS server file/directory permission for /var/www/

Create a user called www-files using useradd command:
# useradd -d /var/www -M -s /sbin/nologin www-files
Make sure you lock www-files account using passwd command:
# passwd -l www-files
Change file owner and group to www-files for /var/www directory use the following passwd command:
# chown -R www-files:www-files /var/www/
Finally changes the file mode bits of each given file and directory according to mode:

## By default all files & dirs permissions are set to read-only ###
chmod -R 0444 /var/www
 
## Allow, apache/nginx/lighttpd to serve files from directory by settings others to x bit ###
find /var/www -type d -print0 | xargs -0 -I {} chmod 0445 "{}"
 
## Optional certain directory may need additional permissions such as /var/www/uploads ###
#chmod  0777 -R /var/www/uploads
 

Use ls -l command to verify file permissions:
# cd /var/www
# ls -l

Sample outputs:

total 32
-r--r--r--. 1 www-files www-files  606 Dec 21  2011 best_resources.php
-r--r--r--. 1 www-files www-files 1068 Sep  4  2011 cdn_info_linux_unix_setup.php
dr--r--r-x. 2 www-files www-files 4096 Aug  5  2012 data
....
...
...
-r--r--r--. 1 www-files www-files 1550 Jun 22  2012 service-per-vm-guide.php
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