Chapter 11. Configuring polyinstantiated directories By default, all programs, services, and users use the /tmp , /var/tmp , and home directories for temporary storage. This makes these directories vulnerable to race condition attacks and information leaks based on file names. You can make /tmp/ , /var/tmp/ , and the home directory instantiated so that they are no longer shared between all users, and each user's /tmp-inst and /var/tmp/tmp-inst is separately mounted to the /tmp and /var/tmp directory. Procedure Enable polyinstantiation in SELinux: You can verify that polyinstantiation is enabled in SELinux by entering the getsebool allow_polyinstantiation command. Create the directory structure for data persistence over reboot with the necessary permissions: Restore the entire security context including the SELinux user part: If your system uses the fapolicyd application control framework, allow fapolicyd to monitor file access events on the underlying file system when they are bind mounted by enabling the allow_filesystem_mark option in the /etc/fapolicyd/fapolicyd.conf configuration file. Enable instantiation of the /tmp , /var/tmp/ , and users' home directories: Important Use /etc/security/namespace.conf instead of a separate file in the /etc/security/namespace.d/ directory because the pam_namespace_helper program does not read additional files in /etc/security/namespace.d . On a system with multi-level security (MLS), uncomment the last three lines in the /etc/security/namespace.conf file: On a system without multi-level security (MLS), add the following lines in the /etc/security/namespace.conf file: Verify that the pam_namespace.so module is configured for the session: Optional: Enable cloud users to access the system with SSH keys: Install the openssh-keycat package. Create a file in the /etc/ssh/sshd_config.d/ directory with the following content: Verify that public key authentication is enabled by checking that the PubkeyAuthentication variable in sshd_config is set to yes . By default, PubkeyAuthentication is set to yes, even though the line in sshd_config is commented out. Add the session required pam_namespace.so unmnt_remnt entry into the module for each service for which polyinstantiation should apply, after the session include system-auth line. For example, in /etc/pam.d/su , /etc/pam.d/sudo , /etc/pam.d/ssh , and /etc/pam.d/sshd : Verification Log in as a non-root user. Users that were logged in before polyinstantiation was configured must log out and log in before the changes take effect for them. Check that the /tmp/ directory is mounted under /tmp-inst/ : The SOURCE output differs based on your environment. * On virutal systems, it shows /dev/vda_<number>_ . * On bare-metal systems it shows /dev/sda_<number>_ or /dev/nvme* Additional resources /usr/share/doc/pam-docs/txts/README.pam_namespace readme file installed with the pam-docs package.

Commands:
setsebool -P allow_polyinstantiation 1
mkdir /tmp-inst /var/tmp/tmp-inst --mode 000
restorecon -Fv /tmp-inst /var/tmp/tmp-inst Relabeled /tmp-inst from unconfined_u:object_r:default_t:s0 to system_u:object_r:tmp_t:s0 Relabeled /var/tmp/tmp-inst from unconfined_u:object_r:tmp_t:s0 to system_u:object_r:tmp_t:s0
allow_filesystem_mark = 1
/tmp /tmp-inst/ level root,adm /var/tmp /var/tmp/tmp-inst/ level root,adm USDHOME USDHOME/USDUSER.inst/ level
/tmp /tmp-inst/ user root,adm /var/tmp /var/tmp/tmp-inst/ user root,adm USDHOME USDHOME/USDUSER.inst/ user
grep namespace /etc/pam.d/login session required pam_namespace.so
AuthorizedKeysCommand /usr/libexec/openssh/ssh-keycat AuthorizedKeysCommandRunAs root
grep -r PubkeyAuthentication /etc/ssh/ /etc/ssh/sshd_config:#PubkeyAuthentication yes
[...] session include system-auth session required pam_namespace.so unmnt_remnt [...]
findmnt --mountpoint /tmp/ TARGET SOURCE FSTYPE OPTIONS /tmp /dev/vda1[/tmp-inst/ <user> ] xfs rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota