danwalsh (danwalsh) wrote,

What is the differences between user_home_dir_t and user_home_t

I just saw this email on the SELinux Fedora Mailing list and figured I would try to answer it here.

Lets look at the labels of content in my  home directory.

> ls -lZd / /home /home/dwalsh /home/dwalsh/.ssh /home/dwalsh/.emacs /home/dwalsh/public_html
dr-xr-xr-x. root   root   system_u:object_r:root_t:s0      /
drwxr-xr-x. root   root   system_u:object_r:home_root_t:s0 /home
drwx------. dwalsh dwalsh unconfined_u:object_r:user_home_dir_t:s0 /home/dwalsh
-rw-r--r--. dwalsh dwalsh unconfined_u:object_r:user_home_t:s0 /home/dwalsh/.emacs
drwxr-xr-x. dwalsh dwalsh unconfined_u:object_r:httpd_user_content_t:s0 /home/dwalsh/public_html
drwx------. dwalsh dwalsh unconfined_u:object_r:ssh_home_t:s0 /home/dwalsh/.ssh

As we go through the different files and directories that make up my home directory we see multiple SELinux labels.  Notice the files that are owned by me have an SELinux user of staff_u.  On most systems these would be labeled unconfined_u.  In SELinux every file that is created by a process running as unconfined_u will get the unconfined_u SELinux user placed on the file label.  Similarly staff_u will get staff_u, ...

This component is ignored on most SELinux systems, system_u on /home here is the default label set by restorecon or at install time.

object_r is just a place holder.  For all SELinux systems other then some experimental systems, every object on the file system gets labeled object_r.

The last field is all s0.  This is the MCS or MLS label depending on your policy.  On most systems this will be s0 (SystemLow)

SELinux is a type enforcement system, and the interesting parts is the types.  Lets look at each directory/file above and the types associated with them.

  • / is labeled with the root_t type.  This should be the only object on the system with the root_t type, if you have other objects on the computer with this label, then you were probably running in permissive mode and a confined process created it, you need to fix the labels.  The main purpose of root_t is for policy writers to define transitions. for system applications that are going to create content in /.  For example boot flags will get created with etc_runtime_t.  Random directories created in / will get labeled default_t.

#  touch /.autorelabel; ls -lZ /.autorelabel
-rw-r--r--. root root unconfined_u:object_r:etc_runtime_t:s0 /.autorelabel
# mkdir /foobar; ls -ldZ /foobar
drwxr-xr-x. root root unconfined_u:object_r:default_t:s0 /foobar

         One thing to note about default_t is that NO confined apps are allowed to read content labeled default_t, because we have no idea what kind of content would be in this directory.  Invariably when some one creates a new directory in / without setting the labels, SELinux will have issues.  If foobar above contained apache content you would need to execute the following in order to allow apache to read the content.

# semanage fcontext -a -t httpd_sys_content_t '/foobar(/.*)?'
# restorecon -R -v /foobar

  • /home is labeled with the home_root_t type.  home_root_t is basically used for the toplevel or root directory for homedirs.   It's main purpose is to be used by policy for applications that need to create home directories or file system quota files.

# sesearch -T -t home_root_t
Found 10 semantic te rules:
   type_transition quota_t home_root_t : file quota_db_t;
   type_transition sysadm_t home_root_t : dir user_home_dir_t;
   type_transition firstboot_t home_root_t : dir user_home_dir_t;
   type_transition useradd_t home_root_t : dir user_home_dir_t;
   type_transition lsassd_t home_root_t : dir user_home_dir_t; 
   type_transition oddjob_mkhomedir_t home_root_t : dir user_home_dir_t;
   type_transition smbd_t home_root_t : dir user_home_dir_t;
   type_transition automount_t home_root_t : dir automount_tmp_t;

  • /home/dwalsh is labeled with the user_home_dir_t type.    Policy writers use this type to write transition rules to get content labeled correctly within the homedir.  Confined applications that create content in a users home directory need transition rules to create the content with the proper label. For example if you use ssh-copy-id on the remote machine, this triggers sshd to create the /home/dwalsh/.ssh directory, which we want labeled ssh_home_t, to protect the content.

# sesearch -T -s sshd_t -t user_home_dir_t | grep "\.ssh"
type_transition sshd_t user_home_dir_t : dir ssh_home_t ".ssh";

sesearch -T -s staff_t -t user_home_dir_t |wc -l

Note there are over 118 transition rules for a confined user creating content in his home directory.  With the advent of file name transition rules, we have exploded the number of these transitions in order to get proper labeling in the users home dir.

  • /home/dwalsh/.emacs is labeled user_home_t, which is the default label for all content in a users home directory.  This is the label that users are allowed to read/write and manage.  We try to prevent confined applications from being able to read and write this content, since this is where users store private content like credit card data, passwords shared secrets etc.  When we have a confined application that needs to access this content we usually wrap it in a boolean like ftp_home_dir.  Or we create a new type for this content, like mozilla_home_t or ssh_home_t.

  • /home/dwalsh/.ssh is labeled ssh_home_t, and contains content that only user types and sshd is allowed to read. Most other confined domains are not allowed to view content in this directory since it contains content like your secret keys.

  • /home/dwalsh/public_html is labeled httpd_user_content_t , which by default is the only place apache process is allowed to read in the home directory.  Allowing apache to read .ssh or say your .mozilla directory is just asking for trouble.

Note:  In order for apache or sshd to read their content they need to be allowed to search and getattr on every directory in the path.  The httpd would need to search root_t, home_root_t, user_home_dir_t, httpd_user_content_t.  It is not allowed to do this by default and needs the httpd_enable_homedirs boolean turned on.

# sesearch -A -b httpd_enable_homedirs -c dir -C | grep -v nfs | grep -v samba
Found 20 semantic av rules:
DT allow httpd_sys_script_t home_root_t : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_sys_script_t user_home_dir_t : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_user_script_t user_home_type : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_t user_home_type : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_user_script_t home_root_t : dir { ioctl read getattr lock search open } ; [ httpd_enable_homedirs ]
DT allow httpd_t home_root_t : dir { ioctl read getattr lock search open } ; [ httpd_enable_homedirs ]
DT allow httpd_user_script_t user_home_dir_t : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_t user_home_dir_t : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_suexec_t user_home_type : dir { getattr search open } ; [ httpd_enable_homedirs ]
DT allow httpd_suexec_t home_root_t : dir { ioctl read getattr lock search open } ; [ httpd_enable_homedirs ]
DT allow httpd_suexec_t user_home_dir_t : dir { getattr search open } ; [ httpd_enable_homedirs ]

Getting back to the question that drove me to create this blog.

What is the differences between user_home_dir_t  and user_home_t?

user_home_dir_t should only be the label of the users home directory but not of any content in the users home directory.
user_home_t is the label for generic content in a users homedir.

If you want to see all the labels that effect the users homedir, you can look at /etc/selinux/targeted/contexts/files/file_contexts.homedirs.

grep /home /etc/selinux/targeted/contexts/files/file_contexts.homedirs |wc -l
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