So I've shown the iterative approach used to develop policies. Again, please be aware that this is my way of developing policies, other policy developers might have a different approach. We were working on the incrontab command, so let's continue with trying to create a new user incrontab:

$ incrontab -e
cannot create temporary file: Permission denied

# tail audit.log
type=AVC msg=audit(1368709633.285:28211): avc:  denied  { setgid } for  pid=8159 comm="incrontab" capability=6  scontext=user_u:user_r:incrontab_t tcontext=user_u:user_r:incrontab_t tclass=capability
type=AVC msg=audit(1368709633.285:28212): avc:  denied  { setuid } for  pid=8159 comm="incrontab" capability=7  scontext=user_u:user_r:incrontab_t tcontext=user_u:user_r:incrontab_t tclass=capability
type=AVC msg=audit(1368709633.287:28213): avc:  denied  { search } for  pid=8159 comm="incrontab" name="/" dev="tmpfs" ino=3927 scontext=user_u:user_r:incrontab_t tcontext=system_u:object_r:tmp_t tclass=dir

The requests for the setuid and setgid capabilities are needed for the application to safely handle the user incrontabs. Note that SELinux does not "remove" the setuid bit on the binary itself, but does govern the related capabilities. Since this is required, we will add these capabilities to the policy. We also notice that incrontab searched in the /tmp location.

allow incrontab_t self:capability { setuid setgid };
...
files_search_tmp(incrontab_t)

In the next round of iteration, we notice the same error message with the following denial:

type=AVC msg=audit(1368728433.521:28215): avc:  denied  { write } for  pid=8913 comm="incrontab" name="/" dev="tmpfs" ino=3927 scontext=user_u:user_r:incrontab_t tcontext=system_u:object_r:tmp_t tclass=dir

It is safe to assume here that the process wants to create a temporary file (if it is a directory, we will find out later and can adapt). But when temporary files are created, we better make those files a specific type, like incrontab_tmp_t. So we define that on top of the policy:

type incrontab_tmp_t;
files_tmp_file(incrontab_tmp_t)

Also, we need to allow the incrontab_t domain write privileges into the tmp_t labeled directory, but with an automatic file transition towards incrontab_tmp_t for every file written. This is done through the files_tmp_filetrans method:

files_tmp_filetrans(incrontab_t, incrontab_tmp_t, file)

What this sais is that, if a domain incrontab_t wants to create a file inside tmp_t, then this file is automatically labeled incrontab_tmp_t. With SELinux, you can make this more precise: if you know what the file name would be, then you can add that as a fourth argument. However, this does not seem necessary now since we definitely want all files created in tmp_t to become incrontab_tmp_t. All that rests us is to allow incrontab to actually manage those files:

allow incrontab_t incrontab_tmp_t:file manage_file_perms;

With those in place, let's look at the outcome:

$ incrontab -e
editor finished with error: No such file or directory

# tail audit.log
type=AVC msg=audit(1368729268.465:28217): avc:  denied  { search } for  pid=8981 comm="incrontab" name="bin" dev="dm-3" ino=524289 scontext=user_u:user_r:incrontab_t tcontext=system_u:object_r:bin_t tclass=dir

Considering that here, incrontab is going to launch the users $EDITOR application to allow him (or her) to create an incrontab, we need to allow incrontab_t not only search privileges inside bin_t directories, but also execute rights: corecmd_exec_bin(incrontab_t). We choose here to execute the editor inside the existing domain (incrontab_t) instead of creating a different domain for the editor for the following reasons:

  • If we would create a separate domain for the editor, the editor would eventually need to have major permissions, depending on when it is used. Editors can be used to modify the sudoers files, passwd files, the /etc/selinux/config file, etc. Instead, it makes much more sense to just be able to launch the editor in the current domain (which is much more confined to its specific purpose)
  • The additional privileges needed to launch the editor are usually very slim, or even nonexistent. It generally only makes sense if, by executing it, the existing domain would need many more privileges, because then a new (confined) domain keeps the privileges for the current domain low.

Let's see if things work now:

$ incrontab -e
(Editor opened, so I added in an incrontab line. Upon closing:)
cannot move temporary table: Permission denied

# tail audit.log
type=AVC msg=audit(1368729825.673:28237): avc:  denied  { dac_read_search } for  pid=9030 comm="incrontab" capability=2  scontext=user_u:user_r:incrontab_t tcontext=user_u:user_r:incrontab_t tclass=capability
type=AVC msg=audit(1368729825.673:28237): avc:  denied  { dac_override } for  pid=9030 comm="incrontab" capability=1  scontext=user_u:user_r:incrontab_t tcontext=user_u:user_r:incrontab_t tclass=capability

From a quick look through ps, I notice that the application runs as the user (luckily, otherwise I could use the editor to escape and get a root shell) after which it tries to do something. Of course, it makes sense that it wants to move this newly created incrontab file somewhere in /var/spool/incron so we grant it the permission to dac_read_search (which is lower than dac_override as explained before):

allow incrontab_t self:capability { dac_read_search setuid setgid };

On to the next failure:

$ incrontab -e 
cannot move temporary table: Permission denied

# tail audit.log
type=AVC msg=audit(1368730155.706:28296): avc:  denied  { write } for  pid=9088 comm="incrontab" name="incron" dev="dm-4" ino=19725 scontext=user_u:user_r:incrontab_t tcontext=root:object_r:incron_spool_t tclass=dir

Now the application wants to write this file there. Now remember we already have search_dir_perms permissions into incron_spool_t? We need to expand those with read/write permissions into the directory, and manage permissions on files (manage because users should be able to create, modify and delete their files). These two permissions are combined in the manage_files_pattern interface, and makes the search one obsolete:

manage_files_pattern(incrontab_t, incron_spool_t, incron_spool_t)

$ incrontab -e
...
table updated

Finally! And looking at the other options in incrontab, it seems that the policy for incrontab_t is finally complete, and looks like so:

###########################################
#
# incrontab policy
#

allow incrontab_t self:capability { setuid setgid dac_read_search };

manage_files_pattern(incrontab_t, incron_spool_t, incron_spool_t)

allow incrontab_t incrontab_tmp_t:file manage_file_perms;
files_tmp_filetrans(incrontab_t, incrontab_tmp_t, file)

corecmd_exec_bin(incrontab_t)

domain_use_interactive_fds(incrontab_t)

files_search_spool(incrontab_t)
files_search_tmp(incrontab_t)

auth_use_nsswitch(incrontab_t)

userdom_use_user_terminals(incrontab_t)

Next on the agenda: the incrond_t domain.


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