Re: lnk_file read permission

["Christian Göttsche" <cgzones@googlemail.com>]

Am Fr., 31. Juli 2020 um 12:03 Uhr schrieb Gionatan Danti <g.danti@assyoma.it>:
>
> Dear list,
> I am writing this email as suggested here:
> https://lists.fedoraproject.org/archives/list/selinux@lists.fedoraproject.org/message/GWEWGDUQS6PERAYEJHL2EE4GDO432IAO/
>
> To recap: I have issue with selinux permission when relocating specific
> daemon data directory, and using symlink in the original location. For
> example, lets consider moving /var/lib/mysql in a new, bigger volume.
>
> After moving /var/lib/mysql in /data/lib/mysql and creating a symlink
> for the new location, I used semanage fcontext to add the relative
> equivalency rules. Moreover, I changed my.cnf to explicitly point to the
> new data dir and socket file. So far, so good.
>
> When restarting apache, I noticed it can't connect to mysql. ausearch -m
> avc showed the following:
> ...
> type=AVC msg=audit(1596055762.070:175569): avc:  denied  { read } for
> pid=72946 comm="httpd" name="mysql" dev="sda2" ino=103
> scontext=system_u:system_r:httpd_t:s0
> tcontext=system_u:object_r:mysqld_db_t:s0 tclass=lnk_file permissive=0
>
> The log above clearly states that httpd policy lacks lnk_read permission
> for mysqld_db_t type. While I solved the issue by leaving the socket
> file inside the original directory (removing the /var/lib/mysql symlink
> and recreating the mysql dir), I was wondering why each symlink type is
> specifically allowed
> rather than giving any processes a generic access to symlinks.
>
> Is this kind of rule not permitted by selinux? Can it open the door to
> other attacks? If so, why? Generally, what is the least invasive
> approach to relocate services?
>

An alternative would be, since these symlinks are trusted and
permanent, to label them as their parent directory (e.g. var_lib_t
(use the '-l' file type specifier)) and allow the applications to read
these lnk types.
This also prevents e.g. mysqld_t to alter the symlink /var/lib/mysqld
(since it probably has write permission to mysql_db_t:lnk_file but not
var_lib_t:lnk_file).