Tor And Selinux

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I’m experimenting with tor hidden services and got it to work nicely on my CentOS7, with tor from epel. That is, until I booted the machine. Then SELinux kicked in and in the logs there’s 

[warn] Directory /var/lib/tor/hidden_service/ cannot be read:
Permission denied

The permissions are drwx——.  2 toranon toranon    4096 Jan 28 23:39 hidden_service

And SELinux gives the following

SELinux is preventing /usr/bin/tor from using the dac_override capability.

*****  Plugin dac_override (91.4 confidence)
suggests   **********************

If you want to help identify if domain needs this access or you have a file with the wrong permissions on your system Then turn on full auditing to get path information about the offending file and generate the error again. Do

Turn on full auditing
# auditctl -w /etc/shadow -p w Try to recreate AVC. Then execute
# ausearch -m avc -ts recent If you see PATH record check ownership/permissions on file, and fix it, otherwise report as a bugzilla.

*****  Plugin catchall (9.59 confidence)
suggests   **************************

If you believe that tor should have the dac_override capability by default. Then you should report this as a bug. You can generate a local policy module to allow this access. Do allow this access for now by executing:
# ausearch -c ‘tor’ –raw | audit2allow -M my-tor
# semodule -i my-tor.pp

Additional Information:
Source Context                system_u:system_r:tor_t:s0
Target Context                system_u:system_r:tor_t:s0
Target Objects                Unknown [ capability ]
Source                        tor Source Path                   /usr/bin/tor Port                          
Source RPM Packages           tor-
Target RPM Packages           
Policy RPM                    selinux-policy-3.13.1-102.el7_3.13.noarch Selinux Enabled               True Policy Type                   targeted Enforcing Mode                Enforcing Host Name                     host Platform                      Linux host 3.10.0-514.6.1.el7.x86_64
                              #1 SMP Wed Jan 18 13:06:36 UTC 2017
x86_64 x86_64
Alert Count                   5
First Seen                    2017-01-29 22:42:46 EST
Last Seen                     2017-01-29 22:42:51 EST
Local ID                      51ceb58e-19cf-4f8f-ab1e-fe48265aaf1d

Raw Audit Messages type=AVC msg=audit(1485747771.709:106): avc:  denied  { dac_override }
for  pid=2253 comm=”tor”
capability=1  scontext=system_u:system_r:tor_t:s0
tcontext=system_u:system_r:tor_t:s0 tclass=capability

type=AVC msg=audit(1485747771.709:106): avc:  denied  { dac_read_search
} for  pid=2253 comm=”tor”
capability=2  scontext=system_u:system_r:tor_t:s0
tcontext=system_u:system_r:tor_t:s0 tclass=capability

type=SYSCALL msg=audit(1485747771.709:106): arch=x86_64 syscall=open success=no exit=EACCES a0=7fcd2c12fe90 a1=20000 a2=0 a3=1 items=0
ppid=1 pid=2253 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0
egid=0 sgid=0 fsgid=0 tty=(none) ses=4294967295 comm=tor exe=/usr/bin/tor subj=system_u:system_r:tor_t:s0 key=(null)


As I don’t know what dac_override is I don’t know if it’s a good idea to give it to tor and the confidence seems quite low. Cheers Mark

3 thoughts on - Tor And Selinux

  • dac_override indicates that you’re running your process as root, and it’s trying to do something on the filesystem which is not explicitly allowed by permissions. DAC is the standard POSIX permission system, and the process is trying to override it. DAC allows access to the toranon user and toranon group only, and the process is trying to override that access by way of root access.

    I’d say that no, giving dac_override is not a good idea, but mostly because that implies that you’d continue running the process as root.
    You should be running the service as the “toranon” user instead, in which case it will not need dac_override.

  • That’s strange, because I started the tor process simply with sudo systemctl start tor

    The only changes I’ve really made was to add two lines of configuration in torrc

    Cheers Mark

  • Yes, it looks like that package runs the service as “root”. That doesn’t seem like a good default, and it could be a packaging bug. Try adding “User toranon” to the configuration file and see if things work.
    If so, file a bug so that the maintainer can change the default.