I had a conversation about this on OFTC #tor:

<ketyn> Is it intended that tor doesn't set supplementary groups when starting as root and then doing setgid? This describes my issue: https://tor.stackexchange.com/questions/23225/tor-cant-read-hiddenservicepor... <ketyn> I see many threads on the internet of people claiming that tor unix socket permissions aren't working for them and while sometimes it turns out to be apparmor/SELinux, I think this might be biting other people too <ketyn> I think significant numbers of people might just be giving up and making their unix socket globally readable, or switching to numbered port based sockets rather than unix domain sockets. <arma> ketyn: we had a series of security surprises in the past with how we did groups and dropping privs, <arma> and we settled on what we do now as the safest option <ketyn> Alright, I guess I'll need to either make the sockets tor listens to owned by the tor user, or use socat to forward. Maybe there's a documentation fix to apply here though. <ketyn> tor forwards to* <bentham> ketyn: What are you doing that would make that necessary? <bentham> Is the default not working for you for some reason? <bentham> Also, which socket(s) is(are) causing issues for you? Is it something in /run/tor? <ketyn> I want to make a tor hidden service listening to port 25, connected to a socket at /var/spool/postfix-test/public/smtpd owned by the user postfix-queue <ketyn> According to postfix, I'm then supposed to add whatever user I want to read to that socket to the group postfix-queue <arma> ketyn: hmmm <arma> for the control socket see the GroupWritable flags <arma> ControlSocket /run/tor/control GroupWritable RelaxDirModeCheck <arma> ControlSocketsGroupWritable 1 <arma> this is how you can add your username to the tor group and then have permissions to connect with a controller <ketyn> This is not a control socket. This is a hidden service. e.g. HiddenServicePort 25 unix:/var/spool/postfix-test/public/smtpd <arma> i don't think anybody has built something similar for the onion service lines <arma> but i think that tor is not dropping the groups, it simply is not letting you use the group permission there <ketyn> I want to be able to receive email over a hidden service. Like in https://github.com/ehloonion/onionmx <ketyn> > <arma> but i think that tor is not dropping the groups, it simply is not letting you use the group permission there <ketyn> Huh, ok. <arma> i say that because being in the group, for the control port, does work <arma> so it's not like the group got dropped <arma> check out the UnixSocksGroupWritable config option <arma> maybe it will help what you want <bentham> ketyn: Out of curiosity, why is your smtpd listening on a socket rather than, say, 127.0.0.1:25? <arma> looks like no, it won't do what you want, because UnixSocksGroupWritable doesn't apply to hiddenservice lines <ketyn> bentham: Because it seems more secure and means I don't have to worry about port conflicts or firewalls, and deploy this setup to computers I don't have full control over. <ketyn> Here's another good reason I saw: https://github.com/freedomofpress/securedrop/issues/1261 <ketyn> "because applications sometimes (fairly) assume that localhost has a different threat model than an externally exposed service. Most recently, this common configuration lead to some hidden services exposing sensitive information through the Apache mod_status module" ... <arma> actually arma said "just barely possible that the network team would take a good patch" <arma> not quite the same as "probably not take" <arma> but yes, more generally, there is vast confusion about what permissions are *supposed* to be, what groups happen by default in debian/ubuntu, what the SELinux and Apparmor policies do, etc <arma> i think most people are probably doing it wrong but i don't even know what right is <arma> i end up putting all my onion service directories in /tmp/ so they work, and that cannot at all be right

I also asked on stackexchange: https://tor.stackexchange.com/questions/23225/tor-cant-read-hiddenservicepor...

------- Original Message ------- On Tuesday, July 5th, 2022 at 5:30 AM, keyandthegate via tor-dev tor-dev@lists.torproject.org wrote:

Permissions are set so tor should be able to access through the `postfix-test-queue` user:

...

$ sudo ls -l /var/spool/postfix-test/public/smtpd srw-rw-rw- 1 postfix-test postfix-test 0 █████ /var/spool/postfix-test/public/smtpd $ sudo ls -l /var/spool/postfix-test # ... drwx--x--- 2 postfix-test postfix-test-queue 4096 █████ public $ sudo -u _tor-test id uid=130(_tor-test) gid=141(_tor-test) groups=141(_tor-test),1006(postfix-test-queue) $ cat /etc/tor/instances/test/torrc | grep HiddenServicePort HiddenServicePort 25 unix:/var/spool/postfix-test/public/smtpd $ cat /run/tor-instances/test.defaults | grep User User _tor-test

Running `tor@test` via the default systemctl config shows:

...

$ ps -ax -o uid,gid,supgid,command | grep /usr/bin/tor 130 141 141 /usr/bin/tor --defaults-torrc /run/tor-instances/test.defaults -f /etc/tor/instances/test/torrc

Which is missing the `postfix-test-queue` `1006` user which, for example shows up if I do:

...

$ sudo -u _tor-test sleep 1000 & ps -ax -o uid,gid,supgid,command | grep sleep [1] 132314 0 141 141,1006 sudo -u _tor-test sleep 1000

Connecting using `sudo -u` works (the message indicates successful connection):

...

$ sudo -u _tor-test curl --unix-socket /var/spool/postfix-test/public/smtpd http://localhost curl: (1) Received HTTP/0.9 when not allowed

But connecting via tor does not:

...

$ torsocks --ipv6 curl http://%E2%96%88%E2%96%88%E2%96%88%E2%96%88%E2%96%88.onion:25 █████ ERROR torsocks[134873]: Host unreachable (in socks5_recv_connect_reply() at socks5.c:539) curl: (7) Couldn't connect to server

But does if I allow access to the socket to everyone:

...

$ sudo chmod "o+x" /var/spool/postfix-test/public/ $ torsocks --ipv6 curl http://%E2%96%88%E2%96%88%E2%96%88%E2%96%88%E2%96%88.onion:25 curl: (1) Received HTTP/0.9 when not allowed

Tor's relevant source code: [Tor: lib/process/setuid.c Source File](https://tpo.pages.torproject.net/core/doc/tor/setuid_8c_source.html)