Tag: root

[Howto] Run programs as non-root user on privileged ports via Systemd [Update]

TuxRunning programs as a non-root user is must in security sensitive environments. However, these programs sometimes need to publish their service on privileged ports like port 80 – which cannot be used by local users. Systemd offers a simple way to solve this problem.

Background

Running services as non-root users is a quite obvious: if it is attacked and a malicious user gets control of the service, the rest of the system should still be secure in terms of access rights.

At the same time plenty programs need to publish their service at ports like 80 or 443 since these are the default ports for http communication and thus for interfaces like REST. But these ports are not available to non-root users.

Problem shown at the example gitea

To show how to solve this with systemd, we take the self hosted git service gitea as an example. Currently there are hardly any available packages, so most people end up installing it from source, for example as the user git. A proper sysmted unit file for such an installation in a local path, running the service as a local user, is:

$ cat /etc/systemd/system/gitea.service
[Unit]
Description=Gitea (Git with a cup of tea)
After=syslog.target
After=network.target
After=postgresql.service

[Service]
RestartSec=2s
Type=simple
User=git
Group=git
WorkingDirectory=/home/git/go/src/code.gitea.io/gitea
ExecStart=/home/git/go/src/code.gitea.io/gitea/gitea web
Restart=always
Environment=USER=git HOME=/home/git

[Install]
WantedBy=multi-user.target

If this service is started, and the application configuration is set to port 80, it fails during the startup with a bind error:

Jan 04 09:12:47 gitea.qxyz.de gitea[8216]: 2018/01/04 09:12:47 [I] Listen: http://0.0.0.0:80
Jan 04 09:12:47 gitea.qxyz.de gitea[8216]: 2018/01/04 09:12:47 [....io/gitea/cmd/web.go:179 runWeb()] [E] Failed to start server: listen tcp 0.0.0.0:80: bind: permission denied

Solution

One way to tackle this would be a reverse proxy, running on port 80 and forwarding traffic to a non-privileged port like 8080. However, it is much more simple to add an additional systemd socket which listens on port 80:

$ cat /etc/systemd/system/gitea.socket
[Unit]
Description=Gitea socket

[Socket]
ListenStream=80
NoDelay=true

As shown above, the definition of a socket is straight forward, and hardly needs any special configuration. We use NoDelay here since this is a default for Go on sockets it opens, and we want to imitate that.

Given this socket definition, we add the socket as requirement to the service definition:

[Unit]
Description=Gitea (Git with a cup of tea)
Requires=gitea.socket
After=syslog.target
After=network.target
After=postgresql.service

[Service]
RestartSec=2s
Type=simple
User=git
Group=git
WorkingDirectory=/home/git/go/src/code.gitea.io/gitea
ExecStart=/home/git/go/src/code.gitea.io/gitea/gitea web
Restart=always
Environment=USER=git HOME=/home/git
NonBlocking=true

[Install]
WantedBy=multi-user.target

As seen above, the unit definition hardly changes, only the requirement for the socket is added – and NonBlocking as well, to imitate Go behavior.

That’s it! Now the service starts up properly and everything is fine:

[...]
Jan 04 09:21:02 gitea.qxyz.de gitea[8327]: 2018/01/04 09:21:02 Listening on init activated [::]:80
Jan 04 09:21:02 gitea.qxyz.de gitea[8327]: 2018/01/04 09:21:02 [I] Listen: http://0.0.0.0:80
Jan 04 09:21:08 gitea.qxyz.de gitea[8327]: [Macaron] 2018-01-04 09:21:08: Started GET / for 192.168.122.1
[...]

The Gitea documentation covers more examples of possible systemd service configurations, including the usage of capabilities instead of sockets.

Sources, further reading

[Short Tip] Workaround MIT-SHM error when starting QT/KDE apps with SUDO

Gnomelogo.svg

Starting GUI programs as root usually is not a problem. In worst case, sudo inside a terminal should do the trick.

However, recently I had to start a QT application as sudo from within GNOME. It was the yubikey configuration GUI, a third party tool thus not part of any desktop environment. Executing the app failed, it only showed a gray window and multiple errors in the command line:

$ sudo /usr/bin/yubikey-personalization-gui 
X Error: BadAccess (attempt to access private resource denied) 10
  Extension:    130 (MIT-SHM)
  Minor opcode: 1 (X_ShmAttach)
  Resource id:  0x142
X Error: BadShmSeg (invalid shared segment parameter) 128
  Extension:    130 (MIT-SHM)
  Minor opcode: 5 (X_ShmCreatePixmap)
  Resource id:  0xfa
X Error: BadDrawable (invalid Pixmap or Window parameter) 9
  Major opcode: 62 (X_CopyArea)
  Resource id:  0x2800015

Workarounds like pkexec and adding a policykit rule didn’t help, either. The error indicates that there is a problem with the MIT Shared Memory Extension of X.

A good workaround is to deactivate the usage of the extension on command line:

$ sudo QT_X11_NO_MITSHM=1 /usr/bin/yubikey-personalization-gui

It works like a charm.