Granting a capability to a service

[Andrew Lutomirski]

On Jul 18, 2015 1:42 AM, "Florian Weimer" <fweimer at redhat.com> wrote:
>
> Let's assume I want to start a service as an ordinary user, but allow to
> bind it to a privileged port.  The program implementing the service does
> not manipulate capabilities in any way.
>
> I came up with with this system unit for testing purposes:
>
> [Unit]
> Description=Test unit
>
> [Service]
> Type=oneshot
> ExecStart=/usr/sbin/getpcaps self
> Capabilities=cap_net_bind_service+ep
> SecureBits=keep-caps
> User=fweimer
> StandardOutput=journal
>
> However, this does not work, the capability set remains empty.  Is there
> a way to achieve what I want?
>
> The algorithm documented in capabilities(7) suggests that retaining
> effective capabilities across an execve system call absolutely requires
> file capabilities (the inheritable part).  The only way to bypass that
> if you perform the execve call with UID 0 (i.e., the literal UID 0, not
> a capability).
>
> This design is really odd because setting file capabilities always
> increases the attack surface (even if it is just the inheritable bits),
> and the only alternative appears to modify the service so that it is
> capability-aware and switches away from UID 0, and grant sufficient
> capabilities so that it can do so.  At that point, you can just skip the
> configuration in the systemd service and do everything capablity-related
> within the program.
>
> What am I missing?

Nothing.  Inheritable capabilities are nearly useless.

The thing you want is called "ambient" capabilities and should be available
in Linux 4.3.

--Andy

>
> --
> Florian Weimer / Red Hat Product Security
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