The chown at startup was the only thing that ever needed root here, and it
served a case this project does not have: keys.dat arriving from a bind mount
owned by somebody else. Every deployment uses a named volume, which takes its
ownership from the image. So the chown goes, and everything that existed to
survive it goes with it.
USER bitmessage in the Dockerfile, from PID 1 onwards. drop_privs.py is
deleted, the supervisor no longer re-executes itself with a trimmed bounding
set, run.sh has no privileged prologue and no wrapper around the eight
commands that used to run through one. What is left of run.sh differs from
the version before any of this by eleven lines: two chowns gone, seven `gosu
bitmessage` prefixes gone, one comment reworded.
keys.dat gets its mode 600 at build time instead of on every start, because
on every start there is now no root to set it. Its mode and ownership reach a
fresh volume from the image, and every volume in service already carries them
-- checked on all four live nodes: nothing under /home/bitmessage is owned by
anyone but 2000.
The setuid strip stays. It is two lines and it closes the one way a taken-over
daemon could still have climbed.
What the caller sets changes too, and in the right direction: `cap_drop: ALL`
with nothing added back, where the previous commit needed seven capabilities
handed in. Confinement that used to be split between the image and the caller
now sits in one place. The image gives up defending itself when run with no
options at all, which is the trade named in the README along with the bind
mount it costs.
PyBitmessage is a Python 2 daemon parsing untrusted data from an open port,
so the question is not whether it can be taken over but what is left once it
has been. Until now: uid 2000, but the full fourteen capabilities Docker
hands a container in the bounding set, no_new_privs off, and a root PID 1
holding all fourteen for the life of the container.
run.sh now needs root exactly once. keys.dat can arrive from a bind mount
owned by anyone, so it is chowned, given mode 600 and read first; the block
that does it ends by re-executing the file with a bounding set of four --
SETUID and SETGID to start the daemon as its own user, KILL for the fallback
stop, SETPCAP to drop the rest. Every start of the daemon then goes through
setpriv rather than gosu: uid 2000, every capability set empty, no_new_privs
on. gosu changed the user and left everything else alone; moreutils went with
it, nothing here ever called any of its tools.
No file in the image carries a setuid or setgid bit any more, which is what
makes no_new_privs worth having: there is nothing left to climb.
Both setpriv lines are checked at build time, in the arrangement run.sh uses,
against uid, capability sets and no_new_privs read back from /proc. The base
image and the PyBitmessage clone are both unpinned, so an option that
quietly changed meaning would otherwise ship as a container that looks
confined and is not. Two things that check caught while it was being written:
Ubuntu 18.04's setpriv refuses the "all" keyword under a kernel that knows
more capabilities than its headers did (40 against 37), and capability names
there carry no cap_ prefix. Hence the lists written out by hand.
A caller that sets cap_drop: ALL now needs seven back, not six: SETPCAP joins
CHOWN, DAC_OVERRIDE, FOWNER, SETUID, SETGID and KILL, because dropping a
bounding set takes it. The example compose file and both READMEs say so, and
say what else only a caller can set: read_only with tmpfs, and pids_limit.
The daemon has always listened on 8444 and nothing published it, so every
node built from this image was outbound-only. Publishing it is now a
documented choice rather than an omission -- including the two things that
are not obvious from the config: peers are told the port from `port` in
keys.dat rather than the one you mapped it to (so only 8444:8444 works),
and the node's own address is never configured at all, because every peer
replaces the hardcoded 127.0.0.1 in the version message with the IP it
sees on the socket.
An open port needs a brake, hence BITMESSAGE_MAXTOTALCONNECTIONS. It
defaults to the PyBitmessage stock 200, so nothing changes for existing
users of the image. The key is inserted when the stock config lacks it
instead of trusting the substitution: the PyBitmessage clone is unpinned,
and a silent no-op would ship a node that looks capped and is not.
The API port in the example compose moves to loopback, which is what the
README already prescribed.
bitdeals
bitdeals git user
private-user