journalctl starts a pager when its output is a terminal, and from less a "!sh"
is a shell with whatever privileges journalctl was given. That is the standard
journalctl escalation, and these rules end in a wildcard:
<user> ALL=(ALL) NOPASSWD: /usr/bin/journalctl -u ledmatrix *
Nothing this project runs needs the pager -- both call sites pass --no-pager,
in web_interface/app.py and api_v3.py. But a sudoers rule cannot require a flag
that sits in the middle of a command line, and reasoning about what a trailing
wildcard does and does not admit is exactly the kind of subtlety that produces
a hole. sudo's NOEXEC tag stops the command executing another program at all,
which closes it without depending on that reasoning.
NOEXEC works by LD_PRELOAD, so it applies to dynamically linked binaries.
Checked on the target hardware: journalctl there is dynamically linked. The
generated rules were run through `visudo -c` -- parsed OK.
Found while auditing the pre-existing wildcard grants, prompted by review
catching a far worse one I had added myself in the same area: `iptables *`,
where --modprobe runs an arbitrary path as root.
Reachability, stated plainly: on a stock Raspberry Pi image none of this
matters, because 010_pi-nopasswd already grants the default user
`ALL=(ALL) NOPASSWD: ALL`. It matters on a hardened install, or where the
service runs as a user without that blanket rule.
Two mutation checks: dropping NOEXEC from a rule fails, and deleting the rules
rather than tagging them fails too -- that second one matters, since "make the
test pass" and "remove the feature" would otherwise look the same.
