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Joachim Wiberg
61b0e0f3e6
Apply a PAM session to run/task/sysv/services Finit starts, pam_limits
above all, so a service running as a given user picks up that user's
limits the way a login does.
Add a new `pam` setting for the new block format (only), like the
per-service directories, naming a file in /etc/pam.d:
service weston {
user = "weston"
pam = "weston-autologin"
command = "/usr/bin/weston --continue-without-input"
}
pam_close_session() has to be called by a process still holding the
handle, and the handle does not survive exec(). Hence the keeper: it
holds the handle, drops to the service's credentials, and waits for a
parent-death signal before closing the session. Same shape as
systemd's (sd-pam), for the same reason, and one per fork, so the
script hooks open and close their own.
The keeper closes the descriptors it inherited from Finit and only
those. Closing everything would also take out what pam_open_session()
opened for itself, a keyring fd or a lock file, and leave the modules
to close a session with those pulled out from under them. Closing
nothing, as (sd-pam) does, would leave it holding the write end of the
notify pipe for the service's whole lifetime and starve notify = "s6"
services of their ready signal. So the fds open before pam_start()
are snapshotted and exactly those are closed, while the ones PAM opens
after are marked close-on-exec so the daemon does not inherit them
either.
A refused value, a denied account stack, an uninstalled pam.d file,
and a build without PAM support all keep the service from starting
rather than running it with the stacks skipped: one that quietly loses
pam_limits and its private /tmp, with nothing said. Capabilities a
module like pam_cap.so granted are merged into the IAB Finit applies
instead of being replaced by it, which only helps a service that also
sets capabilities, the other arm being a plain setuid() with nothing
left to restore once permitted is empty.
The test sysroot gains pam_permit.so, pam_deny.so and pam_limits.so,
which ldd cannot see, libpam dlopen()s them, and the test skips when
the host has none to stage. The negative cases pin the exit status
rather than only asserting crashed, which serv reports for any early
exit, so a bad command or an unwritable pidfile cannot pass for a
rejected session.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
152 lines
5.9 KiB
Markdown
152 lines
5.9 KiB
Markdown
PAM Sessions
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============
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`pam = "NAME"` runs a service inside a PAM session set up from
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`/etc/pam.d/NAME`. The `session` stack in that file runs for the
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process that goes on to become the daemon, so modules like `pam_limits`,
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`pam_env`, or `pam_keyinit` see the service the way they see a login.
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## Basic Usage
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A display server that needs the session a login would have arranged for
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it:
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```conf
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service weston {
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user = "weston"
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pam = "weston-autologin"
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command = "/usr/bin/weston --continue-without-input"
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}
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```
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With `/etc/pam.d/weston-autologin`:
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```
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auth required pam_permit.so
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account required pam_unix.so
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session required pam_unix.so
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session required pam_limits.so
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```
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The `auth` line is needed even though nothing is ever authenticated.
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Opening the session goes through `pam_setcred()`, which consults the
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`auth` stack, and an empty stack comes back as a permission denial, so a
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pam.d file with only `account` and `session` lines keeps the service
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from starting.
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The value names a file in `/etc/pam.d`, it is not a path. A value
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holding `/` or `..`, or one too long to fit, is refused, with an error
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in the log when the .conf file is read. The service does not start
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either, `initctl status` reports it as missing. Like the per-service
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directory keys, `pam` exists in the block format only.
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## No Authentication
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Finit runs the account and session stacks, `pam_acct_mgmt()`,
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`pam_setcred()`, and `pam_open_session()`, and never
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`pam_authenticate()`. A module that asks a question gets a
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conversation error back and its entry in the stack fails.
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A service whose account is denied, an expired account say, does not
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start, and neither does one naming a pam.d file that is not installed.
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The child exits 71 (`EX_OSERR`), `initctl status` shows the service as
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crashed, and PAM's own reason is in the log.
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## Requirements
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PAM support is built by default, see [Building Finit](../build.md).
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Without it, e.g. after `--disable-pam`, a service declaring `pam` is
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refused rather than started with the stacks skipped:
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weston: pam weston-autologin requires Finit built with --enable-pam
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## Which Blocks Take It
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`service`, `task`, `run`, and `sysv`. Not `tty`: `login` opens a
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session of its own there.
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Every fork gets its own session, so the `pre:`, `post:`, `ready:`, and
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`cleanup:` scripts each open and close one too, as do the stop and
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reload scripts and the `stop` call on a `sysv` script.
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A refused value stops those too. A `pre:` script forks before the
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start-time check runs, so it exits 71 without running instead of the
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service being reported missing.
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## The User
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`user` decides who the session is for. Without it the session is for
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root.
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systemd documents the opposite for its equivalent. `systemd.exec(5)`
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says `PAMName=` is "only useful in conjunction with the `User=`
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setting, and is otherwise ignored". That was true of systemd up to and
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including v256, v257 changed it to open a session for the manager's own
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user, and the man page was never updated. Finit does what v257 does.
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A service with a [controlling tty](tty.md#controlling-tty-for-services)
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has it passed to PAM as `PAM_TTY`, for the modules that care which
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terminal a session is on.
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## Precedence
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Three places where PAM and a Finit setting cover the same ground:
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- `pam_limits` overrides a per-service `rlimit`. A block asking for
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`nofile = 4096` under a `limits.conf` that says 512 gets 512.
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- `pam_env` overrides Finit's own environment defaults, `PATH`, `USER`,
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`LOGNAME`, and `HOME`, and `envfile` in turn overrides `pam_env`. A
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`HOME` from the session stack also moves the working directory, which
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otherwise is the home directory from `/etc/passwd`.
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- The groups from `/etc/group` and `extra-groups` override `pam_group`.
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## The Session Keeper
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A helper is forked next to the service to close the session when the
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service exits, `(finit-pam)`:
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```
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CGroup : /system/weston cpu 0 [100, max] mem [--.--, max]
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|- 312 /usr/bin/weston --continue-without-input
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`- 313 (finit-pam)
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```
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There is one per fork. It runs as the service's user, in the service's
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cgroup, and stopping the service takes it along.
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A daemon that reaps children in its own `wait()` loop will see a child
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it never forked. systemd has the same property, with `(sd-pam)`.
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## Limitations
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- `type = "forking"` closes the session early. The initial process
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exits by design, the keeper's parent-death signal fires with it, and
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the session is closed while the real daemon runs on. Finit warns
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about the combination when it reads the .conf file, and starts the
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service anyway:
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/etc/finit.d/foo.conf: foo: pam with type = forking closes the
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session when the initial process exits
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- A daemon whose initial thread exits while the process lives closes the
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session the same way. The parent-death signal follows the thread that
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forked the keeper, not the process.
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- A capability granted by `pam_cap.so` is only kept when the service
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also sets [`capabilities`](capabilities.md).
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- The keeper shares the service's cgroup, so an empty cgroup directory
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can outlive a stop until the next start reuses it. Cosmetic.
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- A module that blocks has no time bound. `pam_open_session()` waits
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for as long as the module does, `pam_ldap` against an unreachable
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server, say, or `pam_mount` on a hung network mount. The fork has already succeeded by then, so the service
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reaches the running state and stays there with no daemon behind it.
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Nothing crashes and nothing restarts.
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## See Also
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- [Service Options](service-opts.md) - the other run/task/service keys
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- [Linux Capabilities](capabilities.md) - the key `pam_cap.so` needs
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- [Building Finit](../build.md) - `--disable-pam` and its dependency
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- [pam(8)](https://man7.org/linux/man-pages/man8/pam.8.html) - the PAM library
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- [pam.d(5)](https://man7.org/linux/man-pages/man5/pam.d.5.html) - the file format
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- [pam_limits(8)](https://man7.org/linux/man-pages/man8/pam_limits.8.html) - limits from `limits.conf`
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