install: Add support for --root-ssh-authorized-keys

The current `bootc install` model is VERY opinionated: we install
the running container image to disk, and that is (almost!) it.

The only non-container out of band state that we support injecting right now
is kargs (via `--karg`) - we know we need this for machine local
kernel arguments.

(We do have a current outstanding PR to add a highly generic mechanism
 to inject arbitrary files in `/etc`, but I want to think about that more)

THis current strict stance is quite painful for
a use case like "take a generic container image and bootc install to-filesystem --alongside"
in a cloud environment, because the generic container may not
have cloud-init.

With this change it becomes extremely convenient to:

- Boot generic cloud image (e.g. AMI with apt/dnf + cloud-init)
- cloud-init fetches SSH keys from hypervisor (instance metadata)
- podman run -v /root/.ssh/authorized_keys:/keys:ro <image> bootc install ... --root-ssh-authorized-keys=/keys`

And then the instance will carry forward those hypervisor-provided
keys but without a dependency on cloud-init.

Another use case for this of course is being the backend of things
like Anaconda's kickstart verbs which support injecting SSH keys.

Signed-off-by: Colin Walters <[email protected]>

.github/workflows/ci.yml

@@ -132,11 +132,13 @@ jobs:
       - name: Integration tests
         run: |
           set -xeuo pipefail
+          echo 'ssh-ed25519 ABC0123 [email protected]' > test_authorized_keys
           sudo podman run --rm -ti --privileged --env RUST_LOG=debug -v /:/target -v /var/lib/containers:/var/lib/containers -v ./usr/bin/bootc:/usr/bin/bootc --pid=host --security-opt label=disable \
             quay.io/centos-bootc/fedora-bootc-dev:eln bootc install to-filesystem \
-            --karg=foo=bar --disable-selinux --replace=alongside /target
+            --karg=foo=bar --disable-selinux --replace=alongside --root-ssh-authorized-keys=test_authorized_keys /target
           ls -al /boot/loader/
           sudo grep foo=bar /boot/loader/entries/*.conf
+          grep authorized_keys /ostree/deploy/default/deploy/*/etc/tmpfiles.d/bootc-root-ssh.conf
           # TODO fix https://github.com/containers/bootc/pull/137
           sudo chattr -i / /ostree/deploy/default/deploy/*
           sudo rm /ostree/deploy/default -rf

lib/src/install.rs

@@ -8,6 +8,7 @@
 // and filesystem setup.
 pub(crate) mod baseline;
 pub(crate) mod config;
+pub(crate) mod osconfig;
 
 use std::io::BufWriter;
 use std::io::Write;
@@ -132,6 +133,16 @@ pub(crate) struct InstallConfigOpts {
     /// Add a kernel argument
     karg: Option<Vec<String>>,
 
+    /// The path to an `authorized_keys` that will be injected into the `root` account.
+    ///
+    /// The implementation of this uses systemd `tmpfiles.d`, writing to a file named
+    /// `/etc/tmpfiles.d/bootc-root-ssh.conf`.  This will have the effect that by default,
+    /// the SSH credentials will be set if not present.  The intention behind this
+    /// is to allow mounting the whole `/root` home directory as a `tmpfs`, while still
+    /// getting the SSH key replaced on boot.
+    #[clap(long)]
+    root_ssh_authorized_keys: Option<Utf8PathBuf>,
+
     /// Perform configuration changes suitable for a "generic" disk image.
     /// At the moment:
     ///
@@ -261,6 +272,8 @@ pub(crate) struct State {
     pub(crate) config_opts: InstallConfigOpts,
     pub(crate) target_imgref: ostree_container::OstreeImageReference,
     pub(crate) install_config: config::InstallConfiguration,
+    /// The parsed contents of the authorized_keys (not the file path)
+    pub(crate) root_ssh_authorized_keys: Option<String>,
 }
 
 impl State {
@@ -596,6 +609,10 @@ async fn initialize_ostree_root_from_self(
     }
     f.flush()?;
 
+    if let Some(contents) = state.root_ssh_authorized_keys.as_deref() {
+        osconfig::inject_root_ssh_authorized_keys(&root, contents)?;
+    }
+
     let uname = rustix::system::uname();
 
     let labels = crate::status::labels_of_config(&imgstate.configuration);
@@ -944,6 +961,14 @@ async fn prepare_install(
     let install_config = config::load_config()?;
     tracing::debug!("Loaded install configuration");
 
+    // Eagerly read the file now to ensure we error out early if e.g. it doesn't exist,
+    // instead of much later after we're 80% of the way through an install.
+    let root_ssh_authorized_keys = config_opts
+        .root_ssh_authorized_keys
+        .as_ref()
+        .map(|p| std::fs::read_to_string(p).with_context(|| format!("Reading {p}")))
+        .transpose()?;
+
     // Create our global (read-only) state which gets wrapped in an Arc
     // so we can pass it to worker threads too. Right now this just
     // combines our command line options along with some bind mounts from the host.
@@ -954,6 +979,7 @@ async fn prepare_install(
         config_opts,
         target_imgref,
         install_config,
+        root_ssh_authorized_keys,
     });
 
     Ok(state)

lib/src/install/osconfig.rs

@@ -0,0 +1,41 @@
+use anyhow::Result;
+use camino::Utf8Path;
+use cap_std::fs::Dir;
+use cap_std_ext::{cap_std, dirext::CapStdExtDirExt};
+use fn_error_context::context;
+
+const ETC_TMPFILES: &str = "etc/tmpfiles.d";
+const ROOT_SSH_TMPFILE: &str = "bootc-root-ssh.conf";
+
+#[context("Injecting root authorized_keys")]
+pub(crate) fn inject_root_ssh_authorized_keys(root: &Dir, contents: &str) -> Result<()> {
+    // While not documented right now, this one looks like it does not newline wrap
+    let b64_encoded = ostree_ext::glib::base64_encode(contents.as_bytes());
+    // See the example in https://systemd.io/CREDENTIALS/
+    // HOWEVER: I chose to use `f` and not `f~` in order to allow this to be overwritten
+    // by something else later - e.g. for later key rotation support.  The real more ideal
+    // model here would be to have e.g .`/usr/lib/ssh/authorized_keys/root` that would be
+    // *merged at runtime* with the home directory (if present) - but would be clearly immutable
+    // underneath `/usr`.
+    let tmpfiles_content = format!("f /root/.ssh/authorized_keys 600 root root - {b64_encoded}");
+
+    let tmpfiles_dir = Utf8Path::new(ETC_TMPFILES);
+    root.create_dir_all(tmpfiles_dir)?;
+    let target = tmpfiles_dir.join(ROOT_SSH_TMPFILE);
+    root.atomic_write(target, &tmpfiles_content)
+        .map_err(Into::into)
+}
+
+#[test]
+fn test_inject_root_ssh() -> Result<()> {
+    let root = &cap_std_ext::cap_tempfile::TempDir::new(cap_std::ambient_authority())?;
+
+    inject_root_ssh_authorized_keys(root, "ssh-ed25519 ABCDE example@demo\n").unwrap();
+
+    let content = root.read_to_string(format!("etc/tmpfiles.d/{ROOT_SSH_TMPFILE}"))?;
+    assert_eq!(
+        content,
+        "f /root/.ssh/authorized_keys 600 root root - c3NoLWVkMjU1MTkgQUJDREUgZXhhbXBsZUBkZW1vCg=="
+    );
+    Ok(())
+}