hyperhive/swarm-secret-client/src/client.rs

//! A logged-in handle on the store, built from this deployment's environment.

use rustify_derive::Endpoint;
use serde::{Serialize, de::DeserializeOwned};
use vaultrs::client::{Client, VaultClient, VaultClientSettingsBuilder};

use crate::{Error, path::MOUNT};

/// The store's address.
pub const ENV_ADDR: &str = "BAO_ADDR";
/// PEM client certificate presented to the store's listener.
pub const ENV_CLIENT_CERT: &str = "BAO_CLIENT_CERT";
/// PEM private key for [`ENV_CLIENT_CERT`].
pub const ENV_CLIENT_KEY: &str = "BAO_CLIENT_KEY";
/// CA bundle the store's own certificate is verified against. Optional:
/// absent means the system trust store, which is what a deployment with a
/// real CA wants and what a self-signed one must not be left with.
pub const ENV_CACERT: &str = "BAO_CACERT";

/// The auth mount a cert login goes through, unless a caller says otherwise.
pub const DEFAULT_CERT_MOUNT: &str = "cert";

/// Where the store is and which identity we present to it.
///
/// Separate from the connect so the environment can be checked without one:
/// every arm below is a misconfiguration an operator has to read an error
/// about, and none of them needs a reachable store to happen.
#[derive(Debug, PartialEq, Eq)]
pub struct Settings {
    address: String,
    cert_path: String,
    key_path: String,
    ca_path: Option<String>,
}

impl Settings {
    /// Read the `BAO_*` variables from the process environment.
    ///
    /// # Errors
    /// [`Error::MissingEnv`] naming the first variable that is unset or empty.
    pub fn from_env() -> Result<Self, Error> {
        Self::from_lookup(|k| std::env::var(k).ok())
    }

    /// [`Settings::from_env`] against an arbitrary lookup.
    ///
    /// [`vaultrs`]'s own defaults are deliberately not used: it looks for
    /// `VAULT_ADDR` / `VAULT_CLIENT_CERT` / `VAULT_CLIENT_KEY`, and every unit
    /// in this tree sets the `BAO_` spellings. Falling through to those
    /// defaults yields a client with **no identity**, which fails at the TLS
    /// handshake rather than anywhere that names the cause.
    ///
    /// # Errors
    /// [`Error::MissingEnv`] naming the first variable that is unset or empty.
    pub fn from_lookup(get: impl Fn(&str) -> Option<String>) -> Result<Self, Error> {
        let required = |var: &'static str| -> Result<String, Error> {
            get(var)
                .filter(|v| !v.is_empty())
                .ok_or(Error::MissingEnv(var))
        };
        Ok(Self {
            address: required(ENV_ADDR)?,
            cert_path: required(ENV_CLIENT_CERT)?,
            key_path: required(ENV_CLIENT_KEY)?,
            ca_path: get(ENV_CACERT).filter(|v| !v.is_empty()),
        })
    }
}

/// A client that has already exchanged its certificate for a token.
pub struct SecretStore {
    inner: VaultClient,
}

fn read_file(var: &'static str, path: &str) -> Result<Vec<u8>, Error> {
    std::fs::read(path).map_err(|source| Error::Identity {
        var,
        path: path.to_owned(),
        source,
    })
}

impl SecretStore {
    /// Connect using the `BAO_*` environment and log in with the certificate
    /// auth method, returning a handle that already holds a token.
    ///
    /// `cert_role` is the role configured on the store's `cert` mount; it
    /// selects which policy the returned token carries.
    ///
    /// # Errors
    /// As [`SecretStore::connect`], plus [`Error::MissingEnv`].
    pub async fn from_env(cert_role: &str) -> Result<Self, Error> {
        Self::connect(&Settings::from_env()?, cert_role, DEFAULT_CERT_MOUNT).await
    }

    /// Present `settings`' identity to the store and log in at `cert_mount`.
    ///
    /// # Errors
    /// [`Error::Identity`] when a named file cannot be read, [`Error::Tls`]
    /// when the cert/key pair does not form a usable identity,
    /// [`Error::Settings`] when the address will not parse, and
    /// [`Error::Vault`] when the store refuses the login — which is what an
    /// unconfigured `cert` mount looks like from here.
    pub async fn connect(
        settings: &Settings,
        cert_role: &str,
        cert_mount: &str,
    ) -> Result<Self, Error> {
        // One PEM blob holding both, which is the shape `Identity::from_pem`
        // wants; the two stay separate on disk because the key is the half
        // that gets `0600`.
        let mut pem = read_file(ENV_CLIENT_CERT, &settings.cert_path)?;
        pem.push(b'\n');
        pem.extend_from_slice(&read_file(ENV_CLIENT_KEY, &settings.key_path)?);
        Self::connect_with_identity(settings, &pem, cert_role, cert_mount).await
    }

    /// Log in at the store `settings` names, presenting an identity the caller
    /// is already holding rather than one named by the environment.
    ///
    /// `identity_pem` is the certificate and its private key concatenated into
    /// one PEM blob — the shape [`reqwest::Identity::from_pem`] takes, and the
    /// shape [`SecretStore::connect`] builds out of the two files
    /// [`ENV_CLIENT_CERT`] and [`ENV_CLIENT_KEY`] name. Only the address and
    /// the CA are read out of `settings` here; its two identity paths are not
    /// touched.
    ///
    /// This is what a **minter** needs. A process that has just issued a leaf
    /// holds the bytes, and the safest place for a freshly minted private key
    /// is the memory it was generated in: writing it to a file purely so that
    /// a `Settings` could name it would put a key on disk for no other reason
    /// than to log in with it once.
    ///
    /// # Errors
    /// [`Error::Tls`] when `identity_pem` is not a usable certificate/key
    /// pair, [`Error::Settings`] when the address will not parse, and
    /// [`Error::Vault`] when the store refuses the login — which is what a
    /// certificate no cert-auth role accepts looks like from here.
    pub async fn connect_with_identity(
        settings: &Settings,
        identity_pem: &[u8],
        cert_role: &str,
        cert_mount: &str,
    ) -> Result<Self, Error> {
        let identity = reqwest::Identity::from_pem(identity_pem).map_err(Error::Tls)?;

        let mut builder = VaultClientSettingsBuilder::default();
        builder
            .address(settings.address.clone())
            .identity(Some(identity));
        if let Some(ca) = &settings.ca_path {
            builder.ca_certs(vec![ca.clone()]);
        }
        let built = builder
            .build()
            .map_err(|e| Error::Settings(e.to_string()))?;

        let mut inner = VaultClient::new(built)?;
        let auth = vaultrs::auth::cert::login(&inner, cert_mount, cert_role).await?;
        inner.set_token(&auth.client_token);
        Ok(Self { inner })
    }

    /// Read the object stored at `path`, decoded as `T`.
    ///
    /// The shape belongs to whoever owns the path — see [`crate::matrix`] for
    /// the one kind of secret that exists — so this moves bytes and knows no
    /// fields.
    ///
    /// # Errors
    /// [`Error::Vault`] when the path does not exist, the token's policy does
    /// not cover it, or the stored object does not decode as `T`.
    pub async fn read<T: DeserializeOwned>(&self, path: &str) -> Result<T, Error> {
        Ok(vaultrs::kv2::read(&self.inner, MOUNT, path).await?)
    }

    /// Write `value` at `path`, creating a new version.
    ///
    /// # Errors
    /// [`Error::Vault`] when the token's policy does not cover the path.
    pub async fn write<T: Serialize + Sync>(&self, path: &str, value: &T) -> Result<(), Error> {
        vaultrs::kv2::set(&self.inner, MOUNT, path, value).await?;
        Ok(())
    }

    /// Replace the ACL policy named `name` with `policy`.
    ///
    /// A whole-document write, not a merge: the store has no other verb, and
    /// the caller renders the document from the current agent set anyway, so
    /// a stanza that is gone from the render is meant to be gone from the
    /// grant. Render the text with [`crate::policy`] rather than by hand.
    ///
    /// # Errors
    /// [`Error::Vault`] when the token's own policy does not cover
    /// `sys/policies/acl/<name>` — which is what a controller scoped to the
    /// `hive-*` namespace gets for any other name.
    pub async fn write_policy(&self, name: &str, policy: &str) -> Result<(), Error> {
        let request = WriteAclPolicy {
            name: name.to_owned(),
            policy: policy.to_owned(),
        };
        vaultrs::api::exec_with_empty(&self.inner, request).await?;
        Ok(())
    }

    /// Create or replace the cert-auth role `name`, so that a certificate
    /// issued by `certificate` and carrying `common_name` logs in with
    /// `policy`.
    ///
    /// `certificate` is the authority by value, not a path: the store keeps
    /// its own copy inside the role and never reads a file of ours.
    ///
    /// # Errors
    /// [`Error::Vault`] when the token's policy does not cover
    /// `auth/<mount>/certs/<name>`, or the store rejects the authority.
    pub async fn write_cert_role(
        &self,
        mount: &str,
        name: &str,
        certificate: &str,
        common_name: &str,
        policy: &str,
    ) -> Result<(), Error> {
        let mut opts =
            vaultrs::api::auth::cert::requests::CreateCaCertificateRoleRequest::builder();
        opts.allowed_common_names(vec![common_name.to_owned()])
            .token_policies(vec![policy.to_owned()]);
        vaultrs::auth::cert::ca_cert_role::set(
            &self.inner,
            mount,
            name,
            certificate,
            Some(&mut opts),
        )
        .await?;
        Ok(())
    }
}

/// Write an ACL policy, spelled out here because [`vaultrs`] does not have it:
/// its `sys::policy` module targets `sys/policy/<name>`, the deprecated alias,
/// and the store ACLs that path separately from `sys/policies/acl/<name>`. A
/// token granted the latter — which is what every grant in this tree names —
/// is refused at the former with a 403.
#[derive(Endpoint, Serialize)]
#[endpoint(path = "sys/policies/acl/{self.name}", method = "PUT")]
struct WriteAclPolicy {
    name: String,
    /// Marked as the body so `name` stays in the path alone: an untagged field
    /// would be serialised into the request too.
    #[endpoint(body)]
    policy: String,
}

#[cfg(test)]
mod tests {
    use super::*;

    /// A lookup standing in for a fully-configured unit's environment.
    fn full(k: &str) -> Option<String> {
        match k {
            ENV_ADDR => Some("https://store.invalid:8200".to_owned()),
            ENV_CLIENT_CERT => Some("/run/c.pem".to_owned()),
            ENV_CLIENT_KEY => Some("/run/k.pem".to_owned()),
            _ => None,
        }
    }

    #[test]
    fn a_complete_environment_is_accepted() {
        // The control: without this, every assertion below could be passing
        // because `from_lookup` rejects everything.
        let s = Settings::from_lookup(full).expect("every required variable is set");
        assert_eq!(s.address, "https://store.invalid:8200");
        assert_eq!(s.ca_path, None, "an absent CA is the system trust store");
    }

    #[test]
    fn each_required_variable_is_named_when_it_is_the_missing_one() {
        for var in [ENV_ADDR, ENV_CLIENT_CERT, ENV_CLIENT_KEY] {
            let e = Settings::from_lookup(|k| if k == var { None } else { full(k) })
                .expect_err("one required variable is absent");
            assert!(
                matches!(e, Error::MissingEnv(v) if v == var),
                "dropping {var} should name {var}, got {e:?}"
            );
        }
    }

    #[test]
    fn an_empty_variable_is_as_absent_as_an_unset_one() {
        // systemd writes `Environment=BAO_CACERT=` for an unset nix option, so
        // empty is the shape this actually arrives in.
        let e = Settings::from_lookup(|k| {
            if k == ENV_ADDR {
                Some(String::new())
            } else {
                full(k)
            }
        })
        .expect_err("an empty address is not an address");
        assert!(matches!(e, Error::MissingEnv(ENV_ADDR)), "got {e:?}");

        let s = Settings::from_lookup(|k| {
            if k == ENV_CACERT {
                Some(String::new())
            } else {
                full(k)
            }
        })
        .expect("an empty CA is optional, not fatal");
        assert_eq!(s.ca_path, None);
    }

    #[test]
    fn an_unreadable_cert_names_the_file_and_the_variable() {
        let e = read_file(ENV_CLIENT_CERT, "/nonexistent/cert.pem")
            .expect_err("the file does not exist");
        match e {
            Error::Identity { var, ref path, .. } => {
                assert_eq!(var, ENV_CLIENT_CERT);
                assert_eq!(path, "/nonexistent/cert.pem");
            }
            other => panic!("wanted an Identity error, got {other:?}"),
        }
    }

    /// The defect this pins: the store gates `sys/policy/<name>` and
    /// `sys/policies/acl/<name>` separately, so a client on the first is
    /// refused by a grant naming the second — and nothing else in the tree
    /// says which one is addressed.
    #[test]
    fn a_policy_write_addresses_the_modern_acl_path() {
        use rustify::endpoint::Endpoint as _;

        let request = WriteAclPolicy {
            name: "hive-pr1ma".to_owned(),
            policy: crate::policy::render("pr1ma").expect("a plain name is legal"),
        };
        assert_eq!(request.path(), "sys/policies/acl/hive-pr1ma");
    }

    #[test]
    fn the_request_body_carries_the_policy_and_not_the_name() {
        use rustify::endpoint::Endpoint as _;

        let request = WriteAclPolicy {
            name: "hive-pr1ma".to_owned(),
            policy: crate::policy::render("pr1ma").expect("a plain name is legal"),
        };
        let body = request
            .body()
            .expect("the body serialises")
            .expect("a policy write sends one");
        let sent: serde_json::Value =
            serde_json::from_slice(&body).expect("the body is the JSON the store parses");
        assert_eq!(
            sent["policy"],
            crate::policy::render("pr1ma").expect("legal")
        );
        assert!(
            sent.get("name").is_none(),
            "`name` addresses the policy in the path; sending it too would make \
             the store's copy of the document disagree with its own name"
        );
    }
}