proxmox-backup/src/config/tfa.rs

use std::collections::HashMap;
use std::fs::File;
use std::time::Duration;

use anyhow::{bail, format_err, Error};
use serde::{de::Deserializer, Deserialize, Serialize};
use serde_json::Value;

use proxmox::api::api;
use proxmox::sys::error::SysError;
use proxmox::tools::tfa::totp::Totp;
use proxmox::tools::tfa::u2f;
use proxmox::tools::uuid::Uuid;

use crate::api2::types::Userid;

/// Mapping of userid to TFA entry.
pub type TfaUsers = HashMap<Userid, TfaUserData>;

const CONF_FILE: &str = configdir!("/tfa.json");
const LOCK_FILE: &str = configdir!("/tfa.json.lock");
const LOCK_TIMEOUT: Duration = Duration::from_secs(5);

/// U2F registration challenges time out after 2 minutes.
const CHALLENGE_TIMEOUT: i64 = 2 * 60;

#[derive(Deserialize, Serialize)]
pub struct U2fConfig {
    appid: String,
}

#[derive(Default, Deserialize, Serialize)]
pub struct TfaConfig {
    #[serde(skip_serializing_if = "Option::is_none")]
    pub u2f: Option<U2fConfig>,
    #[serde(skip_serializing_if = "TfaUsers::is_empty", default)]
    pub users: TfaUsers,
}

/// Heper to get a u2f instance from a u2f config, or `None` if there isn't one configured.
fn get_u2f(u2f: &Option<U2fConfig>) -> Option<u2f::U2f> {
    u2f.as_ref().map(|cfg| u2f::U2f::new(cfg.appid.clone(), cfg.appid.clone()))
}

/// Heper to get a u2f instance from a u2f config.
// deduplicate error message while working around self-borrow issue
fn need_u2f(u2f: &Option<U2fConfig>) -> Result<u2f::U2f, Error> {
    get_u2f(u2f).ok_or_else(|| format_err!("no u2f configuration available"))
}

impl TfaConfig {
    fn u2f(&self) -> Option<u2f::U2f> {
        get_u2f(&self.u2f)
    }

    fn need_u2f(&self) -> Result<u2f::U2f, Error> {
        need_u2f(&self.u2f)
    }

    /// Get a two factor authentication challenge for a user, if the user has TFA set up.
    pub fn login_challenge(&self, userid: &Userid) -> Result<Option<TfaChallenge>, Error> {
        match self.users.get(userid) {
            Some(udata) => udata.challenge(self.u2f().as_ref()),
            None => Ok(None),
        }
    }

    /// Get a u2f registration challenge.
    fn u2f_registration_challenge(
        &mut self,
        user: &Userid,
        description: String,
    ) -> Result<String, Error> {
        let u2f = self.need_u2f()?;

        self.users
            .entry(user.clone())
            .or_default()
            .u2f_registration_challenge(&u2f, description)
    }

    /// Finish a u2f registration challenge.
    fn u2f_registration_finish(
        &mut self,
        user: &Userid,
        challenge: &str,
        response: &str,
    ) -> Result<String, Error> {
        let u2f = self.need_u2f()?;

        match self.users.get_mut(user) {
            Some(user) => user.u2f_registration_finish(&u2f, challenge, response),
            None => bail!("no such challenge"),
        }
    }

    /// Verify a TFA response.
    fn verify(
        &mut self,
        userid: &Userid,
        challenge: &TfaChallenge,
        response: TfaResponse,
    ) -> Result<(), Error> {
        match self.users.get_mut(userid) {
            Some(user) => {
                match response {
                    TfaResponse::Totp(value) => user.verify_totp(&value),
                    TfaResponse::U2f(value) => match &challenge.u2f {
                        Some(challenge) => {
                            let u2f = need_u2f(&self.u2f)?;
                            user.verify_u2f(u2f, &challenge.challenge, value)
                        }
                        None => bail!("no u2f factor available for user '{}'", userid),
                    }
                    TfaResponse::Recovery(value) => user.verify_recovery(&value),
                }
            }
            None => bail!("no 2nd factor available for user '{}'", userid),
        }
    }
}

#[api]
/// Over the API we only provide this part when querying a user's second factor list.
#[derive(Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
pub struct TfaInfo {
    /// The id used to reference this entry.
    pub id: String,

    /// User chosen description for this entry.
    pub description: String,

    /// Whether this TFA entry is currently enabled.
    #[serde(skip_serializing_if = "is_default_tfa_enable")]
    #[serde(default = "default_tfa_enable")]
    pub enable: bool,
}

impl TfaInfo {
    /// For recovery keys we have a fixed entry.
    pub(crate) fn recovery() -> Self {
        Self {
            id: "recovery".to_string(),
            description: "recovery keys".to_string(),
            enable: true,
        }
    }
}

/// A TFA entry for a user.
///
/// This simply connects a raw registration to a non optional descriptive text chosen by the user.
#[derive(Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
pub struct TfaEntry<T> {
    #[serde(flatten)]
    pub info: TfaInfo,

    /// The actual entry.
    entry: T,
}

impl<T> TfaEntry<T> {
    /// Create an entry with a description. The id will be autogenerated.
    fn new(description: String, entry: T) -> Self {
        Self {
            info: TfaInfo {
                id: Uuid::generate().to_string(),
                enable: true,
                description,
            },
            entry,
        }
    }
}

/// A u2f registration challenge.
#[derive(Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
pub struct U2fRegistrationChallenge {
    /// JSON formatted challenge string.
    challenge: String,

    /// The description chosen by the user for this registration.
    description: String,

    /// When the challenge was created as unix epoch. They are supposed to be short-lived.
    created: i64,
}

impl U2fRegistrationChallenge {
    pub fn new(challenge: String, description: String) -> Self {
        Self {
            challenge,
            description,
            created: proxmox::tools::time::epoch_i64(),
        }
    }

    fn is_expired(&self, at_epoch: i64) -> bool {
        self.created < at_epoch
    }
}

/// TFA data for a user.
#[derive(Default, Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
#[serde(rename_all = "kebab-case")]
pub struct TfaUserData {
    /// Totp keys for a user.
    #[serde(skip_serializing_if = "Vec::is_empty", default)]
    pub(crate) totp: Vec<TfaEntry<Totp>>,

    /// Registered u2f tokens for a user.
    #[serde(skip_serializing_if = "Vec::is_empty", default)]
    pub(crate) u2f: Vec<TfaEntry<u2f::Registration>>,

    /// Recovery keys. (Unordered OTP values).
    #[serde(skip_serializing_if = "Vec::is_empty", default)]
    pub(crate) recovery: Vec<String>,

    /// Active u2f registration challenges for a user.
    ///
    /// Expired values are automatically filtered out while parsing the tfa configuration file.
    #[serde(skip_serializing_if = "Vec::is_empty", default)]
    #[serde(deserialize_with = "filter_expired_registrations")]
    u2f_registrations: Vec<U2fRegistrationChallenge>,
}

/// Serde helper using our `FilteredVecVisitor` to filter out expired entries directly at load
/// time.
fn filter_expired_registrations<'de, D>(
    deserializer: D,
) -> Result<Vec<U2fRegistrationChallenge>, D::Error>
where
    D: Deserializer<'de>,
{
    let expire_before = proxmox::tools::time::epoch_i64() - CHALLENGE_TIMEOUT;
    Ok(
        deserializer.deserialize_seq(crate::tools::serde_filter::FilteredVecVisitor::new(
            "a u2f registration challenge entry",
            move |reg: &U2fRegistrationChallenge| !reg.is_expired(expire_before),
        ))?,
    )
}

impl TfaUserData {
    /// `true` if no second factors exist
    pub fn is_empty(&self) -> bool {
        self.totp.is_empty() && self.u2f.is_empty() && self.recovery.is_empty()
    }

    /// Find an entry by id, except for the "recovery" entry which we're currently treating
    /// specially.
    pub fn find_entry_mut<'a>(&'a mut self, id: &str) -> Option<&'a mut TfaInfo> {
        for entry in &mut self.totp {
            if entry.info.id == id {
                return Some(&mut entry.info);
            }
        }

        for entry in &mut self.u2f {
            if entry.info.id == id {
                return Some(&mut entry.info);
            }
        }

        None
    }

    /// Create a u2f registration challenge.
    ///
    /// The description is required at this point already mostly to better be able to identify such
    /// challenges in the tfa config file if necessary. The user otherwise has no access to this
    /// information at this point, as the challenge is identified by its actual challenge data
    /// instead.
    fn u2f_registration_challenge(
        &mut self,
        u2f: &u2f::U2f,
        description: String,
    ) -> Result<String, Error> {
        let challenge = serde_json::to_string(&u2f.registration_challenge()?)?;

        self.u2f_registrations.push(U2fRegistrationChallenge::new(
            challenge.clone(),
            description,
        ));

        Ok(challenge)
    }

    /// Finish a u2f registration. The challenge should correspond to an output of
    /// `u2f_registration_challenge` (which is a stringified `RegistrationChallenge`). The response
    /// should come directly from the client.
    fn u2f_registration_finish(
        &mut self,
        u2f: &u2f::U2f,
        challenge: &str,
        response: &str,
    ) -> Result<String, Error> {
        let expire_before = proxmox::tools::time::epoch_i64() - CHALLENGE_TIMEOUT;

        let index = self
            .u2f_registrations
            .iter()
            .position(|r| r.challenge == challenge)
            .ok_or_else(|| format_err!("no such challenge"))?;

        let reg = &self.u2f_registrations[index];
        if reg.is_expired(expire_before) {
            bail!("no such challenge");
        }

        // the verify call only takes the actual challenge string, so we have to extract it
        // (u2f::RegistrationChallenge did not always implement Deserialize...)
        let chobj: Value = serde_json::from_str(challenge)
            .map_err(|err| format_err!("error parsing original registration challenge: {}", err))?;
        let challenge = chobj["challenge"]
            .as_str()
            .ok_or_else(|| format_err!("invalid registration challenge"))?;

        let (mut reg, description) = match u2f.registration_verify(challenge, response)? {
            None => bail!("verification failed"),
            Some(reg) => {
                let entry = self.u2f_registrations.remove(index);
                (reg, entry.description)
            }
        };

        // we do not care about the attestation certificates, so don't store them
        reg.certificate.clear();

        let entry = TfaEntry::new(description, reg);
        let id = entry.info.id.clone();
        self.u2f.push(entry);
        Ok(id)
    }

    /// Generate a generic TFA challenge. See the [`TfaChallenge`] description for details.
    pub fn challenge(&self, u2f: Option<&u2f::U2f>) -> Result<Option<TfaChallenge>, Error> {
        if self.is_empty() {
            return Ok(None);
        }

        Ok(Some(TfaChallenge {
            totp: self.totp.iter().any(|e| e.info.enable),
            recovery: RecoveryState::from_count(self.recovery.len()),
            u2f: match u2f {
                Some(u2f) => self.u2f_challenge(u2f)?,
                None => None,
            },
        }))
    }

    /// Helper to iterate over enabled totp entries.
    fn enabled_totp_entries(&self) -> impl Iterator<Item = &Totp> {
        self.totp
            .iter()
            .filter_map(|e| {
                if e.info.enable {
                    Some(&e.entry)
                } else {
                    None
                }
            })
    }

    /// Helper to iterate over enabled u2f entries.
    fn enabled_u2f_entries(&self) -> impl Iterator<Item = &u2f::Registration> {
        self.u2f
            .iter()
            .filter_map(|e| {
                if e.info.enable {
                    Some(&e.entry)
                } else {
                    None
                }
            })
    }

    /// Generate an optional u2f challenge.
    fn u2f_challenge(&self, u2f: &u2f::U2f) -> Result<Option<U2fChallenge>, Error> {
        if self.u2f.is_empty() {
            return Ok(None);
        }

        let keys: Vec<u2f::RegisteredKey> = self
            .enabled_u2f_entries()
            .map(|registration| registration.key.clone())
            .collect();

        if keys.is_empty() {
            return Ok(None);
        }

        Ok(Some(U2fChallenge {
            challenge: u2f.auth_challenge()?,
            keys,
        }))
    }

    /// Verify a totp challenge. The `value` should be the totp digits as plain text.
    fn verify_totp(&self, value: &str) -> Result<(), Error> {
        let now = std::time::SystemTime::now();

        for entry in self.enabled_totp_entries() {
            if entry.verify(value, now, -1..=1)?.is_some() {
                return Ok(());
            }
        }

        bail!("totp verification failed");
    }

    /// Verify a u2f response.
    fn verify_u2f(
        &self,
        u2f: u2f::U2f,
        challenge: &u2f::AuthChallenge,
        response: Value,
    ) -> Result<(), Error> {
        let response: u2f::AuthResponse = serde_json::from_value(response)
            .map_err(|err| format_err!("invalid u2f response: {}", err))?;

        if let Some(entry) = self
            .enabled_u2f_entries()
            .find(|e| e.key.key_handle == response.key_handle)
        {
            if u2f.auth_verify_obj(&entry.public_key, &challenge.challenge, response)?.is_some() {
                return Ok(());
            }
        }

        bail!("u2f verification failed");
    }

    /// Verify a recovery key.
    ///
    /// NOTE: If successful, the key will automatically be removed from the list of available
    /// recovery keys, so the configuration needs to be saved afterwards!
    fn verify_recovery(&mut self, value: &str) -> Result<(), Error> {
        match self.recovery.iter().position(|v| v == value) {
            Some(idx) => {
                self.recovery.remove(idx);
                Ok(())
            }
            None => bail!("recovery verification failed"),
        }
    }

    /// Add a new set of recovery keys. There can only be 1 set of keys at a time.
    fn add_recovery(&mut self) -> Result<Vec<String>, Error> {
        if !self.recovery.is_empty() {
            bail!("user already has recovery keys");
        }

        let mut key_data = [0u8; 40]; // 10 keys of 32 bits
        proxmox::sys::linux::fill_with_random_data(&mut key_data)?;
        for b in key_data.chunks(4) {
            self.recovery.push(format!("{:02x}{:02x}{:02x}{:02x}", b[0], b[1], b[2], b[3]));
        }

        Ok(self.recovery.clone())
    }
}

/// Read the TFA entries.
pub fn read() -> Result<TfaConfig, Error> {
    let file = match File::open(CONF_FILE) {
        Ok(file) => file,
        Err(ref err) if err.not_found() => return Ok(TfaConfig::default()),
        Err(err) => return Err(err.into()),
    };

    Ok(serde_json::from_reader(file)?)
}

/// Requires the write lock to be held.
pub fn write(data: &TfaConfig) -> Result<(), Error> {
    let options = proxmox::tools::fs::CreateOptions::new()
        .perm(nix::sys::stat::Mode::from_bits_truncate(0o0600));

    let json = serde_json::to_vec(data)?;
    proxmox::tools::fs::replace_file(CONF_FILE, &json, options)
}

pub fn read_lock() -> Result<File, Error> {
    proxmox::tools::fs::open_file_locked(LOCK_FILE, LOCK_TIMEOUT, false)
}

pub fn write_lock() -> Result<File, Error> {
    proxmox::tools::fs::open_file_locked(LOCK_FILE, LOCK_TIMEOUT, true)
}

/// Add a TOTP entry for a user. Returns the ID.
pub fn add_totp(userid: &Userid, description: String, value: Totp) -> Result<String, Error> {
    let _lock = crate::config::tfa::write_lock();
    let mut data = read()?;
    let entry = TfaEntry::new(description, value);
    let id = entry.info.id.clone();
    data.users
        .entry(userid.clone())
        .or_default()
        .totp
        .push(entry);
    write(&data)?;
    Ok(id)
}

/// Add recovery tokens for the user. Returns the token list.
pub fn add_recovery(userid: &Userid) -> Result<Vec<String>, Error> {
    let _lock = crate::config::tfa::write_lock();

    let mut data = read()?;
    let out = data.users.entry(userid.clone()).or_default().add_recovery()?;
    write(&data)?;
    Ok(out)
}

/// Add a u2f registration challenge for a user.
pub fn add_u2f_registration(userid: &Userid, description: String) -> Result<String, Error> {
    let _lock = crate::config::tfa::write_lock();
    let mut data = read()?;
    let challenge = data.u2f_registration_challenge(userid, description)?;
    write(&data)?;
    Ok(challenge)
}

/// Finish a u2f registration challenge for a user.
pub fn finish_u2f_registration(
    userid: &Userid,
    challenge: &str,
    response: &str,
) -> Result<String, Error> {
    let _lock = crate::config::tfa::write_lock();
    let mut data = read()?;
    let challenge = data.u2f_registration_finish(userid, challenge, response)?;
    write(&data)?;
    Ok(challenge)
}

/// Verify a TFA challenge.
pub fn verify_challenge(
    userid: &Userid,
    challenge: &TfaChallenge,
    response: TfaResponse,
) -> Result<(), Error> {
    let _lock = crate::config::tfa::write_lock();
    let mut data = read()?;
    data.verify(userid, challenge, response)?;
    write(&data)?;
    Ok(())
}

/// Used to inform the user about the recovery code status.
#[derive(Clone, Copy, Eq, PartialEq, Deserialize, Serialize)]
#[serde(rename_all = "kebab-case")]
pub enum RecoveryState {
    Unavailable,
    Low,
    Available,
}

impl RecoveryState {
    fn from_count(count: usize) -> Self {
        match count {
            0 => RecoveryState::Unavailable,
            1..=3 => RecoveryState::Low,
            _ => RecoveryState::Available,
        }
    }

    // serde needs `&self` but this is a tiny Copy type, so we mark this as inline
    #[inline]
    fn is_unavailable(&self) -> bool {
        *self == RecoveryState::Unavailable
    }
}

impl Default for RecoveryState {
    fn default() -> Self {
        RecoveryState::Unavailable
    }
}

/// When sending a TFA challenge to the user, we include information about what kind of challenge
/// the user may perform. If u2f devices are available, a u2f challenge will be included.
#[derive(Deserialize, Serialize)]
#[serde(rename_all = "kebab-case")]
pub struct TfaChallenge {
    /// True if the user has TOTP devices.
    totp: bool,

    /// Whether there are recovery keys available.
    #[serde(skip_serializing_if = "RecoveryState::is_unavailable", default)]
    recovery: RecoveryState,

    /// If the user has any u2f tokens registered, this will contain the U2F challenge data.
    #[serde(skip_serializing_if = "Option::is_none")]
    u2f: Option<U2fChallenge>,
}

/// Data used for u2f challenges.
#[derive(Deserialize, Serialize)]
pub struct U2fChallenge {
    /// AppID and challenge data.
    challenge: u2f::AuthChallenge,

    /// Available tokens/keys.
    keys: Vec<u2f::RegisteredKey>,
}

/// A user's response to a TFA challenge.
pub enum TfaResponse {
    Totp(String),
    U2f(Value),
    Recovery(String),
}

impl std::str::FromStr for TfaResponse {
    type Err = Error;

    fn from_str(s: &str) -> Result<Self, Error> {
        Ok(if s.starts_with("totp:") {
            TfaResponse::Totp(s[5..].to_string())
        } else if s.starts_with("u2f:") {
            TfaResponse::U2f(serde_json::from_str(&s[4..])?)
        } else if s.starts_with("recovery:") {
            TfaResponse::Recovery(s[9..].to_string())
        } else {
            bail!("invalid tfa response");
        })
    }
}

const fn default_tfa_enable() -> bool {
    true
}

const fn is_default_tfa_enable(v: &bool) -> bool {
    *v
}