[PATCH proxmox 1/7] notify (smtp): Introduce xoauth2 module

[Arthur Bied-Charreton <a.bied-charreton@proxmox.com>]

Prepare proxmox-notify to use the oauth2 crate for SMTP XOAUTH2 support.

The xoauth2 module handles some of the implementation details related to
supporting XOAUTH2 for SMTP notification targets.

* Add a ureq::Agent newtype wrapper implementing the SyncHttpClient
  trait to allow using ureq as oauth2 backend, since OAuth2 dropped the
  ureq feature. Debian seems to have patched it out due to a ureq 2/3
  version mismatch [1].

* Add get_{google,microsoft}_token functions

Signed-off-by: Arthur Bied-Charreton <a.bied-charreton@proxmox.com>
---
 proxmox-notify/Cargo.toml                    |   6 +-
 proxmox-notify/debian/control                |  14 +-
 proxmox-notify/src/endpoints/smtp.rs         |   2 +
 proxmox-notify/src/endpoints/smtp/xoauth2.rs | 167 +++++++++++++++++++
 4 files changed, 186 insertions(+), 3 deletions(-)
 create mode 100644 proxmox-notify/src/endpoints/smtp/xoauth2.rs

diff --git a/proxmox-notify/Cargo.toml b/proxmox-notify/Cargo.toml
index bc63e19d..d816c695 100644
--- a/proxmox-notify/Cargo.toml
+++ b/proxmox-notify/Cargo.toml
@@ -19,6 +19,9 @@ http = { workspace = true, optional = true }
 lettre = { workspace = true, optional = true }
 tracing.workspace = true
 mail-parser = { workspace = true, optional = true }
+oauth2 = { version = "5.0.0", default-features = false, optional = true }
+ureq = { version = "3.0.11", features = ["platform-verifier"], optional = true }
+
 openssl.workspace = true
 percent-encoding = { workspace = true, optional = true }
 regex.workspace = true
@@ -36,6 +39,7 @@ proxmox-sendmail = { workspace = true, optional = true }
 proxmox-sys = { workspace = true, optional = true }
 proxmox-time.workspace = true
 proxmox-uuid = { workspace = true, features = ["serde"] }
+nix.workspace = true
 
 [features]
 default = ["sendmail", "gotify", "smtp", "webhook"]
@@ -44,5 +48,5 @@ sendmail = ["dep:proxmox-sys", "dep:proxmox-sendmail"]
 gotify = ["dep:proxmox-http", "dep:http"]
 pve-context = ["dep:proxmox-sys"]
 pbs-context = ["dep:proxmox-sys"]
-smtp = ["dep:lettre"]
+smtp = ["dep:lettre", "dep:oauth2", "dep:ureq", "dep:http", "dep:proxmox-sys"]
 webhook = ["dep:http", "dep:percent-encoding", "dep:proxmox-base64", "dep:proxmox-http"]
diff --git a/proxmox-notify/debian/control b/proxmox-notify/debian/control
index e588e485..a84af040 100644
--- a/proxmox-notify/debian/control
+++ b/proxmox-notify/debian/control
@@ -11,6 +11,8 @@ Build-Depends-Arch: cargo:native <!nocheck>,
  librust-handlebars-5+default-dev <!nocheck>,
  librust-http-1+default-dev <!nocheck>,
  librust-lettre-0.11+default-dev (>= 0.11.1-~~) <!nocheck>,
+ librust-nix-0.29+default-dev <!nocheck>,
+ librust-oauth2-5-dev <!nocheck>,
  librust-openssl-0.10+default-dev <!nocheck>,
  librust-percent-encoding-2+default-dev (>= 2.1-~~) <!nocheck>,
  librust-proxmox-base64-1+default-dev <!nocheck>,
@@ -33,7 +35,9 @@ Build-Depends-Arch: cargo:native <!nocheck>,
  librust-serde-1+default-dev <!nocheck>,
  librust-serde-1+derive-dev <!nocheck>,
  librust-serde-json-1+default-dev <!nocheck>,
- librust-tracing-0.1+default-dev <!nocheck>
+ librust-tracing-0.1+default-dev <!nocheck>,
+ librust-ureq-3+default-dev (>= 3.0.11-~~) <!nocheck>,
+ librust-ureq-3+platform-verifier-dev (>= 3.0.11-~~) <!nocheck>
 Maintainer: Proxmox Support Team <support@proxmox.com>
 Standards-Version: 4.7.2
 Vcs-Git: git://git.proxmox.com/git/proxmox.git
@@ -49,6 +53,7 @@ Depends:
  librust-anyhow-1+default-dev,
  librust-const-format-0.2+default-dev,
  librust-handlebars-5+default-dev,
+ librust-nix-0.29+default-dev,
  librust-openssl-0.10+default-dev,
  librust-proxmox-http-error-1+default-dev,
  librust-proxmox-human-byte-1+default-dev,
@@ -177,7 +182,12 @@ Multi-Arch: same
 Depends:
  ${misc:Depends},
  librust-proxmox-notify-dev (= ${binary:Version}),
- librust-lettre-0.11+default-dev (>= 0.11.1-~~)
+ librust-http-1+default-dev,
+ librust-lettre-0.11+default-dev (>= 0.11.1-~~),
+ librust-oauth2-5-dev,
+ librust-proxmox-sys-1+default-dev,
+ librust-ureq-3+default-dev (>= 3.0.11-~~),
+ librust-ureq-3+platform-verifier-dev (>= 3.0.11-~~)
 Provides:
  librust-proxmox-notify-1+smtp-dev (= ${binary:Version}),
  librust-proxmox-notify-1.0+smtp-dev (= ${binary:Version}),
diff --git a/proxmox-notify/src/endpoints/smtp.rs b/proxmox-notify/src/endpoints/smtp.rs
index c888dee7..277b70f4 100644
--- a/proxmox-notify/src/endpoints/smtp.rs
+++ b/proxmox-notify/src/endpoints/smtp.rs
@@ -23,6 +23,8 @@ const SMTP_SUBMISSION_STARTTLS_PORT: u16 = 587;
 const SMTP_SUBMISSION_TLS_PORT: u16 = 465;
 const SMTP_TIMEOUT: u16 = 5;
 
+mod xoauth2;
+
 #[api]
 #[derive(Debug, Serialize, Deserialize, Default, Clone, Copy)]
 #[serde(rename_all = "kebab-case")]
diff --git a/proxmox-notify/src/endpoints/smtp/xoauth2.rs b/proxmox-notify/src/endpoints/smtp/xoauth2.rs
new file mode 100644
index 00000000..90ee630f
--- /dev/null
+++ b/proxmox-notify/src/endpoints/smtp/xoauth2.rs
@@ -0,0 +1,167 @@
+use oauth2::{
+    basic::BasicClient, AccessToken, AuthUrl, ClientId, ClientSecret, RefreshToken, TokenResponse,
+    TokenUrl,
+};
+
+use crate::Error;
+
+/// This newtype implements the `SyncHttpClient` trait for [`ureq::Agent`]. This allows
+/// us to avoid pulling in a different backend like `reqwest`.
+///
+/// Debian patched out `[0]` the `ureq` backend due to a `ureq` 2-3 version
+/// mismatch in the `oauth2` crate.
+///
+/// There is an open PR `[1]` in `oauth2`, once/if this is merged, we can drop the
+/// custom client implementation.
+///
+/// `[0]`
+/// https://git.proxmox.com/?p=debcargo-conf.git;a=blob;f=src/oauth2/debian/patches/disable-ureq.patch;h=828b883a83a86927c5cd32df055226a5e78e8bea;hb=refs/heads/proxmox/trixie
+///
+/// `[1]` https://github.com/ramosbugs/oauth2-rs/pull/338
+pub(crate) struct UreqSyncHttpClient(ureq::Agent);
+
+impl Default for UreqSyncHttpClient {
+    /// Set `max_redirects` to 0 to prevent SSRF, see
+    /// https://docs.rs/oauth2/latest/oauth2/#security-warning
+    fn default() -> Self {
+        Self(ureq::Agent::new_with_config(
+            ureq::Agent::config_builder().max_redirects(0).build(),
+        ))
+    }
+}
+
+impl oauth2::SyncHttpClient for UreqSyncHttpClient {
+    type Error = oauth2::HttpClientError<ureq::Error>;
+
+    fn call(&self, request: oauth2::HttpRequest) -> Result<oauth2::HttpResponse, Self::Error> {
+        let uri = request.uri().to_string();
+
+        let response = match request.method() {
+            &http::Method::POST => {
+                let req = request
+                    .headers()
+                    .iter()
+                    .fold(self.0.post(&uri), |req, (name, value)| {
+                        req.header(name, value)
+                    });
+                req.send(request.body()).map_err(Box::new)?
+            }
+            &http::Method::GET => {
+                let req = request
+                    .headers()
+                    .iter()
+                    .fold(self.0.get(&uri), |req, (name, value)| {
+                        req.header(name, value)
+                    });
+                req.call().map_err(Box::new)?
+            }
+            m => {
+                return Err(oauth2::HttpClientError::Other(format!(
+                    "unexpected method: {m}"
+                )));
+            }
+        };
+
+        let mut builder = http::Response::builder().status(response.status());
+
+        if let Some(content_type) = response.headers().get(http::header::CONTENT_TYPE) {
+            builder = builder.header(http::header::CONTENT_TYPE, content_type);
+        }
+
+        let (_, mut body) = response.into_parts();
+
+        let body = body.read_to_vec().map_err(Box::new)?;
+
+        builder.body(body).map_err(oauth2::HttpClientError::Http)
+    }
+}
+
+/// The result yielded by an OAuth2 token exchange.
+///
+/// A successful OAuth2 token exchange will always return an access token to be
+/// used for authentication.
+///
+/// Some providers additionally yield a new refresh token that should replace the
+/// old one.
+pub(crate) struct TokenExchangeResult {
+    pub access_token: AccessToken,
+    pub refresh_token: Option<RefreshToken>,
+}
+
+/// Perform a Microsoft OAuth2 token exchange.
+///
+/// This always yields a new refresh token, which should be persisted on a best-effort
+/// basis, replacing the one that was passed to this function.
+///
+/// Microsoft Identity Platform refresh tokens have static lifetimes of 90 days, with each
+/// token exchange yielding a new refresh token. The new refresh token is assigned a new
+/// static lifetime, starting from the moment the token exchange was performed.
+///
+/// The old refresh token is not invalidated, rather it keeps the static lifetime it was
+/// assigned at generation time. This means that at any given point in time, there can be
+/// many different refresh tokens that are *all* valid.
+///
+/// https://learn.microsoft.com/en-us/entra/identity-platform/refresh-tokens#token-lifetime
+pub(crate) fn get_microsoft_token(
+    client_id: ClientId,
+    client_secret: ClientSecret,
+    tenant_id: &str,
+    refresh_token: RefreshToken,
+) -> Result<TokenExchangeResult, Error> {
+    let client = BasicClient::new(client_id)
+        .set_client_secret(client_secret)
+        .set_auth_uri(
+            AuthUrl::new(format!(
+                "https://login.microsoftonline.com/{tenant_id}/oauth2/v2.0/authorize"
+            ))
+            .map_err(|e| Error::Generic(format!("invalid auth URL: {e}")))?,
+        )
+        .set_token_uri(
+            TokenUrl::new(format!(
+                "https://login.microsoftonline.com/{tenant_id}/oauth2/v2.0/token"
+            ))
+            .map_err(|e| Error::Generic(format!("invalid token URL: {e}")))?,
+        );
+
+    let token_result = client
+        .exchange_refresh_token(&refresh_token)
+        .request(&UreqSyncHttpClient::default())
+        .map_err(|e| Error::Generic(format!("could not get access token: {e}")))?;
+
+    Ok(TokenExchangeResult {
+        access_token: token_result.access_token().clone(),
+        refresh_token: token_result.refresh_token().cloned(),
+    })
+}
+
+/// Google refresh tokens' TTL is extended at every use. As long as
+/// a token has been used at least once in the past 6 months, and no
+/// other expiration reason applies, the same token can be kept.
+///
+/// https://developers.google.com/identity/protocols/oauth2#expiration
+pub(crate) fn get_google_token(
+    client_id: ClientId,
+    client_secret: ClientSecret,
+    refresh_token: RefreshToken,
+) -> Result<TokenExchangeResult, Error> {
+    let client = BasicClient::new(client_id)
+        .set_client_secret(client_secret)
+        .set_auth_uri(
+            AuthUrl::new("https://accounts.google.com/o/oauth2/v2/auth".into())
+                .map_err(|e| Error::Generic(format!("invalid auth URL: {e}")))?,
+        )
+        .set_token_uri(
+            TokenUrl::new("https://oauth2.googleapis.com/token".into())
+                .map_err(|e| Error::Generic(format!("invalid token URL: {e}")))?,
+        );
+
+    let token_result = client
+        .exchange_refresh_token(&refresh_token)
+        .request(&UreqSyncHttpClient::default())
+        .map_err(|e| Error::Generic(format!("could not get access token: {e}")))?;
+
+    Ok(TokenExchangeResult {
+        access_token: token_result.access_token().clone(),
+        refresh_token: token_result.refresh_token().cloned(),
+    })
+}
-- 
2.47.3