Add cross-platform support for SSL_CERT_FILE

Cargo.toml

@@ -20,6 +20,7 @@ ring = "0.16.5"
 untrusted = "0.7.0"
 rustls = "0.20"
 x509-parser = "0.9.2"
+serial_test = "0.5.1"
 
 [target.'cfg(windows)'.dependencies]
 schannel = "0.1.15"

README.md

@@ -5,6 +5,10 @@ platform's native certificate store when operating as a TLS client.
 
 This is supported on Windows, macOS and Linux:
 
+- On all platforms, the `SSL_CERT_FILE` environment variable is checked first.
+  If that's set, certificates are loaded from the path specified by that variable,
+  or an error is returned if certificates cannot be loaded from the given path.
+  If it's not set, then the platform-specific certificate source is used.
 - On Windows, certificates are loaded from the system certificate store.
   The [`schannel`](https://github.com/steffengy/schannel-rs) crate is used to access
   the Windows certificate store APIs.
@@ -45,18 +49,13 @@ If you'd like to help out, please see [CONTRIBUTING.md](CONTRIBUTING.md).
 This library exposes a single function with this signature:
 
 ```rust
-pub fn load_native_certs() -> Result<rustls::RootCertStore, (Option<rustls::RootCertStore>, std::io::Error)>
+pub fn load_native_certs() -> Result<Vec<Certificate>, std::io::Error>
 ```
 
-On success, this returns a `rustls::RootCertStore` loaded with a
-snapshop of the root certificates found on this platform.  This
+On success, this returns a `Vec<Certificate>` loaded with a
+snapshot of the root certificates found on this platform.  This
 function fails in a platform-specific way, expressed in a `std::io::Error`.
 
-When an error is returned, optionally a `rustls::RootCertStore` is also
-returned containing the certificates which *could* be loaded.  This means
-callers can opt-in to "best-effort" behaviour even in the presence of invalid
-certificates.
-
 This function can be expensive: on some platforms it involves loading
 and parsing a ~300KB disk file.  It's therefore prudent to call
 this sparingly.

src/lib.rs

@@ -25,7 +25,11 @@ mod macos;
 #[cfg(target_os = "macos")]
 use macos as platform;
 
-use std::io::Error;
+use std::io::{Error, ErrorKind};
+use std::io::BufReader;
+use std::fs::File;
+use std::path::{Path,PathBuf};
+use std::env;
 
 /// Loads root certificates found in the platform's native certificate
 /// store, executing callbacks on the provided builder.
@@ -36,7 +40,37 @@ use std::io::Error;
 /// and parsing a ~300KB disk file.  It's therefore prudent to call
 /// this sparingly.
 pub fn load_native_certs() -> Result<Vec<Certificate>, Error> {
-    platform::load_native_certs()
+    load_certs_from_env()
+    .unwrap_or_else(platform::load_native_certs)
 }
 
 pub struct Certificate(pub Vec<u8>);
+
+const ENV_CERT_FILE: &str = "SSL_CERT_FILE";
+
+/// Returns None if SSL_CERT_FILE is not defined in the current environment.
+///
+/// If it is defined, it is always used, so it must be a path to a real
+/// file from which certificates can be loaded successfully.
+fn load_certs_from_env() -> Option<Result<Vec<Certificate>, Error>> {
+    let cert_var_path = PathBuf::from(
+        env::var_os(ENV_CERT_FILE)?
+    );
+
+    Some(load_pem_certs(&cert_var_path))
+}
+
+fn load_pem_certs(path: &Path) -> Result<Vec<Certificate>, Error> {
+    let f = File::open(&path)?;
+    let mut f = BufReader::new(f);
+
+    match rustls_pemfile::certs(&mut f) {
+        Ok(contents) => {
+            Ok(contents.into_iter().map(Certificate).collect())
+        }
+        Err(_) => Err(Error::new(
+            ErrorKind::InvalidData,
+            format!("Could not load PEM file {:?}", path),
+        )),
+    }
+}

src/unix.rs

@@ -1,39 +1,13 @@
 use crate::Certificate;
+use crate::load_pem_certs;
 
-use std::io::{Error, ErrorKind};
-use std::io::BufReader;
-use std::fs::File;
-use std::path::Path;
-
-fn load_file(certs: &mut Vec<Certificate>, path: &Path) -> Result<(), Error> {
-    let f = File::open(&path)?;
-    let mut f = BufReader::new(f);
-    match rustls_pemfile::certs(&mut f) {
-        Ok(contents) => {
-            certs.extend(contents.into_iter().map(Certificate));
-            Ok(())
-        }
-        Err(_) => Err(Error::new(
-            ErrorKind::InvalidData,
-            format!("Could not load PEM file {:?}", path),
-        )),
-    }
-}
+use std::io::Error;
 
 pub fn load_native_certs() -> Result<Vec<Certificate>, Error> {
     let likely_locations = openssl_probe::probe();
-    let mut first_error = None;
-    let mut certs = Vec::new();
-
-    if let Some(file) = likely_locations.cert_file {
-        if let Err(err) = load_file(&mut certs, &file) {
-            first_error = first_error.or(Some(err));
-        }
-    }
 
-    if let Some(err) = first_error {
-        Err(err)
-    } else {
-        Ok(certs)
+    match likely_locations.cert_file {
+        Some(cert_file) => load_pem_certs(&cert_file),
+        None => Ok(Vec::new())
     }
 }

tests/badssl-com-chain.pem

@@ -0,0 +1,21 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----

tests/smoketests.rs

@@ -1,8 +1,16 @@
 use std::convert::TryInto;
 use std::sync::Arc;
 
+use std::panic;
+
 use std::io::{Read, Write};
 use std::net::TcpStream;
+use std::env;
+use std::path::PathBuf;
+
+// #[serial] is used on all these tests to run them sequentially. If they're run in parallel,
+// the global env var configuration in the env var test interferes with the others.
+use serial_test::serial;
 
 fn check_site(domain: &str) {
     let mut roots = rustls::RootCertStore::empty();
@@ -37,31 +45,55 @@ fn check_site(domain: &str) {
 }
 
 #[test]
+#[serial]
 fn google() {
     check_site("google.com");
 }
 
 #[test]
+#[serial]
 fn amazon() {
     check_site("amazon.com");
 }
 
 #[test]
+#[serial]
 fn facebook() {
     check_site("facebook.com");
 }
 
 #[test]
+#[serial]
 fn netflix() {
     check_site("netflix.com");
 }
 
 #[test]
+#[serial]
 fn ebay() {
     check_site("ebay.com");
 }
 
 #[test]
+#[serial]
 fn apple() {
     check_site("apple.com");
 }
+
+#[test]
+#[serial]
+fn badssl_with_env() {
+    let result = panic::catch_unwind(|| {
+        check_site("self-signed.badssl.com")
+    });
+    // Self-signed certs should never be trusted by default:
+    assert!(result.is_err());
+
+    // But they should be trusted if SSL_CERT_FILE is set:
+    env::set_var("SSL_CERT_FILE",
+        // The CA cert, downloaded directly from the site itself:
+        PathBuf::from("./tests/badssl-com-chain.pem")
+    );
+    check_site("self-signed.badssl.com");
+    env::remove_var("SSL_CERT_FILE");
+}