Validate SSH host keys

Cargo.toml

@@ -17,6 +17,7 @@ path = "src/cargo/lib.rs"
 
 [dependencies]
 atty = "0.2"
+base64 = "0.13.1"
 bytesize = "1.0"
 cargo-platform = { path = "crates/cargo-platform", version = "0.1.2" }
 cargo-util = { path = "crates/cargo-util", version = "0.2.1" }
@@ -28,8 +29,8 @@ pretty_env_logger = { version = "0.4", optional = true }
 anyhow = "1.0"
 filetime = "0.2.9"
 flate2 = { version = "1.0.3", default-features = false, features = ["zlib"] }
-git2 = "0.15.0"
-git2-curl = "0.16.0"
+git2 = "0.16.0"
+git2-curl = "0.17.0"
 glob = "0.3.0"
 hex = "0.4"
 home = "0.5"
@@ -41,7 +42,7 @@ jobserver = "0.1.24"
 lazycell = "1.2.0"
 libc = "0.2"
 log = "0.4.6"
-libgit2-sys = "0.14.0"
+libgit2-sys = "0.14.1"
 memchr = "2.1.3"
 opener = "0.5"
 os_info = "3.5.0"

crates/cargo-test-support/Cargo.toml

@@ -15,7 +15,7 @@ crates-io = { path = "../crates-io" }
 snapbox = { version = "0.3.0", features = ["diff", "path"] }
 filetime = "0.2"
 flate2 = { version = "1.0", default-features = false, features = ["zlib"] }
-git2 = "0.15.0"
+git2 = "0.16.0"
 glob = "0.3"
 itertools = "0.10.0"
 lazy_static = "1.0"

src/cargo/sources/git/known_hosts.rs

@@ -0,0 +1,439 @@
+//! SSH host key validation support.
+//!
+//! A primary goal with this implementation is to provide user-friendly error
+//! messages, guiding them to understand the issue and how to resolve it.
+//!
+//! Note that there are a lot of limitations here. This reads OpenSSH
+//! known_hosts files from well-known locations, but it does not read OpenSSH
+//! config files. The config file can change the behavior of how OpenSSH
+//! handles known_hosts files. For example, some things we don't handle:
+//!
+//! - `GlobalKnownHostsFile` — Changes the location of the global host file.
+//! - `UserKnownHostsFile` — Changes the location of the user's host file.
+//! - `KnownHostsCommand` — A command to fetch known hosts.
+//! - `CheckHostIP` — DNS spoofing checks.
+//! - `VisualHostKey` — Shows a visual ascii-art key.
+//! - `VerifyHostKeyDNS` — Uses SSHFP DNS records to fetch a host key.
+//!
+//! There's also a number of things that aren't supported but could be easily
+//! added (it just adds a little complexity). For example, hashed hostnames,
+//! hostname patterns, and revoked markers. See "FIXME" comments littered in
+//! this file.
+
+use git2::cert::Cert;
+use git2::CertificateCheckStatus;
+use std::collections::HashSet;
+use std::fmt::Write;
+use std::path::{Path, PathBuf};
+
+/// These are host keys that are hard-coded in cargo to provide convenience.
+///
+/// If GitHub ever publishes new keys, the user can add them to their own
+/// configuration file to use those instead.
+///
+/// The GitHub keys are sourced from <https://api.github.com/meta> or
+/// <https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/githubs-ssh-key-fingerprints>.
+///
+/// These will be ignored if the user adds their own entries for `github.com`,
+/// which can be useful if GitHub ever revokes their old keys.
+static BUNDLED_KEYS: &[(&str, &str, &str)] = &[
+    ("github.com", "ssh-ed25519", "AAAAC3NzaC1lZDI1NTE5AAAAIOMqqnkVzrm0SdG6UOoqKLsabgH5C9okWi0dh2l9GKJl"),
+    ("github.com", "ecdsa-sha2-nistp256", "AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEmKSENjQEezOmxkZMy7opKgwFB9nkt5YRrYMjNuG5N87uRgg6CLrbo5wAdT/y6v0mKV0U2w0WZ2YB/++Tpockg="),
+    ("github.com", "ssh-rsa", "AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ=="),
+];
+
+enum KnownHostError {
+    /// Some general error happened while validating the known hosts.
+    CheckError(anyhow::Error),
+    /// The host key was not found.
+    HostKeyNotFound {
+        hostname: String,
+        key_type: git2::cert::SshHostKeyType,
+        remote_host_key: String,
+        remote_fingerprint: String,
+        other_hosts: Vec<KnownHost>,
+    },
+    /// The host key was found, but does not match the remote's key.
+    HostKeyHasChanged {
+        hostname: String,
+        key_type: git2::cert::SshHostKeyType,
+        old_known_host: KnownHost,
+        remote_host_key: String,
+        remote_fingerprint: String,
+    },
+}
+
+impl From<anyhow::Error> for KnownHostError {
+    fn from(err: anyhow::Error) -> KnownHostError {
+        KnownHostError::CheckError(err.into())
+    }
+}
+
+/// The location where a host key was located.
+#[derive(Clone)]
+enum KnownHostLocation {
+    /// Loaded from a file from disk.
+    File { path: PathBuf, lineno: u32 },
+    /// Part of the hard-coded bundled keys in Cargo.
+    Bundled,
+}
+
+/// The git2 callback used to validate a certificate (only ssh known hosts are validated).
+pub fn certificate_check(
+    cert: &Cert<'_>,
+    host: &str,
+    port: Option<u16>,
+) -> Result<CertificateCheckStatus, git2::Error> {
+    let Some(host_key) = cert.as_hostkey() else {
+        // Return passthrough for TLS X509 certificates to use whatever validation
+        // was done in git2.
+        return Ok(CertificateCheckStatus::CertificatePassthrough)
+    };
+    // If a nonstandard port is in use, check for that first.
+    // The fallback to check without a port is handled in the HostKeyNotFound handler.
+    let host_maybe_port = match port {
+        Some(port) if port != 22 => format!("[{host}]:{port}"),
+        _ => host.to_string(),
+    };
+    // The error message must be constructed as a string to pass through the libgit2 C API.
+    let err_msg = match check_ssh_known_hosts(host_key, &host_maybe_port) {
+        Ok(()) => {
+            return Ok(CertificateCheckStatus::CertificateOk);
+        }
+        Err(KnownHostError::CheckError(e)) => {
+            format!("error: failed to validate host key:\n{:#}", e)
+        }
+        Err(KnownHostError::HostKeyNotFound {
+            hostname,
+            key_type,
+            remote_host_key,
+            remote_fingerprint,
+            other_hosts,
+        }) => {
+            // Try checking without the port.
+            if port.is_some()
+                && !matches!(port, Some(22))
+                && check_ssh_known_hosts(host_key, host).is_ok()
+            {
+                return Ok(CertificateCheckStatus::CertificateOk);
+            }
+            let key_type_short_name = key_type.short_name();
+            let key_type_name = key_type.name();
+            let known_hosts_location = user_known_host_location_to_add();
+            let other_hosts_message = if other_hosts.is_empty() {
+                String::new()
+            } else {
+                let mut msg = String::from(
+                    "Note: This host key was found, \
+                    but is associated with a different host:\n",
+                );
+                for known_host in other_hosts {
+                    let loc = match known_host.location {
+                        KnownHostLocation::File { path, lineno } => {
+                            format!("{} line {lineno}", path.display())
+                        }
+                        KnownHostLocation::Bundled => format!("bundled with cargo"),
+                    };
+                    write!(msg, "    {loc}: {}\n", known_host.patterns).unwrap();
+                }
+                msg
+            };
+            format!("error: unknown SSH host key\n\
+                The SSH host key for `{hostname}` is not known and cannot be validated.\n\
+                \n\
+                To resolve this issue, add the host key to {known_hosts_location}\n\
+                \n\
+                The key to add is:\n\
+                \n\
+                {hostname} {key_type_name} {remote_host_key}\n\
+                \n\
+                The {key_type_short_name} key fingerprint is: SHA256:{remote_fingerprint}\n\
+                This fingerprint should be validated with the server administrator that it is correct.\n\
+                {other_hosts_message}\n\
+                See https://doc.rust-lang.org/nightly/cargo/appendix/git-authentication.html#ssh-known-hosts \
+                for more information.\n\
+                ")
+        }
+        Err(KnownHostError::HostKeyHasChanged {
+            hostname,
+            key_type,
+            old_known_host,
+            remote_host_key,
+            remote_fingerprint,
+        }) => {
+            let key_type_short_name = key_type.short_name();
+            let key_type_name = key_type.name();
+            let known_hosts_location = user_known_host_location_to_add();
+            let old_key_resolution = match old_known_host.location {
+                KnownHostLocation::File { path, lineno } => {
+                    let old_key_location = path.display();
+                    format!(
+                        "removing the old {key_type_name} key for `{hostname}` \
+                        located at {old_key_location} line {lineno}, \
+                        and adding the new key to {known_hosts_location}",
+                    )
+                }
+                KnownHostLocation::Bundled => {
+                    format!(
+                        "adding the new key to {known_hosts_location}\n\
+                        The current host key is bundled as part of Cargo."
+                    )
+                }
+            };
+            format!("error: SSH host key has changed for `{hostname}`\n\
+                *********************************\n\
+                * WARNING: HOST KEY HAS CHANGED *\n\
+                *********************************\n\
+                This may be caused by a man-in-the-middle attack, or the \
+                server may have changed its host key.\n\
+                \n\
+                The {key_type_short_name} fingerprint for the key from the remote host is:\n\
+                    SHA256:{remote_fingerprint}\n\
+                \n\
+                You are strongly encouraged to contact the server \
+                administrator for `{hostname}` to verify that this new key is \
+                correct.\n\
+                \n\
+                If you can verify that the server has a new key, you can \
+                resolve this error by {old_key_resolution}\n\
+                \n\
+                The key provided by the remote host is:\n\
+                \n\
+                {hostname} {key_type_name} {remote_host_key}\n\
+                \n\
+                See https://doc.rust-lang.org/nightly/cargo/appendix/git-authentication.html#ssh-known-hosts \
+                for more information.\n\
+                ")
+        }
+    };
+    Err(git2::Error::new(
+        git2::ErrorCode::GenericError,
+        git2::ErrorClass::Callback,
+        err_msg,
+    ))
+}
+
+/// Checks if the given host/host key pair is known.
+fn check_ssh_known_hosts(
+    cert_host_key: &git2::cert::CertHostkey<'_>,
+    host: &str,
+) -> Result<(), KnownHostError> {
+    let Some(remote_host_key) = cert_host_key.hostkey() else {
+        return Err(anyhow::format_err!("remote host key is not available").into());
+    };
+    let remote_key_type = cert_host_key.hostkey_type().unwrap();
+    // `changed_key` keeps track of any entries where the key has changed.
+    let mut changed_key = None;
+    // `other_hosts` keeps track of any entries that have an identical key,
+    // but a different hostname.
+    let mut other_hosts = Vec::new();
+
+    // Collect all the known host entries from disk.
+    let mut known_hosts = Vec::new();
+    for path in known_host_files() {
+        if !path.exists() {
+            continue;
+        }
+        let hosts = load_hostfile(&path)?;
+        known_hosts.extend(hosts);
+    }
+    // Load the bundled keys. Don't add keys for hosts that the user has
+    // configured, which gives them the option to override them. This could be
+    // useful if the keys are ever revoked.
+    let configured_hosts: HashSet<_> = known_hosts
+        .iter()
+        .flat_map(|known_host| {
+            known_host
+                .patterns
+                .split(',')
+                .map(|pattern| pattern.to_lowercase())
+        })
+        .collect();
+    for (patterns, key_type, key) in BUNDLED_KEYS {
+        if !configured_hosts.contains(*patterns) {
+            let key = base64::decode(key).unwrap();
+            known_hosts.push(KnownHost {
+                location: KnownHostLocation::Bundled,
+                patterns: patterns.to_string(),
+                key_type: key_type.to_string(),
+                key,
+            });
+        }
+    }
+
+    for known_host in known_hosts {
+        // The key type from libgit2 needs to match the key type from the host file.
+        if known_host.key_type != remote_key_type.name() {
+            continue;
+        }
+        let key_matches = known_host.key == remote_host_key;
+        if !known_host.host_matches(host) {
+            // `name` can be None for hashed hostnames (which libgit2 does not expose).
+            if key_matches {
+                other_hosts.push(known_host.clone());
+            }
+            continue;
+        }
+        if key_matches {
+            return Ok(());
+        }
+        // The host and key type matched, but the key itself did not.
+        // This indicates the key has changed.
+        // This is only reported as an error if no subsequent lines have a
+        // correct key.
+        changed_key = Some(known_host.clone());
+    }
+    // Older versions of OpenSSH (before 6.8, March 2015) showed MD5
+    // fingerprints (see FingerprintHash ssh config option). Here we only
+    // support SHA256.
+    let mut remote_fingerprint = cargo_util::Sha256::new();
+    remote_fingerprint.update(remote_host_key);
+    let remote_fingerprint =
+        base64::encode_config(remote_fingerprint.finish(), base64::STANDARD_NO_PAD);
+    let remote_host_key = base64::encode(remote_host_key);
+    // FIXME: Ideally the error message should include the IP address of the
+    // remote host (to help the user validate that they are connecting to the
+    // host they were expecting to). However, I don't see a way to obtain that
+    // information from libgit2.
+    match changed_key {
+        Some(old_known_host) => Err(KnownHostError::HostKeyHasChanged {
+            hostname: host.to_string(),
+            key_type: remote_key_type,
+            old_known_host,
+            remote_host_key,
+            remote_fingerprint,
+        }),
+        None => Err(KnownHostError::HostKeyNotFound {
+            hostname: host.to_string(),
+            key_type: remote_key_type,
+            remote_host_key,
+            remote_fingerprint,
+            other_hosts,
+        }),
+    }
+}
+
+/// Returns a list of files to try loading OpenSSH-formatted known hosts.
+fn known_host_files() -> Vec<PathBuf> {
+    let mut result = Vec::new();
+    if cfg!(unix) {
+        result.push(PathBuf::from("/etc/ssh/ssh_known_hosts"));
+    } else if cfg!(windows) {
+        // The msys/cygwin version of OpenSSH uses `/etc` from the posix root
+        // filesystem there (such as `C:\msys64\etc\ssh\ssh_known_hosts`).
+        // However, I do not know of a way to obtain that location from
+        // Windows-land. The ProgramData version here is what the PowerShell
+        // port of OpenSSH does.
+        if let Some(progdata) = std::env::var_os("ProgramData") {
+            let mut progdata = PathBuf::from(progdata);
+            progdata.push("ssh");
+            progdata.push("ssh_known_hosts");
+            result.push(progdata)
+        }
+    }
+    result.extend(user_known_host_location());
+    result
+}
+
+/// The location of the user's known_hosts file.
+fn user_known_host_location() -> Option<PathBuf> {
+    // NOTE: This is a potentially inaccurate prediction of what the user
+    // actually wants. The actual location depends on several factors:
+    //
+    // - Windows OpenSSH Powershell version: I believe this looks up the home
+    //   directory via ProfileImagePath in the registry, falling back to
+    //   `GetWindowsDirectoryW` if that fails.
+    // - OpenSSH Portable (under msys): This is very complicated. I got lost
+    //   after following it through some ldap/active directory stuff.
+    // - OpenSSH (most unix platforms): Uses `pw->pw_dir` from `getpwuid()`.
+    //
+    // This doesn't do anything close to that. home_dir's behavior is:
+    // - Windows: $USERPROFILE, or SHGetFolderPathW()
+    // - Unix: $HOME, or getpwuid_r()
+    //
+    // Since there is a mismatch here, the location returned here might be
+    // different than what the user's `ssh` CLI command uses. We may want to
+    // consider trying to align it better.
+    home::home_dir().map(|mut home| {
+        home.push(".ssh");
+        home.push("known_hosts");
+        home
+    })
+}
+
+/// The location to display in an error message instructing the user where to
+/// add the new key.
+fn user_known_host_location_to_add() -> String {
+    // Note that we don't bother with the legacy known_hosts2 files.
+    match user_known_host_location() {
+        Some(path) => path.to_str().expect("utf-8 home").to_string(),
+        None => "~/.ssh/known_hosts".to_string(),
+    }
+}
+
+/// A single known host entry.
+#[derive(Clone)]
+struct KnownHost {
+    location: KnownHostLocation,
+    /// The hostname. May be comma separated to match multiple hosts.
+    patterns: String,
+    key_type: String,
+    key: Vec<u8>,
+}
+
+impl KnownHost {
+    /// Returns whether or not the given host matches this known host entry.
+    fn host_matches(&self, host: &str) -> bool {
+        let mut match_found = false;
+        let host = host.to_lowercase();
+        // FIXME: support hashed hostnames
+        for pattern in self.patterns.split(',') {
+            let pattern = pattern.to_lowercase();
+            // FIXME: support * and ? wildcards
+            if let Some(pattern) = pattern.strip_prefix('!') {
+                if pattern == host {
+                    return false;
+                }
+            } else {
+                match_found = pattern == host;
+            }
+        }
+        match_found
+    }
+}
+
+/// Loads an OpenSSH known_hosts file.
+fn load_hostfile(path: &Path) -> Result<Vec<KnownHost>, anyhow::Error> {
+    let contents = cargo_util::paths::read(path)?;
+    let entries = contents
+        .lines()
+        .enumerate()
+        .filter_map(|(lineno, line)| {
+            let location = KnownHostLocation::File {
+                path: path.to_path_buf(),
+                lineno: lineno as u32 + 1,
+            };
+            parse_known_hosts_line(line, location)
+        })
+        .collect();
+    Ok(entries)
+}
+
+fn parse_known_hosts_line(line: &str, location: KnownHostLocation) -> Option<KnownHost> {
+    let line = line.trim();
+    // FIXME: @revoked and @cert-authority is currently not supported.
+    if line.is_empty() || line.starts_with('#') || line.starts_with('@') {
+        return None;
+    }
+    let mut parts = line.split([' ', '\t']).filter(|s| !s.is_empty());
+    let Some(patterns) = parts.next() else { return None };
+    let Some(key_type) = parts.next() else { return None };
+    let Some(key) = parts.next() else { return None };
+    let Ok(key) = base64::decode(key) else { return None };
+    Some(KnownHost {
+        location,
+        patterns: patterns.to_string(),
+        key_type: key_type.to_string(),
+        key,
+    })
+}

src/cargo/sources/git/mod.rs

@@ -1,4 +1,5 @@
 pub use self::source::GitSource;
 pub use self::utils::{fetch, GitCheckout, GitDatabase, GitRemote};
+mod known_hosts;
 mod source;
 mod utils;

src/cargo/sources/git/utils.rs

@@ -683,7 +683,6 @@ where
             | ErrorClass::Submodule
             | ErrorClass::FetchHead
             | ErrorClass::Ssh
-            | ErrorClass::Callback
             | ErrorClass::Http => {
                 let mut msg = "network failure seems to have happened\n".to_string();
                 msg.push_str(
@@ -694,6 +693,13 @@ where
                 );
                 err = err.context(msg);
             }
+            ErrorClass::Callback => {
+                // This unwraps the git2 error. We're using the callback error
+                // specifically to convey errors from Rust land through the C
+                // callback interface. We don't need the `; class=Callback
+                // (26)` that gets tacked on to the git2 error message.
+                err = anyhow::format_err!("{}", e.message());
+            }
             _ => {}
         }
     }
@@ -722,12 +728,16 @@ pub fn with_fetch_options(
     let mut progress = Progress::new("Fetch", config);
     network::with_retry(config, || {
         with_authentication(url, git_config, |f| {
+            let port = Url::parse(url).ok().and_then(|url| url.port());
             let mut last_update = Instant::now();
             let mut rcb = git2::RemoteCallbacks::new();
             // We choose `N=10` here to make a `300ms * 10slots ~= 3000ms`
             // sliding window for tracking the data transfer rate (in bytes/s).
             let mut counter = MetricsCounter::<10>::new(0, last_update);
             rcb.credentials(f);
+            rcb.certificate_check(|cert, host| {
+                super::known_hosts::certificate_check(cert, host, port)
+            });
             rcb.transfer_progress(|stats| {
                 let indexed_deltas = stats.indexed_deltas();
                 let msg = if indexed_deltas > 0 {

src/doc/src/appendix/git-authentication.md

@@ -58,9 +58,32 @@ on how to start `ssh-agent` and to add keys.
 > used by Cargo's built-in SSH library. More advanced requirements should use
 > [`net.git-fetch-with-cli`].
 
+### SSH Known Hosts
+
+When connecting to an SSH host, Cargo must verify the identity of the host
+using "known hosts", which are a list of host keys. Cargo can look for these
+known hosts in OpenSSH-style `known_hosts` files located in their standard
+locations (`.ssh/known_hosts` in your home directory, or
+`/etc/ssh/ssh_known_hosts` on Unix-like platforms or
+`%PROGRAMDATA%\ssh\ssh_known_hosts` on Windows). More information about these
+files can be found in the [sshd man page].
+
+When connecting to an SSH host before the known hosts has been configured,
+Cargo will display an error message instructing you how to add the host key.
+This also includes a "fingerprint", which is a smaller hash of the host key,
+which should be easier to visually verify. The server administrator can get
+the fingerprint by running `ssh-keygen` against the public key (for example,
+`ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub`). Well-known sites may
+publish their fingerprints on the web; for example GitHub posts theirs at
+<https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/githubs-ssh-key-fingerprints>.
+
+Cargo comes with the host keys for [github.com](https://github.com) built-in.
+If those ever change, you can add the new keys to your known_hosts file.
+
 [`credential.helper`]: https://git-scm.com/book/en/v2/Git-Tools-Credential-Storage
 [`net.git-fetch-with-cli`]: ../reference/config.md#netgit-fetch-with-cli
 [GCM]: https://github.com/microsoft/Git-Credential-Manager-Core/
 [PuTTY]: https://www.chiark.greenend.org.uk/~sgtatham/putty/
 [Microsoft installation documentation]: https://docs.microsoft.com/en-us/windows-server/administration/openssh/openssh_install_firstuse
 [key management]: https://docs.microsoft.com/en-us/windows-server/administration/openssh/openssh_keymanagement
+[sshd man page]: https://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT