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author | Carlos MartÃn Nieto <carlosmn@github.com> | 2022-11-02 13:47:25 +0100 |
---|---|---|
committer | Edward Thomson <ethomson@edwardthomson.com> | 2023-01-18 21:36:29 +0000 |
commit | 63b083e5d8ecea0750eb5767018b1cdad94c2382 (patch) | |
tree | 61dadc975b0166c0276b5f9adf0da7374e85182f | |
parent | bbcadbbcd8184d1c04b12bcbecd162df2c2ed0b0 (diff) | |
download | libgit2-63b083e5d8ecea0750eb5767018b1cdad94c2382.tar.gz |
ssh: look for a key in known_hosts to set the key type for the handshake
The server and client negotiate a single hostkey, but the "best" cipher may not
be the one for which we have an entry in `known_hosts`. This can lead to us not
finding the key in known_hosts even though we should be connecting.
Instead here we look up the hostname with a nonsense key to perform a lookup in
the known hosts and set that. This is roughly what the OpenSSH client does as
well.
-rw-r--r-- | src/transports/ssh.c | 206 |
1 files changed, 153 insertions, 53 deletions
diff --git a/src/transports/ssh.c b/src/transports/ssh.c index 185d53959..85e779744 100644 --- a/src/transports/ssh.c +++ b/src/transports/ssh.c @@ -421,15 +421,119 @@ static int request_creds(git_credential **out, ssh_subtransport *t, const char * return 0; } +#define KNOWN_HOSTS_FILE ".ssh/known_hosts" + +/* + * Load the known_hosts file. + * + * Returns success but leaves the output NULL if we couldn't find the file. + */ +static int load_known_hosts(LIBSSH2_KNOWNHOSTS **hosts, LIBSSH2_SESSION *session) +{ + git_str path = GIT_STR_INIT, home = GIT_STR_INIT; + LIBSSH2_KNOWNHOSTS *known_hosts = NULL; + int error; + + GIT_ASSERT_ARG(hosts); + + if ((error = git__getenv(&home, "HOME")) < 0) + return error; + + if ((error = git_str_joinpath(&path, git_str_cstr(&home), KNOWN_HOSTS_FILE)) < 0) + goto out; + + if ((known_hosts = libssh2_knownhost_init(session)) == NULL) { + ssh_error(session, "error initializing known hosts"); + error = -1; + goto out; + } + + /* + * Try to read the file and consider not finding it as not trusting the + * host rather than an error. + */ + error = libssh2_knownhost_readfile(known_hosts, git_str_cstr(&path), LIBSSH2_KNOWNHOST_FILE_OPENSSH); + if (error == LIBSSH2_ERROR_FILE) + error = 0; + if (error < 0) + ssh_error(session, "error reading known_hosts"); + +out: + *hosts = known_hosts; + + git_str_clear(&home); + git_str_clear(&path); + + return error; +} + +static const char *hostkey_type_to_string(int type) +{ + switch (type) { + case LIBSSH2_KNOWNHOST_KEY_SSHRSA: + return "ssh-rsa"; + case LIBSSH2_KNOWNHOST_KEY_SSHDSS: + return "ssh-dss"; +#ifdef LIBSSH2_KNOWNHOST_KEY_ECDSA_256 + case LIBSSH2_KNOWNHOST_KEY_ECDSA_256: + return "ecdsa-sha2-nistp256"; + case LIBSSH2_KNOWNHOST_KEY_ECDSA_384: + return "ecdsa-sha2-nistp384"; + case LIBSSH2_KNOWNHOST_KEY_ECDSA_521: + return "ecdsa-sha2-nistp521"; +#endif +#ifdef LIBSSH2_KNOWNHOST_KEY_ED25519 + case LIBSSH2_KNOWNHOST_KEY_ED25519: + return "ssh-ed25519"; +#endif + } + + return NULL; +} + +/* + * We figure out what kind of key we want to ask the remote for by trying to + * look it up with a nonsense key and using that mismatch to figure out what key + * we do have stored for the host. + * + * Returns the string to pass to libssh2_session_method_pref or NULL if we were + * unable to find anything or an error happened. + */ +static const char *find_hostkey_preference(LIBSSH2_KNOWNHOSTS *known_hosts, const char *hostname, int port) +{ + struct libssh2_knownhost *host = NULL; + /* Specify no key type so we don't filter on that */ + int type = LIBSSH2_KNOWNHOST_TYPE_PLAIN | LIBSSH2_KNOWNHOST_KEYENC_RAW; + const char key = '\0'; + int error; + + /* + * In case of mismatch, we can find the type of key from known_hosts in + * the returned host's information as it means that an entry was found + * but our nonsense key obviously didn't match. + */ + error = libssh2_knownhost_checkp(known_hosts, hostname, port, &key, 1, type, &host); + if (error == LIBSSH2_KNOWNHOST_CHECK_MISMATCH) + return hostkey_type_to_string(host->typemask & LIBSSH2_KNOWNHOST_KEY_MASK); + + return NULL; +} + static int _git_ssh_session_create( LIBSSH2_SESSION **session, + LIBSSH2_KNOWNHOSTS **hosts, + const char *hostname, + int port, git_stream *io) { int rc = 0; LIBSSH2_SESSION *s; + LIBSSH2_KNOWNHOSTS *known_hosts; git_socket_stream *socket = GIT_CONTAINER_OF(io, git_socket_stream, parent); + const char *keytype = NULL; GIT_ASSERT_ARG(session); + GIT_ASSERT_ARG(hosts); s = libssh2_session_init(); if (!s) { @@ -437,23 +541,46 @@ static int _git_ssh_session_create( return -1; } + if ((rc = load_known_hosts(&known_hosts, s)) < 0) { + ssh_error(s, "error loading known_hosts"); + libssh2_session_free(s); + return -1; + } + + if ((keytype = find_hostkey_preference(known_hosts, hostname, port)) != NULL) { + do { + rc = libssh2_session_method_pref(s, LIBSSH2_METHOD_HOSTKEY, keytype); + } while (LIBSSH2_ERROR_EAGAIN == rc || LIBSSH2_ERROR_TIMEOUT == rc); + if (rc != LIBSSH2_ERROR_NONE) { + ssh_error(s, "failed to set hostkey preference"); + goto on_error; + } + } + + do { rc = libssh2_session_handshake(s, socket->s); } while (LIBSSH2_ERROR_EAGAIN == rc || LIBSSH2_ERROR_TIMEOUT == rc); if (rc != LIBSSH2_ERROR_NONE) { ssh_error(s, "failed to start SSH session"); - libssh2_session_free(s); - return -1; + goto on_error; } libssh2_session_set_blocking(s, 1); *session = s; + *hosts = known_hosts; return 0; + +on_error: + libssh2_knownhost_free(known_hosts); + libssh2_session_free(s); + return -1; } + /* * Returns the typemask argument to pass to libssh2_knownhost_check{,p} based on * the type of key that libssh2_session_hostkey returns. @@ -491,8 +618,6 @@ static int fingerprint_type_mask(int keytype) return mask; } -#define KNOWN_HOSTS_FILE ".ssh/known_hosts" - /* * Check the host against the user's known_hosts file. * @@ -500,62 +625,28 @@ static int fingerprint_type_mask(int keytype) */ static int check_against_known_hosts( LIBSSH2_SESSION *session, + LIBSSH2_KNOWNHOSTS *known_hosts, const char *hostname, int port, const char *key, size_t key_len, int key_type) { - int error, check, typemask, ret = 0; - git_str path = GIT_STR_INIT, home = GIT_STR_INIT; - LIBSSH2_KNOWNHOSTS *known_hosts = NULL; + int check, typemask, ret = 0; struct libssh2_knownhost *host = NULL; - if ((error = git__getenv(&home, "HOME")) < 0) { - return error; - } - - if ((error = git_str_joinpath(&path, git_str_cstr(&home), KNOWN_HOSTS_FILE)) < 0) { - ret = error; - goto out; - } - - if ((known_hosts = libssh2_knownhost_init(session)) == NULL) { - ssh_error(session, "error initializing known hosts"); - ret = -1; - goto out; - } - - /* - * Try to read the file and consider not finding it as not trusting the - * host rather than an error. - */ - error = libssh2_knownhost_readfile(known_hosts, git_str_cstr(&path), LIBSSH2_KNOWNHOST_FILE_OPENSSH); - if (error == LIBSSH2_ERROR_FILE) { - ret = 0; - goto out; - } - if (error < 0) { - ssh_error(session, "error reading known_hosts"); - ret = -1; - goto out; - } + if (known_hosts == NULL) + return 0; typemask = fingerprint_type_mask(key_type); check = libssh2_knownhost_checkp(known_hosts, hostname, port, key, key_len, typemask, &host); if (check == LIBSSH2_KNOWNHOST_CHECK_FAILURE) { ssh_error(session, "error checking for known host"); - ret = -1; - goto out; + return -1; } ret = check == LIBSSH2_KNOWNHOST_CHECK_MATCH ? 1 : 0; -out: - libssh2_knownhost_free(known_hosts); - git_str_clear(&path); - git_str_clear(&home); - return ret; } @@ -567,26 +658,23 @@ out: */ static int check_certificate( LIBSSH2_SESSION *session, + LIBSSH2_KNOWNHOSTS *known_hosts, git_transport_certificate_check_cb check_cb, void *check_cb_payload, const char *host, - const char *portstr) + int port) { git_cert_hostkey cert = {{ 0 }}; const char *key; size_t cert_len; - int cert_type, port, cert_valid = 0, error = 0; + int cert_type, cert_valid = 0, error = 0; if ((key = libssh2_session_hostkey(session, &cert_len, &cert_type)) == NULL) { ssh_error(session, "failed to retrieve hostkey"); return -1; } - /* Try to parse the port as a number, if we can't then fall back to default */ - if (git__strntol32(&port, portstr, strlen(portstr), NULL, 10) < 0) - port = -1; - - if ((cert_valid = check_against_known_hosts(session, host, port, key, cert_len, cert_type)) < 0) + if ((cert_valid = check_against_known_hosts(session, known_hosts, host, port, key, cert_len, cert_type)) < 0) return -1; cert.parent.cert_type = GIT_CERT_HOSTKEY_LIBSSH2; @@ -682,11 +770,12 @@ static int _git_ssh_setup_conn( const char *cmd, git_smart_subtransport_stream **stream) { - int auth_methods, error = 0; + int auth_methods, error = 0, port; ssh_stream *s; git_credential *cred = NULL; LIBSSH2_SESSION *session=NULL; LIBSSH2_CHANNEL *channel=NULL; + LIBSSH2_KNOWNHOSTS *known_hosts = NULL; t->current_stream = NULL; @@ -710,10 +799,19 @@ static int _git_ssh_setup_conn( (error = git_stream_connect(s->io)) < 0) goto done; - if ((error = _git_ssh_session_create(&session, s->io)) < 0) + /* + * Try to parse the port as a number, if we can't then fall back to + * default. It would be nice if we could get the port that was resolved + * as part of the stream connection, but that's not something that's + * exposed. + */ + if (git__strntol32(&port, s->url.port, strlen(s->url.port), NULL, 10) < 0) + port = -1; + + if ((error = _git_ssh_session_create(&session, &known_hosts, s->url.host, port, s->io)) < 0) goto done; - if ((error = check_certificate(session, t->owner->connect_opts.callbacks.certificate_check, t->owner->connect_opts.callbacks.payload, s->url.host, s->url.port)) < 0) + if ((error = check_certificate(session, known_hosts, t->owner->connect_opts.callbacks.certificate_check, t->owner->connect_opts.callbacks.payload, s->url.host, port)) < 0) goto done; /* we need the username to ask for auth methods */ @@ -786,6 +884,8 @@ done: if (error < 0) { ssh_stream_free(*stream); + if (known_hosts) + libssh2_knownhost_free(known_hosts); if (session) libssh2_session_free(session); } |