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ssh: look for a key in known_hosts to set the key type for the handshake

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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.
This commit is contained in:
Carlos Martín Nieto
2022-11-02 13:47:25 +01:00
committed by Edward Thomson
parent 52ebc04774
commit 63212a929d
1 changed files with 153 additions and 53 deletions
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206
src/libgit2/transports/ssh.c
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@@ -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);
}