ssh: make sure to ask for a username and use the same one

In order to know which authentication methods are supported/allowed by
the ssh server, we need to send a NONE auth request, which needs a
username associated with it.

Most ssh server implementations do not allow switching the username
between authentication attempts, which means we cannot use a dummy
username and then switch. There are two ways around this.

The first is to use a different connection, which an earlier commit
implements, but this increases how long it takes to get set up, and
without knowing the right username, we cannot guarantee that the
list we get in response is the right one.

The second is what's implemented here: if there is no username specified
in the url, ask for it first. We can then ask for the list of auth
methods and use the user's credentials in the same connection.

2 files changed, 38 insertions, 34 deletions

diff --git a/src/transports/cred_helpers.c b/src/transports/cred_helpers.c
index d420e3e3c..5cc9b0869 100644
--- a/src/transports/cred_helpers.c
+++ b/src/transports/cred_helpers.c
@@ -41,6 +41,9 @@ int git_cred_userpass(
 	else
 		return -1;
 
+	if (GIT_CREDTYPE_USERNAME & allowed_types)
+		return git_cred_username_new(cred, effective_username);
+
 	if ((GIT_CREDTYPE_USERPASS_PLAINTEXT & allowed_types) == 0 ||
 			git_cred_userpass_plaintext_new(cred, effective_username, userpass->password) < 0)
 		return -1;
diff --git a/src/transports/ssh.c b/src/transports/ssh.c
index d4c39bd39..6a7f67e99 100644
--- a/src/transports/ssh.c
+++ b/src/transports/ssh.c
@@ -9,6 +9,7 @@
 #include "buffer.h"
 #include "netops.h"
 #include "smart.h"
+#include "cred.h"
 
 #ifdef GIT_SSH
 
@@ -36,7 +37,7 @@ typedef struct {
 	ssh_stream *current_stream;
 } ssh_subtransport;
 
-static int list_auth_methods(int *out, const char *host, const char *port);
+static int list_auth_methods(int *out, LIBSSH2_SESSION *session, const char *username);
 
 static void ssh_error(LIBSSH2_SESSION *session, const char *errmsg)
 {
@@ -377,6 +378,12 @@ static int request_creds(git_cred **out, ssh_subtransport *t, const char *user,
 		return -1;
 	}
 
+	if (!(cred->credtype & auth_methods)) {
+		cred->free(cred);
+		giterr_set(GITERR_SSH, "callback returned unsupported credentials type");
+		return -1;
+	}
+
 	*out = cred;
 
 	return 0;
@@ -444,26 +451,33 @@ static int _git_ssh_setup_conn(
 		GITERR_CHECK_ALLOC(port);
 	}
 
-	if (list_auth_methods(&auth_methods, host, port) < 0) {
-		auth_methods = GIT_CREDTYPE_USERPASS_PLAINTEXT |
-			GIT_CREDTYPE_SSH_KEY | GIT_CREDTYPE_SSH_CUSTOM |
-			GIT_CREDTYPE_SSH_INTERACTIVE;
-	}
-
-	if ((error = gitno_connect(&s->socket, host, port, 0)) < 0)
-		goto on_error;
+	/* we need the username to ask for auth methods */
+	if (!user) {
+		if ((error = request_creds(&cred, t, NULL, GIT_CREDTYPE_USERNAME)) < 0)
+			goto on_error;
 
-	if (user && pass) {
+		user = git__strdup(((git_cred_username *) cred)->username);
+		cred->free(cred);
+		cred = NULL;
+		if (!user)
+			goto on_error;
+	} else if (user && pass) {
 		if ((error = git_cred_userpass_plaintext_new(&cred, user, pass)) < 0)
 			goto on_error;
 	}
 
+	if ((error = gitno_connect(&s->socket, host, port, 0)) < 0)
+		goto on_error;
+
 	if ((error = _git_ssh_session_create(&session, s->socket)) < 0)
 		goto on_error;
 
+	if ((error = list_auth_methods(&auth_methods, session, user)) < 0)
+		goto on_error;
+
 	error = GIT_EAUTH;
 	/* if we already have something to try */
-	if (cred)
+	if (cred && auth_methods & cred->credtype)
 		error = _git_ssh_authenticate_session(session, cred);
 
 	while (error == GIT_EAUTH) {
@@ -475,6 +489,12 @@ static int _git_ssh_setup_conn(
 		if ((error = request_creds(&cred, t, user, auth_methods)) < 0)
 			goto on_error;
 
+		if (strcmp(user, git_cred__username(cred))) {
+			giterr_set(GITERR_SSH, "username does not match previous request");
+			error = -1;
+			goto on_error;
+		}
+
 		error = _git_ssh_authenticate_session(session, cred);
 	}
 
@@ -628,26 +648,17 @@ static void _ssh_free(git_smart_subtransport *subtransport)
 #define SSH_AUTH_PASSWORD "password"
 #define SSH_AUTH_KEYBOARD_INTERACTIVE "keyboard-interactive"
 
-static int list_auth_methods(int *out, const char *host, const char *port)
+static int list_auth_methods(int *out, LIBSSH2_SESSION *session, const char *username)
 {
-	gitno_socket s;
-	LIBSSH2_SESSION *session = NULL;
 	const char *list, *ptr;
 
 	*out = 0;
 
-	if (gitno_connect(&s, host, port, 0) < 0)
-		return -1;
-
-	if (_git_ssh_session_create(&session, s) < 0)
-		goto on_error;
-
-	/* the username is dummy, we're throwing away the connection anyway */
-	list = libssh2_userauth_list(session, "git", strlen("git"));
+	list = libssh2_userauth_list(session, username, strlen(username));
 
 	/* either error, or the remote accepts NONE auth, which is bizarre, let's punt */
-	if (list == NULL)
-		goto out;
+	if (list == NULL && !libssh2_userauth_authenticated(session))
+		return -1;
 
 	ptr = list;
 	while (ptr) {
@@ -677,17 +688,7 @@ static int list_auth_methods(int *out, const char *host, const char *port)
 		ptr = strchr(ptr, ',');
 	}
 
-out:
-	libssh2_session_free(session);
-	gitno_close(&s);
-
 	return 0;
-
-on_error:
-	libssh2_session_free(session);
-	gitno_close(&s);
-
-	return -1;
 }
 #endif