CVE-2023-1667:kex: Add support for sending first_kex_packet_follows flag

This is not completely straightforward as it requires us to do some state
shuffling.

We introduce internal flag that can turn this on in client side, so far for
testing only as we do not want to universally enable this. We also repurpose the
server flag indicating the guess was wrong also for the client to make desired
decisions.

If we found out our guess was wrong, we need to hope the server was able to
figure out this much, we need to revert the DH FSM state, drop the callbacks
from the "wrong" key exchange method and initiate the right one.

The server side is already tested by the pkd_hello_i1, which is executing tests
against dropbrear clients, which is using this flag by default out of the box.

Tested manually also with the pkd_hello --rekey to make sure the server is able
to handle the rekeying with all key exchange methods.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Norbert Pocs <npocs@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

Author: Jakuje

include/libssh/dh.h

@@ -77,6 +77,7 @@ int ssh_dh_get_current_server_publickey_blob(ssh_session session,
 ssh_key ssh_dh_get_next_server_publickey(ssh_session session);
 int ssh_dh_get_next_server_publickey_blob(ssh_session session,
                                           ssh_string *pubkey_blob);
+int dh_handshake(ssh_session session);
 
 int ssh_client_dh_init(ssh_session session);
 void ssh_client_dh_remove_callbacks(ssh_session session);

include/libssh/session.h

@@ -169,14 +169,21 @@ struct ssh_session_struct {
         uint32_t current_method;
     } auth;
 
+    /* Sending this flag before key exchange to save one round trip during the
+     * key exchange. This might make sense on high-latency connections.
+     * So far internal only for testing. Usable only on the client side --
+     * there is no key exchange method that would start with server message */
+    bool send_first_kex_follows;
     /*
      * RFC 4253, 7.1: if the first_kex_packet_follows flag was set in
      * the received SSH_MSG_KEXINIT, but the guess was wrong, this
      * field will be set such that the following guessed packet will
-     * be ignored.  Once that packet has been received and ignored,
-     * this field is cleared.
+     * be ignored on the receiving side.  Once that packet has been received and
+     * ignored, this field is cleared.
+     * On the sending side, this is set after we got peer KEXINIT message and we
+     * need to resend the initial message of the negotiated KEX algorithm.
      */
-    int first_kex_follows_guess_wrong;
+    bool first_kex_follows_guess_wrong;
 
     ssh_buffer in_hashbuf;
     ssh_buffer out_hashbuf;

src/client.c

@@ -246,10 +246,13 @@ int ssh_send_banner(ssh_session session, int server)
  * @warning this function returning is no proof that DH handshake is
  * completed
  */
-static int dh_handshake(ssh_session session)
+int dh_handshake(ssh_session session)
 {
   int rc = SSH_AGAIN;
 
+  SSH_LOG(SSH_LOG_TRACE, "dh_handshake_state = %d, kex_type = %d",
+          session->dh_handshake_state,  session->next_crypto->kex_type);
+
   switch (session->dh_handshake_state) {
     case DH_STATE_INIT:
       switch(session->next_crypto->kex_type){
@@ -391,6 +394,8 @@ static void ssh_client_connection_callback(ssh_session session)
 {
     int rc;
 
+    SSH_LOG(SSH_LOG_DEBUG, "session_state=%d", session->session_state);
+
     switch (session->session_state) {
     case SSH_SESSION_STATE_NONE:
     case SSH_SESSION_STATE_CONNECTING:
@@ -453,6 +458,9 @@ static void ssh_client_connection_callback(ssh_session session)
             goto error;
         set_status(session, 0.8f);
         session->session_state = SSH_SESSION_STATE_DH;
+
+        /* If the init packet was already sent in previous step, this will be no
+         * operation */
         if (dh_handshake(session) == SSH_ERROR) {
             goto error;
         }

src/kex.c

@@ -28,6 +28,7 @@
 #include <stdio.h>
 #include <stdbool.h>
 
+#include "libssh/libssh.h"
 #include "libssh/priv.h"
 #include "libssh/buffer.h"
 #include "libssh/dh.h"
@@ -374,14 +375,19 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit)
     (void)type;
     (void)user;
 
+    SSH_LOG(SSH_LOG_TRACE, "KEXINIT received");
+
     if (session->session_state == SSH_SESSION_STATE_AUTHENTICATED) {
         if (session->dh_handshake_state == DH_STATE_FINISHED) {
             SSH_LOG(SSH_LOG_DEBUG, "Peer initiated key re-exchange");
             /* Reset the sent flag if the re-kex was initiated by the peer */
             session->flags &= ~SSH_SESSION_FLAG_KEXINIT_SENT;
-        } else if (session->dh_handshake_state == DH_STATE_INIT_SENT) {
-            SSH_LOG(SSH_LOG_DEBUG, "Receeved peer kexinit answer");
-        } else {
+        } else if (session->flags & SSH_SESSION_FLAG_KEXINIT_SENT &&
+                   session->dh_handshake_state == DH_STATE_INIT_SENT) {
+            /* This happens only when we are sending our-guessed first kex
+             * packet right after our KEXINIT packet. */
+            SSH_LOG(SSH_LOG_DEBUG, "Received peer kexinit answer.");
+        } else if (session->session_state != SSH_SESSION_STATE_INITIAL_KEX) {
             ssh_set_error(session, SSH_FATAL,
                           "SSH_KEXINIT received in wrong state");
             goto error;
@@ -495,16 +501,19 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit)
     /*
      * Remember whether 'first_kex_packet_follows' was set and the client
      * guess was wrong: in this case the next SSH_MSG_KEXDH_INIT message
-     * must be ignored.
+     * must be ignored on the server side.
+     * Client needs to start the Key exchange over with the correct method
      */
-    if (first_kex_packet_follows) {
+    if (first_kex_packet_follows || session->send_first_kex_follows) {
         char **client_methods = crypto->client_kex.methods;
         char **server_methods = crypto->server_kex.methods;
         session->first_kex_follows_guess_wrong =
             cmp_first_kex_algo(client_methods[SSH_KEX],
                                server_methods[SSH_KEX]) ||
             cmp_first_kex_algo(client_methods[SSH_HOSTKEYS],
                                server_methods[SSH_HOSTKEYS]);
+            SSH_LOG(SSH_LOG_DEBUG, "The initial guess was %s.",
+                    session->first_kex_follows_guess_wrong ? "wrong" : "right");
     }
 
     if (server_kex) {
@@ -583,7 +592,12 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit)
 
     /* Note, that his overwrites authenticated state in case of rekeying */
     session->session_state = SSH_SESSION_STATE_KEXINIT_RECEIVED;
-    session->dh_handshake_state = DH_STATE_INIT;
+    /* if we already sent our initial key exchange packet, do not reset the
+     * DH state. We will know if we were right with our guess only in
+     * dh_handshake_state() */
+    if (session->send_first_kex_follows == false) {
+        session->dh_handshake_state = DH_STATE_INIT;
+    }
     session->ssh_connection_callback(session);
     return SSH_PACKET_USED;
 
@@ -892,8 +906,9 @@ int ssh_kex_select_methods (ssh_session session)
     struct ssh_crypto_struct *crypto = session->next_crypto;
     struct ssh_kex_struct *server = &crypto->server_kex;
     struct ssh_kex_struct *client = &crypto->client_kex;
-    char *ext_start = NULL, *kex;
+    char *ext_start = NULL;
     const char *aead_hmac = NULL;
+    enum ssh_key_exchange_e kex_type;
     int i;
 
     /* Here we should drop the  ext-info-c  from the list so we avoid matching.
@@ -925,8 +940,18 @@ int ssh_kex_select_methods (ssh_session session)
             crypto->kex_methods[i] = strdup("");
         }
     }
-    kex = crypto->kex_methods[SSH_KEX];
-    crypto->kex_type = kex_select_kex_type(kex);
+
+    /* We can not set this value directly as the old value is needed to revert
+     * callbacks if we are client */
+    kex_type = kex_select_kex_type(crypto->kex_methods[SSH_KEX]);
+    if (session->client && session->first_kex_follows_guess_wrong) {
+        SSH_LOG(SSH_LOG_DEBUG, "Our guess was wrong. Restarting the KEX");
+        /* We need to remove the wrong callbacks and start kex again */
+        revert_kex_callbacks(session);
+        session->dh_handshake_state = DH_STATE_INIT;
+        session->first_kex_follows_guess_wrong = false;
+    }
+    crypto->kex_type = kex_type;
 
     SSH_LOG(SSH_LOG_INFO, "Negotiated %s,%s,%s,%s,%s,%s,%s,%s,%s,%s",
             session->next_crypto->kex_methods[SSH_KEX],
@@ -953,6 +978,19 @@ int ssh_send_kex(ssh_session session)
     ssh_string str = NULL;
     int i;
     int rc;
+    int first_kex_packet_follows = 0;
+
+    /* Only client can initiate the handshake methods we implement. If we
+     * already received the peer mechanisms, there is no point in guessing */
+    if (session->client &&
+        session->session_state != SSH_SESSION_STATE_KEXINIT_RECEIVED &&
+        session->send_first_kex_follows) {
+        first_kex_packet_follows = 1;
+    }
+
+    SSH_LOG(SSH_LOG_TRACE,
+            "Sending KEXINIT packet, first_kex_packet_follows = %d",
+            first_kex_packet_follows);
 
     rc = ssh_buffer_pack(session->out_buffer,
                          "bP",
@@ -987,14 +1025,14 @@ int ssh_send_kex(ssh_session session)
 
     rc = ssh_buffer_pack(session->out_buffer,
                          "bd",
-                         0,
+                         first_kex_packet_follows,
                          0);
     if (rc != SSH_OK) {
         goto error;
     }
 
     /* Prepare also the first_kex_packet_follows and reserved to 0 */
-    rc = ssh_buffer_add_u8(session->out_hashbuf, 0);
+    rc = ssh_buffer_add_u8(session->out_hashbuf, first_kex_packet_follows);
     if (rc < 0) {
         goto error;
     }
@@ -1010,6 +1048,30 @@ int ssh_send_kex(ssh_session session)
 
     session->flags |= SSH_SESSION_FLAG_KEXINIT_SENT;
     SSH_LOG(SSH_LOG_PACKET, "SSH_MSG_KEXINIT sent");
+
+    /* If we indicated that we are sending the guessed key exchange packet,
+     * do it now. The packet is simple, but we need to do some preparations */
+    if (first_kex_packet_follows) {
+        char *list = kex->methods[SSH_KEX];
+        char *colon = strchr(list, ',');
+        size_t kex_name_len = colon ? (size_t)(colon - list) : strlen(list);
+        char *kex_name = calloc(kex_name_len + 1, 1);
+        if (kex_name == NULL) {
+            ssh_set_error_oom(session);
+            goto error;
+        }
+        snprintf(kex_name, kex_name_len + 1, "%.*s", (int)kex_name_len, list);
+        SSH_LOG(SSH_LOG_TRACE, "Sending the first kex packet for %s", kex_name);
+
+        session->next_crypto->kex_type = kex_select_kex_type(kex_name);
+        free(kex_name);
+
+        /* run the first step of the DH handshake */
+        session->dh_handshake_state = DH_STATE_INIT;
+        if (dh_handshake(session) == SSH_ERROR) {
+            goto error;
+        }
+    }
     return 0;
 
 error: