From d4634630432594b139b3af6b9f254b890c0f275d Mon Sep 17 00:00:00 2001
From: Michael Buckley <michael@buckleyisms.com>
Date: Thu, 30 Nov 2023 15:08:02 -0800
Subject: [PATCH] src: add 'strict KEX' to fix CVE-2023-48795 "Terrapin Attack"

Refs:
https://terrapin-attack.com/
https://seclists.org/oss-sec/2023/q4/292
https://osv.dev/list?ecosystem=&q=CVE-2023-48795
https://github.com/advisories/GHSA-45x7-px36-x8w8
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-48795

Fixes #1290
Closes #1291

CVE: CVE-2023-48795
Upstream-Status: Backport
Signed-off-by: Ross Burton <ross.burton@arm.com>
---
 src/kex.c          | 63 +++++++++++++++++++++++------------
 src/libssh2_priv.h | 18 +++++++---
 src/packet.c       | 83 +++++++++++++++++++++++++++++++++++++++++++---
 src/packet.h       |  2 +-
 src/session.c      |  3 ++
 src/transport.c    | 12 ++++++-
 6 files changed, 149 insertions(+), 32 deletions(-)

diff --git a/src/kex.c b/src/kex.c
index d4034a0a..b4b748ca 100644
--- a/src/kex.c
+++ b/src/kex.c
@@ -3037,6 +3037,13 @@ kex_method_extension_negotiation = {
     0,
 };
 
+static const LIBSSH2_KEX_METHOD
+kex_method_strict_client_extension = {
+    "kex-strict-c-v00@openssh.com",
+    NULL,
+    0,
+};
+
 static const LIBSSH2_KEX_METHOD *libssh2_kex_methods[] = {
 #if LIBSSH2_ED25519
     &kex_method_ssh_curve25519_sha256,
@@ -3055,6 +3062,7 @@ static const LIBSSH2_KEX_METHOD *libssh2_kex_methods[] = {
     &kex_method_diffie_helman_group1_sha1,
     &kex_method_diffie_helman_group_exchange_sha1,
     &kex_method_extension_negotiation,
+    &kex_method_strict_client_extension,
     NULL
 };
 
@@ -3307,13 +3315,13 @@ static int kexinit(LIBSSH2_SESSION * session)
     return 0;
 }
 
-/* kex_agree_instr
+/* _libssh2_kex_agree_instr
  * Kex specific variant of strstr()
  * Needle must be preceded by BOL or ',', and followed by ',' or EOL
  */
-static unsigned char *
-kex_agree_instr(unsigned char *haystack, size_t haystack_len,
-                const unsigned char *needle, size_t needle_len)
+unsigned char *
+_libssh2_kex_agree_instr(unsigned char *haystack, size_t haystack_len,
+                         const unsigned char *needle, size_t needle_len)
 {
     unsigned char *s;
     unsigned char *end_haystack;
@@ -3398,7 +3406,7 @@ static int kex_agree_hostkey(LIBSSH2_SESSION * session,
         while(s && *s) {
             unsigned char *p = (unsigned char *) strchr((char *) s, ',');
             size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
-            if(kex_agree_instr(hostkey, hostkey_len, s, method_len)) {
+            if(_libssh2_kex_agree_instr(hostkey, hostkey_len, s, method_len)) {
                 const LIBSSH2_HOSTKEY_METHOD *method =
                     (const LIBSSH2_HOSTKEY_METHOD *)
                     kex_get_method_by_name((char *) s, method_len,
@@ -3432,9 +3440,9 @@ static int kex_agree_hostkey(LIBSSH2_SESSION * session,
     }
 
     while(hostkeyp && (*hostkeyp) && (*hostkeyp)->name) {
-        s = kex_agree_instr(hostkey, hostkey_len,
-                            (unsigned char *) (*hostkeyp)->name,
-                            strlen((*hostkeyp)->name));
+        s = _libssh2_kex_agree_instr(hostkey, hostkey_len,
+                                     (unsigned char *) (*hostkeyp)->name,
+                                     strlen((*hostkeyp)->name));
         if(s) {
             /* So far so good, but does it suit our purposes? (Encrypting vs
                Signing) */
@@ -3468,6 +3476,12 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
 {
     const LIBSSH2_KEX_METHOD **kexp = libssh2_kex_methods;
     unsigned char *s;
+    const unsigned char *strict =
+        (unsigned char *)"kex-strict-s-v00@openssh.com";
+
+    if(_libssh2_kex_agree_instr(kex, kex_len, strict, 28)) {
+        session->kex_strict = 1;
+    }
 
     if(session->kex_prefs) {
         s = (unsigned char *) session->kex_prefs;
@@ -3475,7 +3489,7 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
         while(s && *s) {
             unsigned char *q, *p = (unsigned char *) strchr((char *) s, ',');
             size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
-            q = kex_agree_instr(kex, kex_len, s, method_len);
+            q = _libssh2_kex_agree_instr(kex, kex_len, s, method_len);
             if(q) {
                 const LIBSSH2_KEX_METHOD *method = (const LIBSSH2_KEX_METHOD *)
                     kex_get_method_by_name((char *) s, method_len,
@@ -3509,9 +3523,9 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
     }
 
     while(*kexp && (*kexp)->name) {
-        s = kex_agree_instr(kex, kex_len,
-                            (unsigned char *) (*kexp)->name,
-                            strlen((*kexp)->name));
+        s = _libssh2_kex_agree_instr(kex, kex_len,
+                                     (unsigned char *) (*kexp)->name,
+                                     strlen((*kexp)->name));
         if(s) {
             /* We've agreed on a key exchange method,
              * Can we agree on a hostkey that works with this kex?
@@ -3555,7 +3569,7 @@ static int kex_agree_crypt(LIBSSH2_SESSION * session,
             unsigned char *p = (unsigned char *) strchr((char *) s, ',');
             size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
 
-            if(kex_agree_instr(crypt, crypt_len, s, method_len)) {
+            if(_libssh2_kex_agree_instr(crypt, crypt_len, s, method_len)) {
                 const LIBSSH2_CRYPT_METHOD *method =
                     (const LIBSSH2_CRYPT_METHOD *)
                     kex_get_method_by_name((char *) s, method_len,
@@ -3577,9 +3591,9 @@ static int kex_agree_crypt(LIBSSH2_SESSION * session,
     }
 
     while(*cryptp && (*cryptp)->name) {
-        s = kex_agree_instr(crypt, crypt_len,
-                            (unsigned char *) (*cryptp)->name,
-                            strlen((*cryptp)->name));
+        s = _libssh2_kex_agree_instr(crypt, crypt_len,
+                                     (unsigned char *) (*cryptp)->name,
+                                     strlen((*cryptp)->name));
         if(s) {
             endpoint->crypt = *cryptp;
             return 0;
@@ -3619,7 +3633,7 @@ static int kex_agree_mac(LIBSSH2_SESSION * session,
             unsigned char *p = (unsigned char *) strchr((char *) s, ',');
             size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
 
-            if(kex_agree_instr(mac, mac_len, s, method_len)) {
+            if(_libssh2_kex_agree_instr(mac, mac_len, s, method_len)) {
                 const LIBSSH2_MAC_METHOD *method = (const LIBSSH2_MAC_METHOD *)
                     kex_get_method_by_name((char *) s, method_len,
                                            (const LIBSSH2_COMMON_METHOD **)
@@ -3640,8 +3654,9 @@ static int kex_agree_mac(LIBSSH2_SESSION * session,
     }
 
     while(*macp && (*macp)->name) {
-        s = kex_agree_instr(mac, mac_len, (unsigned char *) (*macp)->name,
-                            strlen((*macp)->name));
+        s = _libssh2_kex_agree_instr(mac, mac_len,
+                                     (unsigned char *) (*macp)->name,
+                                     strlen((*macp)->name));
         if(s) {
             endpoint->mac = *macp;
             return 0;
@@ -3672,7 +3687,7 @@ static int kex_agree_comp(LIBSSH2_SESSION *session,
             unsigned char *p = (unsigned char *) strchr((char *) s, ',');
             size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
 
-            if(kex_agree_instr(comp, comp_len, s, method_len)) {
+            if(_libssh2_kex_agree_instr(comp, comp_len, s, method_len)) {
                 const LIBSSH2_COMP_METHOD *method =
                     (const LIBSSH2_COMP_METHOD *)
                     kex_get_method_by_name((char *) s, method_len,
@@ -3694,8 +3709,9 @@ static int kex_agree_comp(LIBSSH2_SESSION *session,
     }
 
     while(*compp && (*compp)->name) {
-        s = kex_agree_instr(comp, comp_len, (unsigned char *) (*compp)->name,
-                            strlen((*compp)->name));
+        s = _libssh2_kex_agree_instr(comp, comp_len,
+                                     (unsigned char *) (*compp)->name,
+                                     strlen((*compp)->name));
         if(s) {
             endpoint->comp = *compp;
             return 0;
@@ -3876,6 +3892,7 @@ _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
                 session->local.kexinit = key_state->oldlocal;
                 session->local.kexinit_len = key_state->oldlocal_len;
                 key_state->state = libssh2_NB_state_idle;
+                session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
                 session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                 session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
                 return -1;
@@ -3901,6 +3918,7 @@ _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
                 session->local.kexinit = key_state->oldlocal;
                 session->local.kexinit_len = key_state->oldlocal_len;
                 key_state->state = libssh2_NB_state_idle;
+                session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
                 session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                 session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
                 return -1;
@@ -3949,6 +3967,7 @@ _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
         session->remote.kexinit = NULL;
     }
 
+    session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
     session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
     session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
 
diff --git a/src/libssh2_priv.h b/src/libssh2_priv.h
index 82c3afe2..ee1d8b5c 100644
--- a/src/libssh2_priv.h
+++ b/src/libssh2_priv.h
@@ -699,6 +699,9 @@ struct _LIBSSH2_SESSION
     /* key signing algorithm preferences -- NULL yields server order */
     char *sign_algo_prefs;
 
+    /* Whether to use the OpenSSH Strict KEX extension */
+    int kex_strict;
+
     /* (remote as source of data -- packet_read ) */
     libssh2_endpoint_data remote;
 
@@ -870,6 +873,7 @@ struct _LIBSSH2_SESSION
     int fullpacket_macstate;
     size_t fullpacket_payload_len;
     int fullpacket_packet_type;
+    uint32_t fullpacket_required_type;
 
     /* State variables used in libssh2_sftp_init() */
     libssh2_nonblocking_states sftpInit_state;
@@ -910,10 +914,11 @@ struct _LIBSSH2_SESSION
 };
 
 /* session.state bits */
-#define LIBSSH2_STATE_EXCHANGING_KEYS   0x00000001
-#define LIBSSH2_STATE_NEWKEYS           0x00000002
-#define LIBSSH2_STATE_AUTHENTICATED     0x00000004
-#define LIBSSH2_STATE_KEX_ACTIVE        0x00000008
+#define LIBSSH2_STATE_INITIAL_KEX       0x00000001
+#define LIBSSH2_STATE_EXCHANGING_KEYS   0x00000002
+#define LIBSSH2_STATE_NEWKEYS           0x00000004
+#define LIBSSH2_STATE_AUTHENTICATED     0x00000008
+#define LIBSSH2_STATE_KEX_ACTIVE        0x00000010
 
 /* session.flag helpers */
 #ifdef MSG_NOSIGNAL
@@ -1144,6 +1149,11 @@ ssize_t _libssh2_send(libssh2_socket_t socket, const void *buffer,
 int _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
                           key_exchange_state_t * state);
 
+unsigned char *_libssh2_kex_agree_instr(unsigned char *haystack,
+                                        size_t haystack_len,
+                                        const unsigned char *needle,
+                                        size_t needle_len);
+
 /* Let crypt.c/hostkey.c expose their method structs */
 const LIBSSH2_CRYPT_METHOD **libssh2_crypt_methods(void);
 const LIBSSH2_HOSTKEY_METHOD **libssh2_hostkey_methods(void);
diff --git a/src/packet.c b/src/packet.c
index b5b41981..35d4d39e 100644
--- a/src/packet.c
+++ b/src/packet.c
@@ -605,14 +605,13 @@ authagent_exit:
  * layer when it has received a packet.
  *
  * The input pointer 'data' is pointing to allocated data that this function
- * is asked to deal with so on failure OR success, it must be freed fine.
- * The only exception is when the return code is LIBSSH2_ERROR_EAGAIN.
+ * will be freed unless return the code is LIBSSH2_ERROR_EAGAIN.
  *
  * This function will always be called with 'datalen' greater than zero.
  */
 int
 _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
-                    size_t datalen, int macstate)
+                    size_t datalen, int macstate, uint32_t seq)
 {
     int rc = 0;
     unsigned char *message = NULL;
@@ -657,6 +656,70 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
         break;
     }
 
+    if(session->state & LIBSSH2_STATE_INITIAL_KEX) {
+        if(msg == SSH_MSG_KEXINIT) {
+            if(!session->kex_strict) {
+                if(datalen < 17) {
+                    LIBSSH2_FREE(session, data);
+                    session->packAdd_state = libssh2_NB_state_idle;
+                    return _libssh2_error(session,
+                                          LIBSSH2_ERROR_BUFFER_TOO_SMALL,
+                                          "Data too short extracting kex");
+                }
+                else {
+                    const unsigned char *strict =
+                    (unsigned char *)"kex-strict-s-v00@openssh.com";
+                    struct string_buf buf;
+                    unsigned char *algs = NULL;
+                    size_t algs_len = 0;
+
+                    buf.data = (unsigned char *)data;
+                    buf.dataptr = buf.data;
+                    buf.len = datalen;
+                    buf.dataptr += 17; /* advance past type and cookie */
+
+                    if(_libssh2_get_string(&buf, &algs, &algs_len)) {
+                        LIBSSH2_FREE(session, data);
+                        session->packAdd_state = libssh2_NB_state_idle;
+                        return _libssh2_error(session,
+                                              LIBSSH2_ERROR_BUFFER_TOO_SMALL,
+                                              "Algs too short");
+                    }
+
+                    if(algs_len == 0 ||
+                       _libssh2_kex_agree_instr(algs, algs_len, strict, 28)) {
+                        session->kex_strict = 1;
+                    }
+                }
+            }
+
+            if(session->kex_strict && seq) {
+                LIBSSH2_FREE(session, data);
+                session->socket_state = LIBSSH2_SOCKET_DISCONNECTED;
+                session->packAdd_state = libssh2_NB_state_idle;
+                libssh2_session_disconnect(session, "strict KEX violation: "
+                                           "KEXINIT was not the first packet");
+
+                return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_DISCONNECT,
+                                      "strict KEX violation: "
+                                      "KEXINIT was not the first packet");
+            }
+        }
+
+        if(session->kex_strict && session->fullpacket_required_type &&
+            session->fullpacket_required_type != msg) {
+            LIBSSH2_FREE(session, data);
+            session->socket_state = LIBSSH2_SOCKET_DISCONNECTED;
+            session->packAdd_state = libssh2_NB_state_idle;
+            libssh2_session_disconnect(session, "strict KEX violation: "
+                                       "unexpected packet type");
+
+            return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_DISCONNECT,
+                                  "strict KEX violation: "
+                                  "unexpected packet type");
+        }
+    }
+
     if(session->packAdd_state == libssh2_NB_state_allocated) {
         /* A couple exceptions to the packet adding rule: */
         switch(msg) {
@@ -1341,6 +1404,15 @@ _libssh2_packet_ask(LIBSSH2_SESSION * session, unsigned char packet_type,
 
             return 0;
         }
+        else if(session->kex_strict &&
+                (session->state & LIBSSH2_STATE_INITIAL_KEX)) {
+            libssh2_session_disconnect(session, "strict KEX violation: "
+                                       "unexpected packet type");
+
+            return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_DISCONNECT,
+                                  "strict KEX violation: "
+                                  "unexpected packet type");
+        }
         packet = _libssh2_list_next(&packet->node);
     }
     return -1;
@@ -1402,7 +1474,10 @@ _libssh2_packet_require(LIBSSH2_SESSION * session, unsigned char packet_type,
     }
 
     while(session->socket_state == LIBSSH2_SOCKET_CONNECTED) {
-        int ret = _libssh2_transport_read(session);
+        int ret;
+        session->fullpacket_required_type = packet_type;
+        ret = _libssh2_transport_read(session);
+        session->fullpacket_required_type = 0;
         if(ret == LIBSSH2_ERROR_EAGAIN)
             return ret;
         else if(ret < 0) {
diff --git a/src/packet.h b/src/packet.h
index 79018bcf..6ea100a5 100644
--- a/src/packet.h
+++ b/src/packet.h
@@ -71,6 +71,6 @@ int _libssh2_packet_burn(LIBSSH2_SESSION * session,
 int _libssh2_packet_write(LIBSSH2_SESSION * session, unsigned char *data,
                           unsigned long data_len);
 int _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
-                        size_t datalen, int macstate);
+                        size_t datalen, int macstate, uint32_t seq);
 
 #endif /* __LIBSSH2_PACKET_H */
diff --git a/src/session.c b/src/session.c
index a4d602ba..f4bafb57 100644
--- a/src/session.c
+++ b/src/session.c
@@ -464,6 +464,8 @@ libssh2_session_init_ex(LIBSSH2_ALLOC_FUNC((*my_alloc)),
         session->abstract = abstract;
         session->api_timeout = 0; /* timeout-free API by default */
         session->api_block_mode = 1; /* blocking API by default */
+        session->state = LIBSSH2_STATE_INITIAL_KEX;
+        session->fullpacket_required_type = 0;
         session->packet_read_timeout = LIBSSH2_DEFAULT_READ_TIMEOUT;
         session->flag.quote_paths = 1; /* default behavior is to quote paths
                                           for the scp subsystem */
@@ -1186,6 +1188,7 @@ libssh2_session_disconnect_ex(LIBSSH2_SESSION *session, int reason,
                               const char *desc, const char *lang)
 {
     int rc;
+    session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
     session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
     BLOCK_ADJUST(rc, session,
                  session_disconnect(session, reason, desc, lang));
diff --git a/src/transport.c b/src/transport.c
index 6d902d33..3b30ff84 100644
--- a/src/transport.c
+++ b/src/transport.c
@@ -187,6 +187,7 @@ fullpacket(LIBSSH2_SESSION * session, int encrypted /* 1 or 0 */ )
     struct transportpacket *p = &session->packet;
     int rc;
     int compressed;
+    uint32_t seq = session->remote.seqno;
 
     if(session->fullpacket_state == libssh2_NB_state_idle) {
         session->fullpacket_macstate = LIBSSH2_MAC_CONFIRMED;
@@ -318,7 +319,7 @@ fullpacket(LIBSSH2_SESSION * session, int encrypted /* 1 or 0 */ )
     if(session->fullpacket_state == libssh2_NB_state_created) {
         rc = _libssh2_packet_add(session, p->payload,
                                  session->fullpacket_payload_len,
-                                 session->fullpacket_macstate);
+                                 session->fullpacket_macstate, seq);
         if(rc == LIBSSH2_ERROR_EAGAIN)
             return rc;
         if(rc) {
@@ -329,6 +330,11 @@ fullpacket(LIBSSH2_SESSION * session, int encrypted /* 1 or 0 */ )
 
     session->fullpacket_state = libssh2_NB_state_idle;
 
+    if(session->kex_strict &&
+        session->fullpacket_packet_type == SSH_MSG_NEWKEYS) {
+        session->remote.seqno = 0;
+    }
+
     return session->fullpacket_packet_type;
 }
 
@@ -1091,6 +1097,10 @@ int _libssh2_transport_send(LIBSSH2_SESSION *session,
 
     session->local.seqno++;
 
+    if(session->kex_strict && data[0] == SSH_MSG_NEWKEYS) {
+        session->local.seqno = 0;
+    }
+
     ret = LIBSSH2_SEND(session, p->outbuf, total_length,
                        LIBSSH2_SOCKET_SEND_FLAGS(session));
     if(ret < 0)
-- 
2.34.1