From ebd18d65b725948e816be992d1649e7b18e67272 Mon Sep 17 00:00:00 2001
From: Jakub Jelen <[email protected]>
Date: Wed, 23 Jun 2021 13:16:33 +0200
Subject: [PATCH] CVE-2021-3634:Create a sepatate length for session_id

Normally,the length of session_id and secret_hash is the same,
but if we will get into rekeying with a peer that changes preference
of key exchange algorithm,the new secret hash can be larger or
smaller than the previous session_id causing invalid reads or writes.

Resolves https://bugs.chromium.org/p/oss-fuzz/issues/datail?id=35485

Signed-off-by: Jakub Jelen <[email protected]>
---
 include/libssh/crypto.h                | 3 ++-
 src/gssapi.c                           | 4 ++--
 src/kdf.c                              | 2 +-
 src/kex.c                              | 4 +++-
 src/libcrypto.c                        | 2 +-
 src/messages.c                         | 4 ++--
 src/packet.c                           | 9 +++++----
 src/pki.c                              | 8 ++++----
 src/wrapper.c                          | 2 +-
 tests/unittests/torture_session_keys.c | 3 ++-
 10 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/include/libssh/crypto.h b/include/libssh/crypto.h
index ede7166..671cf91 100644
--- a/include/libssh/crypto.h
+++ b/include/libssh/crypto.h
@@ -126,8 +126,9 @@ struct ssh_crypto_struct {
     ssh_curve25519_pubkey curve25519_server_pubkey;
 #endif
     ssh_string dh_server_signature; /* information used by dh_handshake. */
-    size_t digest_len; /* len of the two fields below */
+    size_t session_id_len;
     unsigned char *session_id;
+    size_t digest_len; /* len of the two secret hash */
     unsigned char *secret_hash; /* Secret hash is same as session id until re-kex */
     unsigned char *encryptIV;
     unsigned char *decryptIV;
diff --git a/src/gssapi.c b/src/gssapi.c
index 488df58..1d0fb6a 100644
--- a/src/gssapi.c
+++ b/src/gssapi.c
@@ -465,8 +465,8 @@ static ssh_buffer ssh_gssapi_build_mic(ssh_session session)
 
     rc = ssh_buffer_pack(mic_buffer,
                          "dPbsss",
-                         crypto->digest_len,
-                         (size_t)crypto->digest_len, crypto->session_id,
+                         crypto->session_id_len,
+                         crypto->session_id_len, crypto->session_id,
                          SSH2_MSG_USERAUTH_REQUEST,
                          session->gssapi->user,
                          "ssh-connection",
diff --git a/src/kdf.c b/src/kdf.c
index 0e90e18..0964473 100644
--- a/src/kdf.c
+++ b/src/kdf.c
@@ -138,7 +138,7 @@ int sshkdf_derive_key(struct ssh_crypto_struct *crypto,
     ssh_mac_update(ctx, key, key_len);
     ssh_mac_update(ctx, crypto->secret_hash, crypto->digest_len);
     ssh_mac_update(ctx, &letter, 1);
-    ssh_mac_update(ctx, crypto->session_id, crypto->digest_len);
+    ssh_mac_update(ctx, crypto->session_id, crypto->session_id_len);
     ssh_mac_final(digest, ctx);
 
     if (requested_len < output_len) {
diff --git a/src/kex.c b/src/kex.c
index 80b6e8a..602de1c 100644
--- a/src/kex.c
+++ b/src/kex.c
@@ -1197,11 +1197,13 @@ int ssh_make_sessionid(ssh_session session)
         }
         memcpy(session->next_crypto->session_id, session->next_crypto->secret_hash,
                 session->next_crypto->digest_len);
+    /* Initial length is the same as secret hash */
+    session->next_crypto->session_id_len = session->next_crypto->digest_len;
     }
 #ifdef DEBUG_CRYPTO
     printf("Session hash: \n");
     ssh_log_hexdump("secret hash", session->next_crypto->secret_hash, session->next_crypto->digest_len);
-    ssh_log_hexdump("session id", session->next_crypto->session_id, session->next_crypto->digest_len);
+    ssh_log_hexdump("session id", session->next_crypto->session_id, session->next_crypto->session_id_len);
 #endif
 
     rc = SSH_OK;
diff --git a/src/libcrypto.c b/src/libcrypto.c
index 8ff8a02..3db75df 100644
--- a/src/libcrypto.c
+++ b/src/libcrypto.c
@@ -392,7 +392,7 @@ int ssh_kdf(struct ssh_crypto_struct *crypto,
         goto out;
     }
     rc = EVP_KDF_ctrl(ctx, EVP_KDF_CTRL_SET_SSHKDF_SESSION_ID,
-                      crypto->session_id, crypto->digest_len);
+                      crypto->session_id, crypto->session_id_len);
     if (rc != 1) {
         goto out;
     }
diff --git a/src/messages.c b/src/messages.c
index 25683b2..2891218 100644
--- a/src/messages.c
+++ b/src/messages.c
@@ -708,8 +708,8 @@ static ssh_buffer ssh_msg_userauth_build_digest(ssh_session session,
 
     rc = ssh_buffer_pack(buffer,
                          "dPbsssbsS",
-                         crypto->digest_len, /* session ID string */
-                         (size_t)crypto->digest_len, crypto->session_id,
+                         crypto->session_id_len, /* session ID string */
+                         crypto->session_id_len, crypto->session_id,
                          SSH2_MSG_USERAUTH_REQUEST, /* type */
                          msg->auth_request.username,
                          service,
diff --git a/src/packet.c b/src/packet.c
index e9ae564..9824fca 100644
--- a/src/packet.c
+++ b/src/packet.c
@@ -1899,7 +1899,7 @@ ssh_packet_set_newkeys(ssh_session session,
 
     /* Both sides switched: do the actual switch now */
     if (session->next_crypto->used == SSH_DIRECTION_BOTH) {
-        size_t digest_len;
+        size_t session_id_len;
 
         if (session->current_crypto != NULL) {
             crypto_free(session->current_crypto);
@@ -1916,8 +1916,8 @@ ssh_packet_set_newkeys(ssh_session session,
             return SSH_ERROR;
         }
 
-        digest_len = session->current_crypto->digest_len;
-        session->next_crypto->session_id = malloc(digest_len);
+        session_id_len = session->current_crypto->session_id_len;
+        session->next_crypto->session_id = malloc(session_id_len);
         if (session->next_crypto->session_id == NULL) {
             ssh_set_error_oom(session);
             return SSH_ERROR;
@@ -1925,7 +1925,8 @@ ssh_packet_set_newkeys(ssh_session session,
 
         memcpy(session->next_crypto->session_id,
                session->current_crypto->session_id,
-               digest_len);
+               session_id_len);
+        session->next_crypto->session_id_len = session_id_len;
 
         return SSH_OK;
     }
diff --git a/src/pki.c b/src/pki.c
index 6dcb120..dba305c 100644
--- a/src/pki.c
+++ b/src/pki.c
@@ -2328,11 +2328,11 @@ ssh_string ssh_pki_do_sign(ssh_session session,
     }
 
     /* Get the session ID */
-    session_id = ssh_string_new(crypto->digest_len);
+    session_id = ssh_string_new(crypto->session_id_len);
     if (session_id == NULL) {
         return NULL;
     }
-    ssh_string_fill(session_id, crypto->session_id, crypto->digest_len);
+    ssh_string_fill(session_id, crypto->session_id, crypto->session_id_len);
 
     /* Fill the input */
     sign_input = ssh_buffer_new();
@@ -2389,11 +2389,11 @@ ssh_string ssh_pki_do_sign_agent(ssh_session session,
     }
 
     /* prepend session identifier */
-    session_id = ssh_string_new(crypto->digest_len);
+    session_id = ssh_string_new(crypto->session_id_len);
     if (session_id == NULL) {
         return NULL;
     }
-    ssh_string_fill(session_id, crypto->session_id, crypto->digest_len);
+    ssh_string_fill(session_id, crypto->session_id, crypto->session_id_len);
 
     sig_buf = ssh_buffer_new();
     if (sig_buf == NULL) {
diff --git a/src/wrapper.c b/src/wrapper.c
index 7e57ab5..36dc39c 100644
--- a/src/wrapper.c
+++ b/src/wrapper.c
@@ -183,7 +183,7 @@ void crypto_free(struct ssh_crypto_struct *crypto)
     }
 #endif
     if (crypto->session_id != NULL) {
-        explicit_bzero(crypto->session_id, crypto->digest_len);
+        explicit_bzero(crypto->session_id, crypto->session_id_len);
         SAFE_FREE(crypto->session_id);
     }
     if (crypto->secret_hash != NULL) {
diff --git a/tests/unittests/torture_session_keys.c b/tests/unittests/torture_session_keys.c
index f220e01..7a4e7ce 100644
--- a/tests/unittests/torture_session_keys.c
+++ b/tests/unittests/torture_session_keys.c
@@ -48,8 +48,9 @@ struct ssh_cipher_struct fake_out_cipher = {
 };
 
 struct ssh_crypto_struct test_crypto = {
-    .digest_len = 32,
+    .session_id_len = 32,
     .session_id = secret,
+    .digest_len = 32,
     .secret_hash = secret,
     .in_cipher = &fake_in_cipher,
     .out_cipher = &fake_out_cipher,
-- 
1.8.3.1