Properly handle null bytes and invalid characters in control messages

This makes OpenVPN more picky in accepting control message in two aspects:
- Characters are checked in the whole buffer and not until the first
  NUL byte
- if the message contains invalid characters, we no longer continue
  evaluating a fixed up version of the message but rather stop
  processing it completely.

Previously it was possible to get invalid characters to end up in log
files or on a terminal.

This also prepares the logic a bit in the direction of having a proper
framing of control messages separated by null bytes instead of relying
on the TLS framing for that. All OpenVPN implementations write the 0
bytes between control commands.

This patch also include several improvement suggestion from Reynir
(thanks!).

CVE: 2024-5594

Reported-By: Reynir Björnsson <reynir@reynir.dk>
Change-Id: I0d926f910637dabc89bf5fa919dc6beef1eb46d9
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20240619103004.56460-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28791.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/buffer.c b/src/openvpn/buffer.c
index 3a8069c563e13bfbe50dc28fa6629cea5a747950..abe6a9c891d5ad473135d66676a4eb9a9a95da18 100644 (file)
--- a/src/openvpn/buffer.c
+++ b/src/openvpn/buffer.c
@@ -1087,6 +1087,23 @@ string_mod(char *str, const unsigned int inclusive, const unsigned int exclusive
     return ret;
 }
 
+bool
+string_check_buf(struct buffer *buf, const unsigned int inclusive, const unsigned int exclusive)
+{
+    ASSERT(buf);
+
+    for (int i = 0; i < BLEN(buf); i++)
+    {
+        char c = BSTR(buf)[i];
+
+        if (!char_inc_exc(c, inclusive, exclusive))
+        {
+            return false;
+        }
+    }
+    return true;
+}
+
 const char *
 string_mod_const(const char *str,
                  const unsigned int inclusive,

diff --git a/src/openvpn/buffer.h b/src/openvpn/buffer.h
index 27c319952319b2aa7d69f7a9772520ba44c843f3..8a40010bc1a2e822ed80ebdc4e656526999a11da 100644 (file)
--- a/src/openvpn/buffer.h
+++ b/src/openvpn/buffer.h
@@ -943,6 +943,18 @@ bool string_class(const char *str, const unsigned int inclusive, const unsigned
  */
 bool string_mod(char *str, const unsigned int inclusive, const unsigned int exclusive, const char replace);
 
+
+/**
+ * Check a buffer if it only consists of allowed characters.
+ *
+ * @param buf The buffer to be checked.
+ * @param inclusive The character classes that are allowed.
+ * @param exclusive Character classes that are not allowed even if they are also in inclusive.
+ * @return True if the string consists only of allowed characters, false otherwise.
+ */
+bool
+string_check_buf(struct buffer *buf, const unsigned int inclusive, const unsigned int exclusive);
+
 /**
  * Returns a copy of a string with certain classes of characters of it replaced with a specified
  * character.

diff --git a/src/openvpn/forward.c b/src/openvpn/forward.c
index 01165b2ed9262efffe7cfeaeea43e2a67202a51e..71b71672801dd80823328ace2f69f2b8e0cae58f 100644 (file)
--- a/src/openvpn/forward.c
+++ b/src/openvpn/forward.c
@@ -230,6 +230,51 @@ check_tls(struct context *c)
     }
 }
 
+static void
+parse_incoming_control_channel_command(struct context *c, struct buffer *buf)
+{
+    if (buf_string_match_head_str(buf, "AUTH_FAILED"))
+    {
+        receive_auth_failed(c, buf);
+    }
+    else if (buf_string_match_head_str(buf, "PUSH_"))
+    {
+        incoming_push_message(c, buf);
+    }
+    else if (buf_string_match_head_str(buf, "RESTART"))
+    {
+        server_pushed_signal(c, buf, true, 7);
+    }
+    else if (buf_string_match_head_str(buf, "HALT"))
+    {
+        server_pushed_signal(c, buf, false, 4);
+    }
+    else if (buf_string_match_head_str(buf, "INFO_PRE"))
+    {
+        server_pushed_info(c, buf, 8);
+    }
+    else if (buf_string_match_head_str(buf, "INFO"))
+    {
+        server_pushed_info(c, buf, 4);
+    }
+    else if (buf_string_match_head_str(buf, "CR_RESPONSE"))
+    {
+        receive_cr_response(c, buf);
+    }
+    else if (buf_string_match_head_str(buf, "AUTH_PENDING"))
+    {
+        receive_auth_pending(c, buf);
+    }
+    else if (buf_string_match_head_str(buf, "EXIT"))
+    {
+        receive_exit_message(c);
+    }
+    else
+    {
+        msg(D_PUSH_ERRORS, "WARNING: Received unknown control message: %s", BSTR(buf));
+    }
+}
+
 /*
  * Handle incoming configuration
  * messages on the control channel.
@@ -245,51 +290,41 @@ check_incoming_control_channel(struct context *c)
     struct buffer buf = alloc_buf_gc(len, &gc);
     if (tls_rec_payload(c->c2.tls_multi, &buf))
     {
-        /* force null termination of message */
-        buf_null_terminate(&buf);
-
-        /* enforce character class restrictions */
-        string_mod(BSTR(&buf), CC_PRINT, CC_CRLF, 0);
 
-        if (buf_string_match_head_str(&buf, "AUTH_FAILED"))
+        while (BLEN(&buf) > 1)
         {
-            receive_auth_failed(c, &buf);
-        }
-        else if (buf_string_match_head_str(&buf, "PUSH_"))
-        {
-            incoming_push_message(c, &buf);
-        }
-        else if (buf_string_match_head_str(&buf, "RESTART"))
-        {
-            server_pushed_signal(c, &buf, true, 7);
-        }
-        else if (buf_string_match_head_str(&buf, "HALT"))
-        {
-            server_pushed_signal(c, &buf, false, 4);
-        }
-        else if (buf_string_match_head_str(&buf, "INFO_PRE"))
-        {
-            server_pushed_info(c, &buf, 8);
-        }
-        else if (buf_string_match_head_str(&buf, "INFO"))
-        {
-            server_pushed_info(c, &buf, 4);
-        }
-        else if (buf_string_match_head_str(&buf, "CR_RESPONSE"))
-        {
-            receive_cr_response(c, &buf);
-        }
-        else if (buf_string_match_head_str(&buf, "AUTH_PENDING"))
-        {
-            receive_auth_pending(c, &buf);
-        }
-        else if (buf_string_match_head_str(&buf, "EXIT"))
-        {
-            receive_exit_message(c);
-        }
-        else
-        {
-            msg(D_PUSH_ERRORS, "WARNING: Received unknown control message: %s", BSTR(&buf));
+            /* commands on the control channel are seperated by 0x00 bytes.
+             * cmdlen does not include the 0 byte of the string */
+            int cmdlen = (int)strnlen(BSTR(&buf), BLEN(&buf));
+
+            if (cmdlen < BLEN(&buf))
+            {
+                /* include the NUL byte and ensure NUL termination */
+                int cmdlen = (int)strlen(BSTR(&buf)) + 1;
+
+                /* Construct a buffer that only holds the current command and
+                 * its closing NUL byte */
+                struct buffer cmdbuf = alloc_buf_gc(cmdlen, &gc);
+                buf_write(&cmdbuf, BPTR(&buf), cmdlen);
+
+                /* check we have only printable characters or null byte in the
+                 * command string and no newlines */
+                if (!string_check_buf(&buf, CC_PRINT | CC_NULL, CC_CRLF))
+                {
+                    msg(D_PUSH_ERRORS, "WARNING: Received control with invalid characters: %s",
+                        format_hex(BPTR(&buf), BLEN(&buf), 256, &gc));
+                }
+                else
+                {
+                    parse_incoming_control_channel_command(c, &cmdbuf);
+                }
+            }
+            else
+            {
+                msg(D_PUSH_ERRORS, "WARNING: Ignoring control channel "
+                    "message command without NUL termination");
+            }
+            buf_advance(&buf, cmdlen);
         }
     }
     else

diff --git a/tests/unit_tests/openvpn/test_buffer.c b/tests/unit_tests/openvpn/test_buffer.c
index fbfb937d12b16247c8ba035f5f06f1b10614788d..6b860b4cd426d8374ccb5ca66c906b561c25a7f8 100644 (file)
--- a/tests/unit_tests/openvpn/test_buffer.c
+++ b/tests/unit_tests/openvpn/test_buffer.c
@@ -354,6 +354,29 @@ test_character_class(void **state)
     assert_string_equal(buf, "There is a .'nice.' \"1234\" [.] year old .tree!");
 }
 
+
+static void
+test_character_string_mod_buf(void **state)
+{
+    struct gc_arena gc = gc_new();
+
+    struct buffer buf = alloc_buf_gc(1024, &gc);
+
+    const char test1[] =  "There is a nice 1234\x00 year old tree!";
+    buf_write(&buf, test1, sizeof(test1));
+
+    /* allow the null bytes and string but not the ! */
+    assert_false(string_check_buf(&buf, CC_ALNUM | CC_SPACE | CC_NULL, 0));
+
+    /* remove final ! and null byte to pass */
+    buf_inc_len(&buf, -2);
+    assert_true(string_check_buf(&buf, CC_ALNUM | CC_SPACE | CC_NULL, 0));
+
+    /* Check excluding digits works */
+    assert_false(string_check_buf(&buf, CC_ALNUM | CC_SPACE | CC_NULL, CC_DIGIT));
+    gc_free(&gc);
+}
+
 static void
 test_snprintf(void **state)
 {
@@ -437,6 +460,7 @@ main(void)
         cmocka_unit_test(test_buffer_free_gc_two),
         cmocka_unit_test(test_buffer_gc_realloc),
         cmocka_unit_test(test_character_class),
+        cmocka_unit_test(test_character_string_mod_buf),
         cmocka_unit_test(test_snprintf)
     };