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>
(cherry picked from commit 414f428)

Author: schwabe

src/openvpn/buffer.c

@@ -1085,6 +1085,23 @@ 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,

src/openvpn/buffer.h

@@ -933,6 +933,31 @@ 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.
+ *
+ * If replace is 0, characters are skipped instead of replaced.
+ *
+ * @param str       The input string to be modified.
+ * @param inclusive Character classes not to be replaced.
+ * @param exclusive Character classes to be replaced even if they are also in inclusive.
+ * @param replace   The character to replace the specified character classes with.
+ * @param gc        The garbage collector arena to allocate memory from.
+ *
+ * @return The modified string with characters replaced within the specified range.
+ */
 const char *string_mod_const(const char *str,
                              const unsigned int inclusive,
                              const unsigned int exclusive,

src/openvpn/forward.c

@@ -184,6 +184,43 @@ check_tls_errors_nco(struct context *c)
 
 #if P2MP
 
+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
+    {
+        msg(D_PUSH_ERRORS, "WARNING: Received unknown control message: %s", BSTR(buf));
+    }
+}
+
 /*
  * Handle incoming configuration
  * messages on the control channel.
@@ -199,43 +236,41 @@ 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"))
-        {
-            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"))
+        while (BLEN(&buf) > 1)
         {
-            server_pushed_info(c, &buf, 4);
-        }
-        else if (buf_string_match_head_str(&buf, "CR_RESPONSE"))
-        {
-            receive_cr_response(c, &buf);
-        }
-        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