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Properly handle null bytes and invalid characters in control messages

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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>
This commit is contained in:
Arne Schwabe 2024-05-27 15:02:41 +02:00 committed by Gert Doering
parent b3a68b85a7
commit 414f428fa2
4 changed files with 131 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
src/openvpn/buffer.c
View File

@ -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,

12
src/openvpn/buffer.h
View File

@ -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.

121
src/openvpn/forward.c
View File

@ -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);
while (BLEN(&buf) > 1)
{
/* 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 (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));
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

24
tests/unit_tests/openvpn/test_buffer.c
View File

@ -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)
};