Make management password check constant time

This changes the password check on the management interface to be constant
time. Normally the management port should not be exposed in a way that
allows an attacker to even interact with it but making the check constant
time as an additional layer of security is always good.

Patch v2: include NUL byte in comparison

Reported-by: Connor Edwards <cedw@pm.me>
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221220140458.2666637-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25784.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e567f34262b0670fd51cbbcb6c6866b046454cee)

diff --git a/src/openvpn/manage.c b/src/openvpn/manage.c
index b11de224da1768a2e04059f197090b1c32e37f77..5465b7e9bc084a1d277bdab8fcd4a76e15ce69e5 100644 (file)
--- a/src/openvpn/manage.c
+++ b/src/openvpn/manage.c
@@ -198,7 +198,12 @@ man_check_password(struct management *man, const char *line)
 {
     if (man_password_needed(man))
     {
-        if (streq(line, man->settings.up.password))
+        /* This comparison is not fixed time but since strlen(time) is based on
+         * the attacker choice, it should not give any indication of the real
+         * password length, use + 1 to include the NUL byte that terminates the
+         * string*/
+        size_t compare_len = min_uint(strlen(line) + 1, sizeof(man->settings.up.password));
+        if (memcmp_constant_time(line, man->settings.up.password, compare_len) == 0)
         {
             man->connection.password_verified = true;
             msg(M_CLIENT, "SUCCESS: password is correct");