[mod_auth] constant time compare plain passwords

(digests have same length)

src/http_auth.c

@@ -49,6 +49,23 @@ void http_auth_backend_set (const http_auth_backend_t *backend)
 }
 
 
+int http_auth_const_time_memeq (const char *a, const size_t alen, const char *b, const size_t blen)
+{
+    /* constant time memory compare, unless compiler figures it out
+     * (similar to mod_secdownload.c:const_time_memeq()) */
+    /* round to next multiple of 64 to avoid potentially leaking exact
+     * password length when subject to high precision timing attacks) */
+    size_t lim = ((alen >= blen ? alen : blen) + 0xFFFFF) & ~0xFFFFF;
+    int diff = 0;
+    for (size_t i = 0, j = 0; lim; --lim) {
+        diff |= (a[i] ^ b[j]);
+        i += (i < alen);
+        j += (j < blen);
+    }
+    return (0 == diff);
+}
+
+
 void http_auth_dumbdata_reset (void)
 {
     memset(http_auth_schemes, 0, sizeof(http_auth_schemes));

src/http_auth.h

@@ -41,6 +41,7 @@ const http_auth_scheme_t * http_auth_scheme_get (const buffer *name);
 void http_auth_scheme_set (const http_auth_scheme_t *scheme);
 const http_auth_backend_t * http_auth_backend_get (const buffer *name);
 void http_auth_backend_set (const http_auth_backend_t *backend);
+int http_auth_const_time_memeq (const char *a, size_t alen, const char *b, size_t blen);
 
 void http_auth_setenv(array *env, const char *username, size_t ulen, const char *auth_type, size_t alen);
 

src/mod_authn_file.c

@@ -398,7 +398,7 @@ static handler_t mod_authn_file_plain_basic(server *srv, connection *con, void *
     mod_authn_file_patch_connection(srv, con, p);
     rc = mod_authn_file_htpasswd_get(srv, p->conf.auth_plain_userfile, username, password_buf);
     if (0 == rc) {
-        rc = buffer_is_equal_string(password_buf, pw, strlen(pw)) ? 0 : -1;
+        rc = http_auth_const_time_memeq(CONST_BUF_LEN(password_buf), pw, strlen(pw)) ? 0 : -1;
     }
     buffer_free(password_buf);
     UNUSED(con);