Use timingsafe_memcmp() to compare MIT-MAGIC-COOKIES CVE-2017-2624

Provide the function definition for systems that don't have it.
Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>

configure.ac

@@ -221,7 +221,8 @@ AC_CHECK_FUNCS([backtrace ffs geteuid getuid issetugid getresuid \
 	mmap posix_fallocate seteuid shmctl64 strncasecmp vasprintf vsnprintf \
 	walkcontext setitimer poll epoll_create1])
 AC_CONFIG_LIBOBJ_DIR([os])
-AC_REPLACE_FUNCS([reallocarray strcasecmp strcasestr strlcat strlcpy strndup])
+AC_REPLACE_FUNCS([reallocarray strcasecmp strcasestr strlcat strlcpy strndup\
+	timingsafe_memcmp])
 AM_CONDITIONAL(POLL, [test "x$ac_cv_func_poll" = "xyes"])
 
 AC_CHECK_DECLS([program_invocation_short_name], [], [], [[#include <errno.h>]])

include/dix-config.h.in

@@ -238,6 +238,9 @@
 /* Define to 1 if you have the <sys/utsname.h> header file. */
 #undef HAVE_SYS_UTSNAME_H
 
+/* Define to 1 if you have the `timingsafe_memcmp' function. */
+#undef HAVE_TIMINGSAFE_MEMCMP
+
 /* Define to 1 if you have the <tslib.h> header file. */
 #undef HAVE_TSLIB_H
 

include/os.h

@@ -590,6 +590,11 @@ extern _X_EXPORT char *
 strndup(const char *str, size_t n);
 #endif
 
+#ifndef HAVE_TIMINGSAFE_MEMCMP
+extern _X_EXPORT int
+timingsafe_memcmp(const void *b1, const void *b2, size_t len);
+#endif
+
 /* Logging. */
 typedef enum _LogParameter {
     XLOG_FLUSH,

os/mitauth.c

@@ -76,7 +76,7 @@ MitCheckCookie(unsigned short data_length,
 
     for (auth = mit_auth; auth; auth = auth->next) {
         if (data_length == auth->len &&
-            memcmp(data, auth->data, (int) data_length) == 0)
+            timingsafe_memcmp(data, auth->data, (int) data_length) == 0)
             return auth->id;
     }
     *reason = "Invalid MIT-MAGIC-COOKIE-1 key";

os/timingsafe_memcmp.c

+/*
+ * Copyright (c) 2014 Google Inc.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <limits.h>
+#include <string.h>
+
+int
+timingsafe_memcmp(const void *b1, const void *b2, size_t len)
+{
+        const unsigned char *p1 = b1, *p2 = b2;
+        size_t i;
+        int res = 0, done = 0;
+
+        for (i = 0; i < len; i++) {
+                /* lt is -1 if p1[i] < p2[i]; else 0. */
+                int lt = (p1[i] - p2[i]) >> CHAR_BIT;
+
+                /* gt is -1 if p1[i] > p2[i]; else 0. */
+                int gt = (p2[i] - p1[i]) >> CHAR_BIT;
+
+                /* cmp is 1 if p1[i] > p2[i]; -1 if p1[i] < p2[i]; else 0. */
+                int cmp = lt - gt;
+
+                /* set res = cmp if !done. */
+                res |= cmp & ~done;
+
+                /* set done if p1[i] != p2[i]. */
+                done |= lt | gt;
+        }
+
+        return (res);
+}