Change cookie signature comparison to resist timing attacks (based on code suggested by Robin Green and Austin Seipp)

2 files changed, 18 insertions, 1 deletions

diff --git a/src/c/urweb.c b/src/c/urweb.c
index 238b27e2..4d2de1dd 100644
--- a/src/c/urweb.c
+++ b/src/c/urweb.c
@@ -3455,6 +3455,23 @@ uw_Basis_string uw_Basis_makeSigString(uw_context ctx, uw_Basis_string sig) {
   return r;
 }
 
+/* This bit of crafty code is intended to prevent GCC from performing
+ * optimizations that would enable timing attacks.  See:
+ *   http://www.impredicative.com/pipermail/ur/2011-July/000659.html
+ */
+int uw_streq(uw_Basis_string s1, uw_Basis_string s2) {
+  int i, x = 0, len1 = strlen(s1);
+
+  if (len1 != strlen(s2)) return 0;
+
+  for (i = 0; i < len1; ++i) {
+        __asm__ __volatile__ ("");
+        x |= s1[i] ^ s2[i];
+  }
+
+  return x == 0;
+}
+
 uw_Basis_string uw_Basis_sigString(uw_context ctx, uw_unit u) {
   return ctx->app->cookie_sig(ctx);
 }
diff --git a/src/cjr_print.sml b/src/cjr_print.sml
index 79d7e7da..340ac9f5 100644
--- a/src/cjr_print.sml
+++ b/src/cjr_print.sml
@@ -2634,7 +2634,7 @@ fun p_file env (ds, ps) =
                               newline,
                               string "if (sig == NULL) uw_error(ctx, FATAL, \"Missing cookie signature\");",
                               newline,
-                              string "if (strcmp(sig, uw_cookie_sig(ctx)))",
+                              string "if (!uw_streq(sig, uw_cookie_sig(ctx)))",
                               newline,
                               box [string "uw_error(ctx, FATAL, \"Wrong cookie signature\");",
                                    newline],