grsecurity: support disabling TCP simultaneous connect

Defaults to OFF because disabling TCP simultaneous connect breaks some
legitimate use cases, notably WebRTC [1], but it's nice to provide the
option for deployments where those features are unneeded anyway.

This is an alternative to https://github.com/NixOS/nixpkgs/pull/4937

[1]: http://article.gmane.org/gmane.linux.documentation/9425

nixos/modules/security/grsecurity.nix

···
           '';
         };
 
+        disableSimultConnect = mkOption {
+          type = types.bool;
+          default = false;
+          description = ''
+            Disable TCP simultaneous connect.  The TCP simultaneous connect
+            feature allows two clients to connect without either of them
+            entering the listening state.  This feature of the TCP specification
+            is claimed to enable an attacker to deny the target access to a given
+            server by guessing the source port the target would use to make the
+            connection.
+
+            This option is OFF by default because TCP simultaneous connect has
+            some legitimate uses.  Enable this option if you know what this TCP
+            feature is for and know that you do not need it.
+          '';
+        };
+
         verboseVersion = mkOption {
           type = types.bool;
           default = false;

pkgs/build-support/grsecurity/default.nix

···
       restrictProcWithGroup = true;
       unrestrictProcGid = 121; # Ugh, an awful hack. See grsecurity NixOS gid
       disableRBAC = false;
+      disableSimultConnect = false;
       verboseVersion = false;
       kernelExtraConfig = "";
     } // grsecOptions.config;
···
         GRKERNSEC_CHROOT_CHMOD ${boolToKernOpt cfg.config.denyChrootChmod}
         GRKERNSEC_DENYUSB ${boolToKernOpt cfg.config.denyUSB}
         GRKERNSEC_NO_RBAC ${boolToKernOpt cfg.config.disableRBAC}
+        GRKERNSEC_NO_SIMULT_CONNECT ${boolToKernOpt cfg.config.disableSimultConnect}
 
         ${cfg.config.kernelExtraConfig}
       '';