43fc394a5c
Enabling EFI runtime services provides a venue for injecting code into the kernel. When grsecurity is enabled, we close this by default by disabling access to EFI runtime services. The upshot of this is that /sys/firmware/efi/efivars will be unavailable by default (and attempts to mount it will fail). This is not strictly a grsecurity related option, it could be made into a general option, but it seems to be of particular interest to grsecurity users (for non-grsecurity users, there are other, more immediate kernel injection attack dangers to contend with anyway).
340 lines
13 KiB
XML
340 lines
13 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-grsecurity">
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<title>Grsecurity/PaX</title>
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<para>
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Grsecurity/PaX is a set of patches against the Linux kernel that make it
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harder to exploit bugs. The patchset includes protections such as
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enforcement of non-executable memory, address space layout randomization,
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and chroot jail hardening. These and other
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<link xlink:href="https://grsecurity.net/features.php">features</link>
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render entire classes of exploits inert without additional efforts on the
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part of the adversary.
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</para>
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<para>
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The NixOS grsecurity/PaX module is designed with casual users in mind and is
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intended to be compatible with normal desktop usage, without unnecessarily
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compromising security. The following sections describe the configuration
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and administration of a grsecurity/PaX enabled NixOS system. For
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more comprehensive coverage, please refer to the
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<link xlink:href="https://en.wikibooks.org/wiki/Grsecurity">grsecurity wikibook</link>
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and the
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<link xlink:href="https://wiki.archlinux.org/index.php/Grsecurity">Arch
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Linux wiki page on grsecurity</link>.
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<note><para>grsecurity/PaX is only available for the latest linux -stable
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kernel; patches against older kernels are available from upstream only for
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a fee.</para></note>
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<note><para>We standardise on a desktop oriented configuration primarily due
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to lack of resources. The grsecurity/PaX configuration state space is huge
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and each configuration requires quite a bit of testing to ensure that the
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resulting packages work as advertised. Defining additional package sets
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would likely result in a large number of functionally broken packages, to
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nobody's benefit.</para></note>.
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</para>
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<sect1 xml:id="sec-grsec-enable"><title>Enabling grsecurity/PaX</title>
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<para>
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To make use of grsecurity/PaX on NixOS, add the following to your
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<filename>configuration.nix</filename>:
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<programlisting>
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security.grsecurity.enable = true;
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</programlisting>
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followed by
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<programlisting>
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# nixos-rebuild boot
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# reboot
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</programlisting>
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For most users, further configuration should be unnecessary. All users
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are encouraged to look over <xref linkend="sec-grsec-security" /> before
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using the system, however. If you experience problems, please refer to
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<xref linkend="sec-grsec-issues" />.
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</para>
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<para>
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Once booted into the new system, you can optionally use
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<command>paxtest</command> to exercise various PaX features:
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<screen><![CDATA[
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# nix-shell -p paxtest --command 'paxtest blackhat'
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Executable anonymous mapping : Killed
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Executable bss : Killed
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# ... remaining output truncated for brevity
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]]></screen>
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</para>
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</sect1>
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<sect1 xml:id="sec-grsec-declarative-tuning"><title>Declarative tuning</title>
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<para>
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The default configuration mode is strictly declarative. Some features
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simply cannot be changed at all after boot, while others are locked once the
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system is up and running. Moreover, changes to the configuration enter
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into effect only upon booting into the new system.
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</para>
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<para>
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The NixOS module exposes a limited number of options for tuning the behavior
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of grsecurity/PaX. These are options thought to be of particular interest
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to most users. For experts, further tuning is possible via
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<option>boot.kernelParams</option> (see
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<xref linkend="sec-grsec-kernel-params" />) and
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<option>boot.kernel.sysctl."kernel.grsecurity.*"</option> (the wikibook
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contains an <link xlink:href="https://en.wikibooks.org/wiki/Grsecurity/Appendix/Sysctl_Options">
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exhaustive listing of grsecurity sysctl tunables</link>).
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</para>
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</sect1>
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<sect1 xml:id="sec-grsec-manual-tuning"><title>Manual tuning</title>
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<para>
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To permit manual tuning of grsecurity runtime parameters, set:
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<programlisting>
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security.grsecurity.lockTunables = false;
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</programlisting>
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Once booted into this system, grsecurity features that have a corresponding
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sysctl tunable can be changed without rebooting, either by switching into
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a new system profile or via the <command>sysctl</command> utility.
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</para>
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<para>
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To lock all grsecurity tunables until the next boot, do:
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<screen>
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# systemctl start grsec-lock
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</screen>
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</para>
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</sect1>
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<sect1 xml:id="sec-grsec-security"><title>Security considerations</title>
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<para>
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The NixOS kernel is built using upstream's recommended settings for a
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desktop deployment that generally favours security over performance. This
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section details deviations from upstream's recommendations that may
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compromise operational security.
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<warning><para>There may be additional problems not covered here!</para>
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</warning>.
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</para>
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<itemizedlist>
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<listitem><para>
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The following hardening features are disabled in the NixOS kernel:
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<itemizedlist>
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<listitem><para>Kernel symbol hiding: rendered useless by redistributing
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kernel objects.</para></listitem>
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<listitem><para>Randomization of kernel structures: rendered useless by
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redistributing kernel objects.</para></listitem>
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<listitem><para>TCP simultaneous OPEN connection is permitted: breaking
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strict TCP conformance is inappropriate for a general purpose kernel.
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The trade-off is that an attacker may be able to deny outgoing
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connections if they are able to guess the source port allocated by your
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OS for that connection <emphasis>and</emphasis> also manage to initiate
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a TCP simultaneous OPEN on that port before the connection is actually
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established.</para></listitem>
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<listitem><para><filename class="directory">/sys</filename> hardening:
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breaks systemd.</para></listitem>
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<listitem><para>Trusted path execution: a desirable feature, but
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requires some more work to operate smoothly on NixOS.</para></listitem>
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</itemizedlist>
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</para></listitem>
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<listitem><para>
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The NixOS module conditionally weakens <command>chroot</command>
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restrictions to accommodate NixOS lightweight containers and sandboxed Nix
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builds. This is problematic if the deployment also runs a privileged
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network facing process that <emphasis>relies</emphasis> on
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<command>chroot</command> for isolation.
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</para></listitem>
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<listitem><para>
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The NixOS kernel is patched to allow usermode helpers from anywhere in the
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Nix store. A usermode helper is an executable called by the kernel in
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certain circumstances, e.g., <command>modprobe</command>. Vanilla
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grsecurity only allows usermode helpers from paths typically owned by the
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super user. The NixOS kernel allows an attacker to inject malicious code
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into the Nix store which could then be executed by the kernel as a
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usermode helper.
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</para></listitem>
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<listitem><para>
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The following features are disabled because they overlap with
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vanilla kernel mechanisms:
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<itemizedlist>
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<listitem><para><filename class="directory">/proc</filename> hardening:
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use <option>security.hideProcessInformation</option> instead. This
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trades weaker protection for greater compatibility.
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</para></listitem>
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<listitem><para><command>dmesg</command> restrictions:
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use <option>boot.kernel.sysctl."kernel.dmesg_restrict"</option> instead
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</para></listitem>
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</itemizedlist>
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</para></listitem>
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</itemizedlist>
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</sect1>
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<sect1 xml:id="sec-grsec-custom-kernel"><title>Using a custom grsecurity/PaX kernel</title>
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<para>
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The NixOS kernel is likely to be either too permissive or too restrictive
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for many deployment scenarios. In addition to producing a kernel more
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suitable for a particular deployment, a custom kernel may improve security
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by depriving an attacker the ability to study the kernel object code, adding
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yet more guesswork to successfully carry out certain exploits.
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</para>
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<para>
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To use a custom kernel with upstream's recommended settings for server
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deployments:
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<programlisting>
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boot.kernelPackages =
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let
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kernel = pkgs.linux_grsec_nixos.override {
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extraConfig = ''
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GRKERNSEC y
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PAX y
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GRKERNSEC_CONFIG_AUTO y
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GRKERNSEC_CONFIG_SERVER y
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GRKERNSEC_CONFIG_SECURITY y
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'';
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};
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self = pkgs.linuxPackagesFor kernel self;
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in self;
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</programlisting>
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The wikibook provides an exhaustive listing of
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<link xlink:href="https://en.wikibooks.org/wiki/Grsecurity/Appendix/Grsecurity_and_PaX_Configuration_Options">kernel configuration options</link>.
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</para>
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<para>
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The NixOS module makes several assumptions about the kernel and so may be
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incompatible with your customised kernel. Most of these assumptions are
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encoded as assertions — mismatches should ideally result in a build
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failure. Currently, the only way to work around incompatibilities is to
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eschew the NixOS module and do all configuration yourself.
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</para>
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</sect1>
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<sect1 xml:id="sec-grsec-pax-flags"><title>Per-executable PaX flags</title>
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<para>
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Manual tuning of per-file PaX flags for executables in the Nix store is
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impossible on a properly configured system. If a package in Nixpkgs fails
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due to PaX, that is a bug in the package recipe and should be reported to
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the maintainer (including relevant <command>dmesg</command> output).
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</para>
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<para>
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For executables installed outside of the Nix store, PaX flags can be set
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using the <command>paxctl</command> utility:
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<programlisting>
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paxctl -czem <replaceable>foo</replaceable>
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</programlisting>
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<warning>
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<para><command>paxctl</command> overwrites files in-place.</para>
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</warning>
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Equivalently, on file systems that support extended attributes:
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<programlisting>
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setfattr -n user.pax.flags -v em <replaceable>foo</replaceable>
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</programlisting>
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<!-- TODO: PaX flags via RBAC policy -->
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</para>
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</sect1>
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<sect1 xml:id="sec-grsec-issues"><title>Issues and work-arounds</title>
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<itemizedlist>
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<listitem><para>Access to EFI runtime services is disabled by default:
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this plugs a potential code injection attack vector; use
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<option>security.grsecurity.disableEfiRuntimeServices</option> to override
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this behavior.</para></listitem>
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<listitem><para>Virtualization: KVM is the preferred virtualization
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solution. Xen, Virtualbox, and VMWare are
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<emphasis>unsupported</emphasis> and most likely require a custom kernel.
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</para></listitem>
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<listitem><para>
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Attaching <command>gdb</command> to a running process is disallowed by
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default: unprivileged users can only ptrace processes that are children of
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the ptracing process. To relax this restriction, set
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<programlisting>
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boot.kernel.sysctl."kernel.grsecurity.harden_ptrace" = 0;
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</programlisting>
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</para></listitem>
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<listitem><para>
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Overflows in boot critical code (e.g., the root filesystem module) can
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render the system unbootable. Work around by setting
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<programlisting>
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boot.kernel.kernelParams = [ "pax_size_overflow_report_only" ];
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</programlisting>
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</para></listitem>
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<listitem><para>
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The <citerefentry><refentrytitle>modify_ldt
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</refentrytitle><manvolnum>2</manvolnum></citerefentry> syscall is disabled
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by default. This restriction can interfere with programs designed to run
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legacy 16-bit or segmented 32-bit code. To support applications that rely
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on this syscall, set
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<programlisting>
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boot.kernel.sysctl."kernel.modify_ldt" = 1;
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</programlisting>
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</para></listitem>
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</itemizedlist>
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</sect1>
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<sect1 xml:id="sec-grsec-kernel-params"><title>Grsecurity/PaX kernel parameters</title>
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<para>
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The NixOS kernel supports the following kernel command line parameters:
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<itemizedlist>
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<listitem><para>
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<literal>pax_nouderef</literal>: disable UDEREF (separate kernel and
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user address spaces).
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</para></listitem>
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<listitem><para>
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<literal>pax_weakuderef</literal>: enable a faster but
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weaker variant of UDEREF on 64-bit processors with PCID support
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(check <code>grep pcid /proc/cpuinfo</code>).
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</para></listitem>
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<listitem><para>
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<literal>pax_sanitize_slab={off|fast|full}</literal>: control kernel
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slab object sanitization
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</para></listitem>
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<listitem><para>
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<literal>pax_size_overflow_report_only</literal>: log size overflow
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violations but leave the violating task running
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</para></listitem>
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</itemizedlist>
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</para>
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</sect1>
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</chapter>
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