mirror of
https://github.com/NixOS/nix
synced 2024-09-19 10:50:24 -04:00
0e4baff868
Instead of running the builds under
`$TMPDIR/{unique-build-directory-owned-by-the-build-user}`, run them
under `$TMPDIR/{unique-build-directory-owned-by-the-daemon}/{subdir-owned-by-the-build-user}`
where the build directory is only readable and traversable by the daemon user.
This achieves two things:
1. It prevents builders from making their build directory world-readable
(or even writeable), which would allow the outside world to interact
with them.
2. It prevents external processes running as the build user (either
because that somehow leaked, maybe as a consequence of 1., or because
`build-users` isn't in use) from gaining access to the build
directory.
130 lines
4.1 KiB
Nix
130 lines
4.1 KiB
Nix
{ config, ... }:
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let
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pkgs = config.nodes.machine.nixpkgs.pkgs;
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attacker = pkgs.runCommandWith {
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name = "attacker";
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stdenv = pkgs.pkgsStatic.stdenv;
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} ''
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$CC -static -o $out ${./attacker.c}
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'';
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try-open-build-dir = pkgs.writeScript "try-open-build-dir" ''
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export PATH=${pkgs.coreutils}/bin:$PATH
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set -x
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chmod 700 .
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# Shouldn't be able to open the root build directory
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(! chmod 700 ..)
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touch foo
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# Synchronisation point: create a world-writable fifo and wait for someone
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# to write into it
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mkfifo syncPoint
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chmod 777 syncPoint
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cat syncPoint
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touch $out
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set +x
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'';
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create-hello-world = pkgs.writeScript "create-hello-world" ''
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export PATH=${pkgs.coreutils}/bin:$PATH
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set -x
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echo "hello, world" > result
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# Synchronisation point: create a world-writable fifo and wait for someone
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# to write into it
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mkfifo syncPoint
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chmod 777 syncPoint
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cat syncPoint
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cp result $out
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set +x
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'';
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in
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{
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name = "sandbox-setuid-leak";
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nodes.machine =
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{ config, lib, pkgs, ... }:
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{ virtualisation.writableStore = true;
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nix.settings.substituters = lib.mkForce [ ];
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nix.nrBuildUsers = 1;
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virtualisation.additionalPaths = [ pkgs.busybox-sandbox-shell attacker try-open-build-dir create-hello-world pkgs.socat ];
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boot.kernelPackages = pkgs.linuxPackages_latest;
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users.users.alice = {
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isNormalUser = true;
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};
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};
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testScript = { nodes }: ''
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start_all()
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with subtest("A builder can't give access to its build directory"):
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# Make sure that a builder can't change the permissions on its build
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# directory to the point of opening it up to external users
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# A derivation whose builder tries to make its build directory as open
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# as possible and wait for someone to hijack it
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machine.succeed(r"""
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nix-build -v -E '
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builtins.derivation {
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name = "open-build-dir";
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system = builtins.currentSystem;
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builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
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args = [ (builtins.storePath "${try-open-build-dir}") ];
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}' >&2 &
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""".strip())
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# Wait for the build to be ready
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# This is OK because it runs as root, so we can access everything
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machine.wait_for_file("/tmp/nix-build-open-build-dir.drv-0/build/syncPoint")
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# But Alice shouldn't be able to access the build directory
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machine.fail("su alice -c 'ls /tmp/nix-build-open-build-dir.drv-0/build'")
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machine.fail("su alice -c 'touch /tmp/nix-build-open-build-dir.drv-0/build/bar'")
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machine.fail("su alice -c 'cat /tmp/nix-build-open-build-dir.drv-0/build/foo'")
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# Tell the user to finish the build
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machine.succeed("echo foo > /tmp/nix-build-open-build-dir.drv-0/build/syncPoint")
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with subtest("Being able to execute stuff as the build user doesn't give access to the build dir"):
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machine.succeed(r"""
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nix-build -E '
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builtins.derivation {
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name = "innocent";
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system = builtins.currentSystem;
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builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
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args = [ (builtins.storePath "${create-hello-world}") ];
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}' >&2 &
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""".strip())
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machine.wait_for_file("/tmp/nix-build-innocent.drv-0/build/syncPoint")
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# The build ran as `nixbld1` (which is the only build user on the
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# machine), but a process running as `nixbld1` outside the sandbox
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# shouldn't be able to touch the build directory regardless
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machine.fail("su nixbld1 --shell ${pkgs.busybox-sandbox-shell}/bin/sh -c 'ls /tmp/nix-build-innocent.drv-0/build'")
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machine.fail("su nixbld1 --shell ${pkgs.busybox-sandbox-shell}/bin/sh -c 'echo pwned > /tmp/nix-build-innocent.drv-0/build/result'")
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# Finish the build
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machine.succeed("echo foo > /tmp/nix-build-innocent.drv-0/build/syncPoint")
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# Check that the build was not affected
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machine.succeed(r"""
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cat ./result
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test "$(cat ./result)" = "hello, world"
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""".strip())
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'';
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}
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