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Nix, the purely functional package manager
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Maximilian Bosch 39b08e3766
libstore/local-derivation-goal: prohibit creating setuid/setgid binaries 
With Linux kernel >=6.6 & glibc 2.39 a `fchmodat2(2)` is available that
isn't filtered away by the libseccomp sandbox.

Being able to use this to bypass that restriction has surprising results
for some builds such as lxc[1]:

> With kernel ≥6.6 and glibc 2.39, lxc's install phase uses fchmodat2,
> which slips through 9b88e52846/src/libstore/build/local-derivation-goal.cc (L1650-L1663).
> The fixupPhase then uses fchmodat, which fails.
> With older kernel or glibc, setting the suid bit fails in the
> install phase, which is not treated as fatal, and then the
> fixup phase does not try to set it again.

Please note that there are still ways to bypass this sandbox[2] and this is
mostly a fix for the breaking builds.

This change works by creating a syscall filter for the `fchmodat2`
syscall (number 452 on most systems). The problem is that glibc 2.39
and seccomp 2.5.5 are needed to have the correct syscall number available
via `__NR_fchmodat2` / `__SNR_fchmodat2`, but this flake is still on
nixpkgs 23.11. To have this change everywhere and not dependent on the
glibc this package is built against, I added a header
"fchmodat2-compat.hh" that sets the syscall number based on the
architecture. On most platforms its 452 according to glibc with a few
exceptions:

    $ rg --pcre2 'define __NR_fchmodat2 (?!452)'
    sysdeps/unix/sysv/linux/x86_64/x32/arch-syscall.h
    58:#define __NR_fchmodat2 1073742276

    sysdeps/unix/sysv/linux/mips/mips64/n32/arch-syscall.h
    67:#define __NR_fchmodat2 6452

    sysdeps/unix/sysv/linux/mips/mips64/n64/arch-syscall.h
    62:#define __NR_fchmodat2 5452

    sysdeps/unix/sysv/linux/mips/mips32/arch-syscall.h
    70:#define __NR_fchmodat2 4452

    sysdeps/unix/sysv/linux/alpha/arch-syscall.h
    59:#define __NR_fchmodat2 562

I tested the change by adding the diff below as patch to
`pkgs/tools/package-management/nix/common.nix` & then built a VM from
the following config using my dirty nixpkgs master:

    {
      vm = { pkgs, ... }: {
        virtualisation.writableStore = true;
        virtualisation.memorySize = 8192;
        virtualisation.diskSize = 12 * 1024;
        nix.package = pkgs.nixVersions.nix_2_21;
      };
    }

The original issue can be triggered via

    nix build -L github:nixos/nixpkgs/d6dc19adbda4fd92fe9a332327a8113eaa843894#lxc \
      --extra-experimental-features 'nix-command flakes'

however the problem disappears with this patch applied.

Closes #10424

[1] https://github.com/NixOS/nixpkgs/issues/300635#issuecomment-2031073804
[2] https://github.com/NixOS/nixpkgs/issues/300635#issuecomment-2030844251

(cherry picked from commit ba68045187)
2024-04-24 23:28:23 +02:00
.github Put functional tests in tests/functional 2023-12-01 12:06:43 -05:00
config Run autoupdate 2021-06-01 11:42:38 +02:00
contrib function-trace: always show the trace 2019-09-18 23:23:21 +02:00
doc show Nix logo in the manual (#10445) 2024-04-09 09:37:44 +00:00
m4 autoconf: Fix C++17 detection not working on Ubuntu 16.04. 2019-07-03 04:32:25 +02:00
maintainers maintainers: add note on marking PRs as draft 2023-06-19 10:55:34 +02:00
misc Choose a reasonable number similar to LimitNOFile 2023-07-07 07:52:16 -07:00
mk Put functional tests in tests/functional 2023-12-01 12:06:43 -05:00
perl Allow dynamic derivation deps in inputDrvs 2023-09-07 10:39:37 -04:00
scripts Merge pull request #8512 from scarf005/install-show-uid 2023-06-15 13:49:44 +02:00
src libstore/local-derivation-goal: prohibit creating setuid/setgid binaries 2024-04-24 23:28:23 +02:00
tests Disallow store path names that are . or .. (plus opt. -) 2024-03-26 13:47:41 +01:00
.dir-locals.el .dir-locals.el: Set c-block-comment-prefix 2020-07-10 11:21:06 +02:00
.editorconfig Add .editorconfig 2017-06-05 22:57:28 +01:00
.gitignore Move tests to separate directories, and document 2023-12-01 13:05:03 -05:00
.version Bump version 2024-03-07 15:03:40 +01:00
boehmgc-coroutine-sp-fallback.diff Always disable GC in a coroutine unless the patch is applied 2023-04-07 14:54:38 +02:00
bootstrap.sh bootstrap: Simplify & make more robust. 2011-09-06 12:11:05 +00:00
configure.ac expand on the system type in hacking guide 2023-07-19 10:37:40 +02:00
CONTRIBUTING.md Put functional tests in tests/functional 2023-12-01 12:06:43 -05:00
COPYING * Change this to LGPL to keep the government happy. 2006-04-25 16:41:06 +00:00
default.nix add flake-compat to flake.nix and use sha256 in default.nix 2023-03-06 21:11:24 +01:00
docker.nix fix "add an option to include flake-registry..." 2023-05-16 14:35:31 +02:00
flake.lock flake.lock: Update 2023-12-01 11:08:15 -05:00
flake.nix Add a NixOS test for the sandbox escape 2024-03-07 09:33:40 +01:00
local.mk Enable -Werror=switch-enum 2023-04-03 18:45:20 +02:00
Makefile Move tests to separate directories, and document 2023-12-01 13:05:03 -05:00
Makefile.config.in Generate API docs with Doxygen 2023-03-10 12:51:06 -05:00
precompiled-headers.h Config: Use nlohmann/json 2020-08-20 11:02:16 +02:00
README.md Improve hacking.md 2023-02-13 12:00:00 +04:00
shell.nix Remove url literals 2022-01-24 13:28:21 +01:00

README.md

Nix

Open Collective supporters Test

Nix is a powerful package manager for Linux and other Unix systems that makes package management reliable and reproducible. Please refer to the Nix manual for more details.

Installation

On Linux and macOS the easiest way to install Nix is to run the following shell command (as a user other than root):

$ curl -L https://nixos.org/nix/install | sh

Information on additional installation methods is available on the Nix download page.

Building And Developing

See our Hacking guide in our manual for instruction on how to to set up a development environment and build Nix from source.

Additional Resources

License

Nix is released under the LGPL v2.1.