lix/maintainers/check-syscalls.nix

{
  runCommandNoCC,
  lib,
  libseccomp,
  writeShellScriptBin,
}:
let
  syscalls-csv = runCommandNoCC "syscalls.csv" { } ''
    echo ${lib.escapeShellArg libseccomp.src}
    tar -xf ${lib.escapeShellArg libseccomp.src} --strip-components=2 ${libseccomp.name}/src/syscalls.csv
    mv syscalls.csv "$out"
  '';
in
writeShellScriptBin "check-syscalls" ''
  ${./check-syscalls.sh} ${syscalls-csv}
''
libstore/build: use an allowlist approach to syscall filtering Previously, system call filtering (to prevent builders from storing files with setuid/setgid permission bits or extended attributes) was performed using a blocklist. While this looks simple at first, it actually carries significant security and maintainability risks: after all, the kernel may add new syscalls to achieve the same functionality one is trying to block, and it can even be hard to actually add the syscall to the blocklist when building against a C library that doesn't know about it yet. For a recent demonstration of this happening in practice to Nix, see the introduction of fchmodat2 [0] [1]. The allowlist approach does not share the same drawback. While it does require a rather large list of harmless syscalls to be maintained in the codebase, failing to update this list (and roll out the update to all users) in time has rather benign effects; at worst, very recent programs that already rely on new syscalls will fail with an error the same way they would on a slightly older kernel that doesn't support them yet. Most importantly, no unintended new ways of performing dangerous operations will be silently allowed. Another possible drawback is reduced system call performance due to the larger filter created by the allowlist requiring more computation [2]. However, this issue has not convincingly been demonstrated yet in practice, for example in systemd or various browsers. To the contrary, it has been measured that the the actual filter constructed here has approximately the same overhead as a very simple filter blocking only one system call. This commit tries to keep the behavior as close to unchanged as possible. The system call list is in line with libseccomp 2.5.5 and glibc 2.39, which are the latest versions at the point of writing. Since libseccomp 2.5.5 is already a requirement and the distributions shipping this together with older versions of glibc are mostly not a thing any more, this should not lead to more build failures any more. [0] https://github.com/NixOS/nixpkgs/issues/300635 [1] https://github.com/NixOS/nix/issues/10424 [2] https://github.com/flatpak/flatpak/pull/4462#issuecomment-1061690607 Change-Id: I541be3ea9b249bcceddfed6a5a13ac10b11e16ad 2024-05-26 10:35:03 +02:00			`{`
			`runCommandNoCC,`
			`lib,`
			`libseccomp,`
			`writeShellScriptBin,`
			`}:`
			`let`
			`syscalls-csv = runCommandNoCC "syscalls.csv" { } ''`
			`echo ${lib.escapeShellArg libseccomp.src}`
			`tar -xf ${lib.escapeShellArg libseccomp.src} --strip-components=2 ${libseccomp.name}/src/syscalls.csv`
			`mv syscalls.csv "$out"`
			`'';`
			`in`
			`writeShellScriptBin "check-syscalls" ''`
			`${./check-syscalls.sh} ${syscalls-csv}`
			`''`