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Chmod build root to make it read-only #11448

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Chmod build root to make it read-only #11448

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b95af5c
Make build root read-only, so `mkdir /homeless-shelter` would fail wi…
Aug 30, 2024
a1337f5
Add test that `mkdir $HOME` fails with permission error
Aug 30, 2024
89ace85
Also allow "Operation not permitted" for `mkdir $HOME`
Sep 1, 2024
d6fd7ee
chmod later
Sep 9, 2024
fdc5d1e
Oops - after chroot, the root is `/`.
Sep 9, 2024
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3 changes: 3 additions & 0 deletions src/libstore/unix/build/local-derivation-goal.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1970,6 +1970,9 @@ void LocalDerivationGoal::runChild()
if (rmdir("real-root") == -1)
throw SysError("cannot remove real-root directory");

// Make build root read-only, so `mkdir /homeless-shelter` would fail.
chmod_("/", 0555);
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Could this break uid-range builds (e.g. tests/nixos/containers/systemd-nspawn.nix)? Those do expect the root to be writable.

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Hmm, I guess it could.

I had another idea. If we actually mkdir /homeless-shelter and then make it read-only, and adjust the "home directory exists" error to be only triggered if the directory exists and is non-empty, I think it may solve the issue. Would you object to always having an empty /homeless-shelter directory in the sandbox?

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Perhaps this should then be controlled by a derivation-provided setting like an "advanced attribute"?


/* Switch to the sandbox uid/gid in the user namespace,
which corresponds to the build user or calling user in
the parent namespace. */
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5 changes: 5 additions & 0 deletions tests/functional/build.sh
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Expand Up @@ -152,6 +152,11 @@ nix build --impure -f multiple-outputs.nix --json e --no-link \
(.outputs | keys == ["a_a", "b"]))
'

# Make sure that `mkdir $HOME` fails with a "Permission denied" or "Operation not permitted" error
out="$(nix build -f mkdir-home-failing.nix -L 2>&1)" && status=0 || status=$?
test "$status" = 1
<<<"$out" grepQuiet -E "Permission denied" || <<<"$out" grepQuiet -E "Operation not permitted"

# Make sure that `--stdin` works and does not apply any defaults
printf "" | nix build --no-link --stdin --json | jq --exit-status '. == []'
printf "%s\n" "$drv^*" | nix build --no-link --stdin --json | jq --exit-status '.[0]|has("drvPath")'
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8 changes: 8 additions & 0 deletions tests/functional/mkdir-home-failing.nix
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@@ -0,0 +1,8 @@
with import ./config.nix;
mkDerivation {
name = "mkdir-home-no-permission";
builder = builtins.toFile "builder.sh"
''
mkdir $HOME
'';
}
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