From af2e1142b17dadf24a0b66d8973033dce6efa32b Mon Sep 17 00:00:00 2001
From: Emily <vcs@emily.moe>
Date: Thu, 4 Jul 2024 16:24:59 +0100
Subject: [PATCH] libstore: fix sandboxed builds on macOS
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The recent fix for CVE-2024-38531 broke the sandbox on macOS
completely. As it’s not practical to use `chroot(2)` on
macOS, the build takes place in the main filesystem tree, and the
world‐unreadable wrapper directory prevents the build from accessing
its `$TMPDIR` at all.

The macOS sandbox probably shouldn’t be treated as any kind of a
security boundary in its current state, but this specific vulnerability
wasn’t possible to exploit on macOS anyway, as creating `set{u,g}id`
binaries is blocked by sandbox policy.

Locking down the build sandbox further may be a good idea in future,
but it already has significant compatibility issues. For now, restore
the previous status quo on macOS.

Thanks to @alois31 for helping me come to a better understanding of
the vulnerability.

Fixes: 1d3696f0fb88d610abc234a60e0d6d424feafdf1
Closes: #11002
---
 src/libstore/unix/build/local-derivation-goal.cc | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/libstore/unix/build/local-derivation-goal.cc b/src/libstore/unix/build/local-derivation-goal.cc
index b20bd2d8c..d5a3e0034 100644
--- a/src/libstore/unix/build/local-derivation-goal.cc
+++ b/src/libstore/unix/build/local-derivation-goal.cc
@@ -504,12 +504,22 @@ void LocalDerivationGoal::startBuilder()
     /* Create a temporary directory where the build will take
        place. */
     topTmpDir = createTempDir(settings.buildDir.get().value_or(""), "nix-build-" + std::string(drvPath.name()), false, false, 0700);
+#if __APPLE__
+    if (false) {
+#else
     if (useChroot) {
+#endif
         /* If sandboxing is enabled, put the actual TMPDIR underneath
            an inaccessible root-owned directory, to prevent outside
-           access. */
+           access.
+
+           On macOS, we don't use an actual chroot, so this isn't
+           possible. Any mitigation along these lines would have to be
+           done directly in the sandbox profile. */
         tmpDir = topTmpDir + "/build";
         createDir(tmpDir, 0700);
+    } else {
+        tmpDir = topTmpDir;
     }
     chownToBuilder(tmpDir);