From 76e4adfaac3083056e79b518ccc197a7645a0f2d Mon Sep 17 00:00:00 2001
From: Emily <vcs@emily.moe>
Date: Thu, 4 Jul 2024 16:19:51 +0100
Subject: [PATCH 1/2] libstore: clean up the build directory properly
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

After the fix for CVE-2024-38531, this was only removing the nested
build directory, rather than the top‐level temporary directory.

Fixes: 1d3696f0fb88d610abc234a60e0d6d424feafdf1
---
 src/libstore/unix/build/local-derivation-goal.cc | 9 +++++----
 src/libstore/unix/build/local-derivation-goal.hh | 8 +++++++-
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/src/libstore/unix/build/local-derivation-goal.cc b/src/libstore/unix/build/local-derivation-goal.cc
index a20ed5300..b20bd2d8c 100644
--- a/src/libstore/unix/build/local-derivation-goal.cc
+++ b/src/libstore/unix/build/local-derivation-goal.cc
@@ -503,12 +503,12 @@ void LocalDerivationGoal::startBuilder()
 
     /* Create a temporary directory where the build will take
        place. */
-    tmpDir = createTempDir(settings.buildDir.get().value_or(""), "nix-build-" + std::string(drvPath.name()), false, false, 0700);
+    topTmpDir = createTempDir(settings.buildDir.get().value_or(""), "nix-build-" + std::string(drvPath.name()), false, false, 0700);
     if (useChroot) {
         /* If sandboxing is enabled, put the actual TMPDIR underneath
            an inaccessible root-owned directory, to prevent outside
            access. */
-        tmpDir = tmpDir + "/build";
+        tmpDir = topTmpDir + "/build";
         createDir(tmpDir, 0700);
     }
     chownToBuilder(tmpDir);
@@ -2980,7 +2980,7 @@ void LocalDerivationGoal::checkOutputs(const std::map<std::string, ValidPathInfo
 
 void LocalDerivationGoal::deleteTmpDir(bool force)
 {
-    if (tmpDir != "") {
+    if (topTmpDir != "") {
         /* Don't keep temporary directories for builtins because they
            might have privileged stuff (like a copy of netrc). */
         if (settings.keepFailed && !force && !drv->isBuiltin()) {
@@ -2988,7 +2988,8 @@ void LocalDerivationGoal::deleteTmpDir(bool force)
             chmod(tmpDir.c_str(), 0755);
         }
         else
-            deletePath(tmpDir);
+            deletePath(topTmpDir);
+        topTmpDir = "";
         tmpDir = "";
     }
 }
diff --git a/src/libstore/unix/build/local-derivation-goal.hh b/src/libstore/unix/build/local-derivation-goal.hh
index 77d07de98..4bcf5c9d4 100644
--- a/src/libstore/unix/build/local-derivation-goal.hh
+++ b/src/libstore/unix/build/local-derivation-goal.hh
@@ -27,10 +27,16 @@ struct LocalDerivationGoal : public DerivationGoal
     std::optional<Path> cgroup;
 
     /**
-     * The temporary directory.
+     * The temporary directory used for the build.
      */
     Path tmpDir;
 
+    /**
+     * The top-level temporary directory. `tmpDir` is either equal to
+     * or a child of this directory.
+     */
+    Path topTmpDir;
+
     /**
      * The path of the temporary directory in the sandbox.
      */

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 2/2] libstore: fix sandboxed builds on macOS
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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);