libstore: Don't default build-dir to temp-dir, store setting

If a build directory is accessible to other users it is possible to
smuggle data in and out of build directories. Usually this is only
a build purity problem, but in combination with other issues it can
be used to break out of a build sandbox. to prevent this we default
to using a subdirectory of nixStateDir (which is more restrictive).

(cherry picked from pennae Lix commit 55b416f6897fb0d8a9315a530a9b7f0914458ded)
(store setting done by roberth)

src/libstore/globals.cc

@@ -6,6 +6,7 @@
 #include "nix/util/abstract-setting-to-json.hh"
 #include "nix/util/compute-levels.hh"
 #include "nix/util/signals.hh"
+#include "nix/util/types.hh"
 
 #include <algorithm>
 #include <map>

src/libstore/include/nix/store/globals.hh

@@ -697,14 +697,7 @@ public:
 
     Setting<std::optional<Path>> buildDir{this, std::nullopt, "build-dir",
         R"(
-            The directory on the host, in which derivations' temporary build directories are created.
-
-            If not set, Nix uses the system temporary directory indicated by the `TMPDIR` environment variable.
-            Note that builds are often performed by the Nix daemon, so its `TMPDIR` is used, and not that of the Nix command line interface.
-
-            This is also the location where [`--keep-failed`](@docroot@/command-ref/opt-common.md#opt-keep-failed) leaves its files.
-
-            If Nix runs without sandbox, or if the platform does not support sandboxing with bind mounts (e.g. macOS), then the [`builder`](@docroot@/language/derivations.md#attr-builder)'s environment contains this directory instead of the virtual location [`sandbox-build-dir`](#conf-sandbox-build-dir).
+            Override the `build-dir` store setting for all stores that have this setting.
         )"};
 
     Setting<PathSet> allowedImpureHostPrefixes{this, {}, "allowed-impure-host-deps",

src/libstore/include/nix/store/local-store.hh

@@ -34,7 +34,38 @@ struct OptimiseStats
     uint64_t bytesFreed = 0;
 };
 
-struct LocalStoreConfig : std::enable_shared_from_this<LocalStoreConfig>, virtual LocalFSStoreConfig
+struct LocalBuildStoreConfig : virtual LocalFSStoreConfig {
+
+private:
+    /**
+      Input for computing the build directory. See `getBuildDir()`.
+     */
+     Setting<std::optional<Path>> buildDir{this, std::nullopt, "build-dir",
+        R"(
+            The directory on the host, in which derivations' temporary build directories are created.
+
+            If not set, Nix will use the `builds` subdirectory of its configured state directory.
+
+            Note that builds are often performed by the Nix daemon, so its `build-dir` applies.
+
+            Nix will create this directory automatically with suitable permissions if it does not exist.
+            Otherwise its permissions must allow all users to traverse the directory (i.e. it must have `o+x` set, in unix parlance) for non-sandboxed builds to work correctly.
+
+            This is also the location where [`--keep-failed`](@docroot@/command-ref/opt-common.md#opt-keep-failed) leaves its files.
+
+            If Nix runs without sandbox, or if the platform does not support sandboxing with bind mounts (e.g. macOS), then the [`builder`](@docroot@/language/derivations.md#attr-builder)'s environment will contain this directory, instead of the virtual location [`sandbox-build-dir`](#conf-sandbox-build-dir).
+
+            > **Warning**
+            >
+            > `build-dir` must not be set to a world-writable directory.
+            > Placing temporary build directories in a world-writable place allows other users to access or modify build data that is currently in use.
+            > This alone is merely an impurity, but combined with another factor this has allowed malicious derivations to escape the build sandbox.
+        )"};
+public:
+    Path getBuildDir() const;
+};
+
+struct LocalStoreConfig : std::enable_shared_from_this<LocalStoreConfig>, virtual LocalFSStoreConfig, virtual LocalBuildStoreConfig
 {
     using LocalFSStoreConfig::LocalFSStoreConfig;
 

src/libstore/local-store.cc

@@ -77,6 +77,18 @@ std::string LocalStoreConfig::doc()
         ;
 }
 
+Path LocalBuildStoreConfig::getBuildDir() const
+{
+    if (settings.buildDir.get().has_value()) {
+        return *settings.buildDir.get();
+    }
+    if (buildDir.get().has_value()) {
+        return *buildDir.get();
+    }
+
+    return stateDir.get() + "/builds";
+}
+
 ref<Store> LocalStore::Config::openStore() const
 {
     return make_ref<LocalStore>(ref{shared_from_this()});

src/libstore/unix/build/derivation-builder.cc

@@ -725,9 +725,13 @@ void DerivationBuilderImpl::startBuilder()
         throw BuildError(msg);
     }
 
+    auto buildDir = getLocalStore(store).config->getBuildDir();
+
+    createDirs(buildDir);
+
     /* Create a temporary directory where the build will take
        place. */
-    topTmpDir = createTempDir(settings.buildDir.get().value_or(""), "nix-build-" + std::string(drvPath.name()), 0700);
+    topTmpDir = createTempDir(buildDir, "nix-build-" + std::string(drvPath.name()), 0700);
     setBuildTmpDir();
     assert(!tmpDir.empty());