From e936861263d9bafdfbe395c12526f2dc48ac17d7 Mon Sep 17 00:00:00 2001
Message-ID: <e936861263d9bafdfbe395c12526f2dc48ac17d7.1729457080.git.reepca@russelstein.xyz>
From: Reepca Russelstein <reepca@russelstein.xyz>
Date: Sun, 20 Oct 2024 15:36:06 -0500
Subject: [PATCH 1/2] nix: build: sanitize failed build outputs prior to
 exposing them.

The only thing keeping a rogue builder and a local user from collaborating to
usurp control over the builder's user during the build is the fact that
whatever files the builder may produce are not accessible to any other users
yet.  If we're going to make them accessible, we should probably do some
sanity checking to ensure that sort of collaborating can't happen.

Currently this isn't happening when failed build outputs are moved from the
chroot as an aid to debugging.

* nix/libstore/build.cc (secureFilePerms): new function.
  (DerivationGoal::buildDone): use it.

Change-Id: I9dce1e3d8813b31cabd87a0e3219bf9830d8be96
---
 nix/libstore/build.cc | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index d23c0944a4..67ebfe2f14 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -1301,6 +1301,34 @@ void replaceValidPath(const Path & storePath, const Path tmpPath)
 MakeError(NotDeterministic, BuildError)
 
 
+/* Recursively make the file permissions of a path safe for exposure to
+   arbitrary users, but without canonicalising its permissions, timestamp, and
+   user.  Throw an exception if a file type that isn't explicitly known to be
+   safe is found. */
+static void secureFilePerms(Path path)
+{
+  struct stat st;
+  if (lstat(path.c_str(), &st)) return;
+
+  switch(st.st_mode & S_IFMT) {
+  case S_IFLNK:
+    return;
+
+  case S_IFDIR:
+    for (auto & i : readDirectory(path)) {
+      secureFilePerms(path + "/" + i.name);
+    }
+    /* FALLTHROUGH */
+
+  case S_IFREG:
+    chmod(path.c_str(), (st.st_mode & ~S_IFMT) & ~(S_ISUID | S_ISGID | S_IWOTH));
+    break;
+
+  default:
+    throw Error(format("file `%1%' has an unsupported type") % path);
+  }
+}
+
 void DerivationGoal::buildDone()
 {
     trace("build done");
@@ -1372,9 +1400,15 @@ void DerivationGoal::buildDone()
                build failures. */
             if (useChroot && buildMode == bmNormal)
                 foreach (PathSet::iterator, i, missingPaths)
-                    if (pathExists(chrootRootDir + *i))
+                    if (pathExists(chrootRootDir + *i)) {
+                      try {
+                        secureFilePerms(chrootRootDir + *i);
                         rename((chrootRootDir + *i).c_str(), i->c_str());
+                      } catch(Error & e) {
+                        printMsg(lvlError, e.msg());
+                      }
+                    }
 
             if (diskFull)
                 printMsg(lvlError, "note: build failure may have been caused by lack of free disk space");

-- 
2.45.2

