Merge pull request #7830 from yorickvP/fix-writable-etc

Don't allow writing to /etc

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

@@ -670,7 +670,8 @@ void LocalDerivationGoal::startBuilder()
            nobody account.  The latter is kind of a hack to support
            Samba-in-QEMU. */
         createDirs(chrootRootDir + "/etc");
-        chownToBuilder(chrootRootDir + "/etc");
+        if (parsedDrv->useUidRange())
+            chownToBuilder(chrootRootDir + "/etc");
 
         if (parsedDrv->useUidRange() && (!buildUser || buildUser->getUIDCount() < 65536))
             throw Error("feature 'uid-range' requires the setting '%s' to be enabled", settings.autoAllocateUids.name);
@@ -970,6 +971,10 @@ void LocalDerivationGoal::startBuilder()
                 "nobody:x:65534:65534:Nobody:/:/noshell\n",
                 sandboxUid(), sandboxGid(), settings.sandboxBuildDir));
 
+        /* Make /etc unwritable */
+        if (!parsedDrv->useUidRange())
+            chmod_(chrootRootDir + "/etc", 0555);
+
         /* Save the mount- and user namespace of the child. We have to do this
            *before* the child does a chroot. */
         sandboxMountNamespace = open(fmt("/proc/%d/ns/mnt", (pid_t) pid).c_str(), O_RDONLY);

tests/linux-sandbox.sh

@@ -37,3 +37,6 @@ nix-build check.nix -A nondeterministic --sandbox-paths /nix/store --no-out-link
 (! nix-build check.nix -A nondeterministic --sandbox-paths /nix/store --no-out-link --check -K 2> $TEST_ROOT/log)
 if grep -q 'error: renaming' $TEST_ROOT/log; then false; fi
 grep -q 'may not be deterministic' $TEST_ROOT/log
+
+# Test that sandboxed builds cannot write to /etc easily
+(! nix-build -E 'with import ./config.nix; mkDerivation { name = "etc-write"; buildCommand = "echo > /etc/test"; }' --no-out-link --sandbox-paths /nix/store)

tests/nixos/containers/systemd-nspawn.nix

@@ -56,12 +56,12 @@ runCommand "test"
     # Make /run a tmpfs to shut up a systemd warning.
     mkdir /run
     mount -t tmpfs none /run
-    chmod 0700 /run
 
     mount -t cgroup2 none /sys/fs/cgroup
 
     mkdir -p $out
 
+    chmod +w /etc
     touch /etc/os-release
     echo a5ea3f98dedc0278b6f3cc8c37eeaeac > /etc/machine-id