fix(oci): harden runtime HOME resolution

oci/remote/install_oci.sh

@@ -1091,7 +1091,7 @@ ensure_runtime_home() {
   passwd_file="${rootfs}/etc/passwd"
   mount_ct_rootfs
   if ! home=$(python3 "$OCI_RUNTIME_RESOLVER" --home "$passwd_file" "$uid" 2>>"${OCI_LOG:-/dev/stderr}"); then
-    oci_quiet pct unmount "$VMID" || true
+    oci_quiet pct unmount "$VMID" || die "$(translate "Could not unmount the container filesystem")"
     return 0
   fi
   oci_quiet pct unmount "$VMID" || die "$(translate "Could not unmount the container filesystem")"

oci/remote/oci_runtime.py

@@ -4,7 +4,9 @@
 from __future__ import annotations
 
 import json
+import os
 import shlex
+import stat
 import sys
 import tarfile
 from pathlib import Path
@@ -90,14 +92,39 @@ def effective_entrypoint(
 
 
 def home_directory(passwd_path: Path, uid: int) -> str | None:
-    """Return the safe home directory recorded for a container UID."""
-    for line in passwd_path.read_text(encoding="utf-8").splitlines():
-        fields = line.split(":")
-        if len(fields) < 7 or fields[2] != str(uid):
-            continue
-        home = fields[5]
-        return home if home.startswith("/") else None
+    """Read a regular image passwd file; only an absent UID defaults to /."""
+    if uid < 0:
         return None
+    # The caller supplies <mounted rootfs>/etc/passwd. Pin etc so an image
+    # symlink or a rename during lookup cannot redirect the host-side read.
+    directory = os.open(passwd_path.parent, os.O_RDONLY | os.O_DIRECTORY | os.O_NOFOLLOW)
+    try:
+        if not stat.S_ISREG(os.stat(passwd_path.name, dir_fd=directory, follow_symlinks=False).st_mode):
+            return None
+        descriptor = os.open(
+            passwd_path.name, os.O_RDONLY | os.O_NOFOLLOW | os.O_NONBLOCK, dir_fd=directory,
+        )
+        try:
+            # Recheck the opened object, not its name; a FIFO replacement
+            # must neither block open() nor reach a read operation.
+            if not stat.S_ISREG(os.fstat(descriptor).st_mode):
+                return None
+            with os.fdopen(descriptor, encoding="utf-8", closefd=False) as stream:
+                content = stream.read()
+        finally:
+            os.close(descriptor)
+    finally:
+        os.close(directory)
+    for line in content.splitlines():
+        fields = line.split(":")
+        if len(fields) < 3 or fields[2] != str(uid):
+            continue
+        if len(fields) != 7:
+            return None
+        home = fields[5]
+        safe = home.startswith("/") and not any(ord(char) < 32 or ord(char) == 127 for char in home)
+        return home if safe else None
+    return "/"
 
 
 def main() -> int:

oci/tests/test_runtime_home_contract.py

@@ -31,14 +31,14 @@ class RuntimeHomeResolutionTests(unittest.TestCase):
         )
         self.assertEqual(oci_runtime.home_directory(passwd, 1001), "/srv/app")
 
-    def test_unknown_or_unsafe_home_is_not_invented(self):
+    def test_unsafe_record_is_rejected_but_unknown_uid_defaults_to_root(self):
         passwd = self.passwd(
             "root:x:0:0:root:relative-home:/bin/sh\n"
             "app:x:1001:1001:App user::/sbin/nologin\n"
         )
         self.assertIsNone(oci_runtime.home_directory(passwd, 0))
         self.assertIsNone(oci_runtime.home_directory(passwd, 1001))
-        self.assertIsNone(oci_runtime.home_directory(passwd, 1234))
+        self.assertEqual(oci_runtime.home_directory(passwd, 1234), "/")
 
 
 class RuntimeHomeInstallerContractTests(unittest.TestCase):

oci/tests/test_runtime_home_hardening.py

@@ -0,0 +1,260 @@
+"""Exercise HOME resolution without reading image symlinks or changing guests."""
+
+from contextlib import redirect_stderr, redirect_stdout
+import io
+import os
+from pathlib import Path
+import re
+import subprocess
+import sys
+import tempfile
+import unittest
+from unittest.mock import patch
+
+ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(ROOT / "remote"))
+import oci_runtime
+
+
+class HomeHardeningTests(unittest.TestCase):
+    def setUp(self):
+        temporary = tempfile.TemporaryDirectory()
+        self.addCleanup(temporary.cleanup)
+        self.root = Path(temporary.name)
+        (self.root / "etc").mkdir()
+        self.passwd = self.root / "etc/passwd"
+        self.passwd.write_text("root:x:0:0:root:/root:/bin/sh\n", encoding="utf-8")
+
+    def cli(self, uid="1234"):
+        output, error = io.StringIO(), io.StringIO()
+        with patch.object(sys, "argv", ["oci_runtime.py", "--home", str(self.passwd), uid]), \
+             redirect_stdout(output), redirect_stderr(error):
+            status = oci_runtime.main()
+        return status, output.getvalue(), error.getvalue()
+
+    def assert_rejected_without_read(self):
+        with patch.object(os, "fdopen", side_effect=AssertionError("unsafe file read")):
+            status, output, _ = self.cli()
+        self.assertEqual(status, 1)
+        self.assertEqual(output, "")
+
+    def test_unknown_uid_uses_root_only_after_reading_a_regular_database(self):
+        self.assertEqual(self.cli(), (0, "/\n", ""))
+
+    def test_empty_regular_database_uses_root(self):
+        self.passwd.write_text("")
+        self.assertEqual(self.cli(), (0, "/\n", ""))
+
+    def test_final_symlink_is_rejected_without_reading_its_target(self):
+        target = self.root / "outside"
+        target.write_text("app:x:1234:1234::/host-secret:/bin/sh\n")
+        self.passwd.unlink()
+        self.passwd.symlink_to(target)
+        self.assert_rejected_without_read()
+        self.assertEqual(target.read_text(), "app:x:1234:1234::/host-secret:/bin/sh\n")
+
+    def test_symlinked_etc_is_rejected_without_reading_its_target(self):
+        directory = self.root / "etc"
+        directory.rename(self.root / "outside-etc")
+        directory.symlink_to(self.root / "outside-etc", target_is_directory=True)
+        self.assert_rejected_without_read()
+        self.assertEqual(self.cli("0")[:2], (1, ""))
+
+    def test_pinned_etc_does_not_follow_a_later_directory_replacement(self):
+        original_open = os.open
+        outside = self.root / "outside"
+        outside.mkdir()
+        (outside / "passwd").write_text("root:x:0:0::/outside:/bin/sh\n")
+        replaced = []
+        def replace(path, flags, *args, **kwargs):
+            if str(path) == "passwd":
+                (self.root / "etc").rename(self.root / "original-etc")
+                (self.root / "etc").symlink_to(outside, target_is_directory=True)
+                replaced.append(True)
+            return original_open(path, flags, *args, **kwargs)
+        with patch.object(os, "open", side_effect=replace):
+            self.assertEqual(self.cli("0"), (0, "/root\n", ""))
+        self.assertEqual(replaced, [True])
+
+    def test_directory_is_rejected_without_reading(self):
+        self.passwd.unlink()
+        self.passwd.mkdir()
+        self.assert_rejected_without_read()
+
+    def test_fifo_is_rejected_without_reading_or_waiting_for_a_writer(self):
+        self.passwd.unlink()
+        os.mkfifo(self.passwd)
+        done = subprocess.run(
+            [sys.executable, "-B", str(ROOT / "remote/oci_runtime.py"), "--home", str(self.passwd), "0"],
+            capture_output=True, text=True, timeout=3,
+        )
+        self.assertEqual(done.returncode, 1)
+        self.assertEqual(done.stdout, "")
+        self.assert_rejected_without_read()
+
+    def test_missing_database_does_not_masquerade_as_unknown_uid(self):
+        self.passwd.unlink()
+        self.assert_rejected_without_read()
+
+    def test_permission_failure_does_not_masquerade_as_unknown_uid(self):
+        with patch.object(os, "open", side_effect=PermissionError("denied")):
+            self.assert_rejected_without_read()
+
+    def test_invalid_encoding_does_not_produce_a_fallback(self):
+        self.passwd.write_bytes(b"\xff\xfe")
+        status, output, _ = self.cli()
+        self.assertEqual((status, output), (1, ""))
+
+    def test_partial_matching_record_does_not_produce_a_fallback(self):
+        self.passwd.write_text("app:x:1234:1234\n")
+        self.assertEqual(self.cli()[:2], (1, ""))
+
+    def test_read_error_closes_both_descriptors_without_a_fallback(self):
+        descriptors = []
+        original_open = os.open
+        def opened(*args, **kwargs):
+            fd = original_open(*args, **kwargs)
+            descriptors.append(fd)
+            return fd
+        with patch.object(os, "open", side_effect=opened), \
+             patch.object(os, "fdopen", side_effect=OSError("read failed")):
+            self.assertEqual(self.cli()[:2], (1, ""))
+        self.assertEqual(len(descriptors), 2)
+        for fd in descriptors:
+            with self.assertRaises(OSError):
+                os.fstat(fd)
+
+    def test_matching_unsafe_home_is_not_replaced_with_root(self):
+        for value in ("", "relative", "/bad\tvalue", "/bad\x00value", "/bad\x7fvalue"):
+            with self.subTest(value=repr(value)):
+                self.passwd.write_text(f"app:x:1234:1234::{value}:/bin/sh\n")
+                self.assertEqual(self.cli()[:2], (1, ""))
+
+    def test_invalid_uid_does_not_open_the_database(self):
+        for uid in ("-1", "nobody"):
+            with self.subTest(uid=uid), patch.object(os, "open") as opened:
+                self.assertEqual(self.cli(uid)[:2], (1, ""))
+                opened.assert_not_called()
+
+    def test_symlink_replacement_between_check_and_open_is_not_read(self):
+        original_open = os.open
+        outside = self.root / "outside"
+        outside.write_text("app:x:1234:1234::/outside:/bin/sh\n")
+        def replace(path, flags, *args, **kwargs):
+            if str(path) == "passwd":
+                self.passwd.unlink()
+                self.passwd.symlink_to(outside)
+            return original_open(path, flags, *args, **kwargs)
+        with patch.object(os, "open", side_effect=replace):
+            self.assert_rejected_without_read()
+
+    def test_fifo_replacement_between_check_and_open_is_not_read(self):
+        original_open = os.open
+        def replace(path, flags, *args, **kwargs):
+            if str(path) == "passwd":
+                self.passwd.unlink()
+                os.mkfifo(self.passwd)
+            return original_open(path, flags, *args, **kwargs)
+        with patch.object(os, "open", side_effect=replace):
+            self.assert_rejected_without_read()
+
+
+class HomeInstallerExecutionTests(unittest.TestCase):
+    """Run the real shell HOME function and Python CLI with fake mount boundaries."""
+    def setUp(self):
+        temporary = tempfile.TemporaryDirectory()
+        self.addCleanup(temporary.cleanup)
+        self.root = Path(temporary.name)
+        (self.root / "etc").mkdir()
+        self.passwd = self.root / "etc/passwd"
+        self.passwd.write_text("root:x:0:0:root:/root:/bin/sh\napp:x:1001:1001::/srv/app:/bin/sh\n")
+        self.conf = self.root / "guest.conf"
+        self.trace = self.root / "trace"
+        self.source = (ROOT / "remote/install_oci.sh").read_text()
+
+    def run_home(self, config="unrelated: keep\n", unmount_status=0, mount_status=0):
+        self.conf.write_text(config)
+        self.trace.write_text("")
+        functions = []
+        for name in ("ensure_runtime_home", "set_runtime_environment"):
+            found = re.search(rf"^{name}\(\) \{{\n.*?^\}}\n", self.source, re.M | re.S)
+            self.assertIsNotNone(found, name)
+            functions.append(found.group(0))
+        script = r'''
+set -euo pipefail
+CONF=$1
+FIXTURE_ROOT=$2
+TRACE=$3
+OCI_RUNTIME_RESOLVER=$4
+PYTHON=$5
+UNMOUNT_STATUS=$6
+MOUNT_STATUS=$7
+OCI_LOG="$FIXTURE_ROOT/resolver.log"
+VMID=123456789
+translate() { printf '%s' "$1"; }
+die() { printf '%s\n' "$*" >&2; exit 1; }
+mount_ct_rootfs() {
+  printf 'mount\n' >> "$TRACE"
+  [ "$MOUNT_STATUS" = 0 ] || die 'mount failed'
+  rootfs=$FIXTURE_ROOT
+  passwd_file="$rootfs/etc/passwd"
+}
+oci_quiet() { printf '%s\n' "$*" >> "$TRACE"; return "$UNMOUNT_STATUS"; }
+python3() { printf 'resolve\n' >> "$TRACE"; command "$PYTHON" -B "$@"; }
+'''
+        done = subprocess.run(
+            ["bash", "-c", script + "\n".join(functions) + "\nensure_runtime_home\nprintf 'continued\\n'\n",
+             "bash", str(self.conf), str(self.root), str(self.trace), str(ROOT / "remote/oci_runtime.py"),
+             sys.executable, str(unmount_status), str(mount_status)],
+            capture_output=True, text=True, timeout=5,
+        )
+        return done, self.conf.read_text(), self.trace.read_text().splitlines()
+
+    def test_explicit_home_including_empty_never_mounts_or_resolves(self):
+        for home in ("/custom", ""):
+            config = f"unrelated: keep\nlxc.environment.runtime: HOME={home}\n"
+            done, after, trace = self.run_home(config)
+            self.assertEqual((done.returncode, after, trace), (0, config, []))
+
+    def test_default_and_effective_uid_write_the_resolved_home_after_unmount(self):
+        for uid, expected in ((None, "/root"), (1001, "/srv/app"), (9999, "/")):
+            config = "unrelated: keep\n" + (f"lxc.init.uid: {uid}\n" if uid is not None else "")
+            done, after, trace = self.run_home(config)
+            self.assertEqual(done.returncode, 0, done.stderr)
+            self.assertEqual(after, config + f"lxc.environment.runtime: HOME={expected}\n")
+            self.assertEqual(trace, ["mount", "resolve", "pct unmount 123456789"])
+
+    def test_invalid_uid_leaves_config_unchanged_without_mount(self):
+        config = "lxc.init.uid: not-a-uid\n"
+        done, after, trace = self.run_home(config)
+        self.assertEqual((done.returncode, after, trace), (0, config, []))
+
+    def test_rejected_input_unmounts_and_preserves_config(self):
+        self.passwd.unlink()
+        self.passwd.symlink_to(self.root / "missing")
+        config = "unrelated: keep\n"
+        done, after, trace = self.run_home(config)
+        self.assertEqual((done.returncode, after), (0, config))
+        self.assertEqual(trace, ["mount", "resolve", "pct unmount 123456789"])
+
+    def test_failed_unmount_stops_before_config_write_for_success_and_failure(self):
+        for valid in (True, False):
+            if not valid:
+                self.passwd.unlink()
+            config = "unrelated: keep\n"
+            done, after, trace = self.run_home(config, unmount_status=1)
+            self.assertNotEqual(done.returncode, 0)
+            self.assertEqual(after, config)
+            self.assertNotIn("continued", done.stdout)
+            self.assertEqual(trace, ["mount", "resolve", "pct unmount 123456789"])
+
+    def test_failed_mount_never_reads_or_writes(self):
+        config = "unrelated: keep\n"
+        done, after, trace = self.run_home(config, mount_status=1)
+        self.assertNotEqual(done.returncode, 0)
+        self.assertEqual(after, config)
+        self.assertEqual(trace, ["mount"])
+
+
+if __name__ == "__main__":
+    unittest.main()