init: Add an selinux transition for snapuserd.
With compressed VAB updates, it is not possible to mount /system without
first running snapuserd, which is the userspace component to the dm-user
kernel module. This poses a problem because as soon as selinux
enforcement is enabled, snapuserd (running in a kernel context) does not
have access to read and decompress the underlying system partition.
To account for this, we split SelinuxInitialize into multiple steps:
First, sepolicy is read into an in-memory string.
Second, the device-mapper tables for all snapshots are rebuilt. This
flushes any pending reads and creates new dm-user devices. The original
kernel-privileged snapuserd is then killed.
Third, sepolicy is loaded from the in-memory string.
Fourth, we re-launch snapuserd and connect it to the newly created
dm-user devices. As part of this step we restorecon device-mapper
devices and /dev/block/by-name/super, since the new snapuserd is in a
limited context.
Finally, we set enforcing mode.
This sequence ensures that snapuserd has appropriate privileges with a
minimal number of permissive audits.
Bug: 173476209
Test: full OTA with VABC applies and boots
Change-Id: Ie4e0f5166b01c31a6f337afc26fc58b96217604e
diff --git a/init/snapuserd_transition.h b/init/snapuserd_transition.h
new file mode 100644
index 0000000..a5ab652
--- /dev/null
+++ b/init/snapuserd_transition.h
@@ -0,0 +1,87 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <sys/types.h>
+
+#include <optional>
+#include <string>
+#include <vector>
+
+#include <libsnapshot/snapshot.h>
+
+#include "block_dev_initializer.h"
+
+namespace android {
+namespace init {
+
+// Fork and exec a new copy of snapuserd.
+void LaunchFirstStageSnapuserd();
+
+class SnapuserdSelinuxHelper final {
+ using SnapshotManager = android::snapshot::SnapshotManager;
+
+ public:
+ SnapuserdSelinuxHelper(std::unique_ptr<SnapshotManager>&& sm, pid_t old_pid);
+
+ void StartTransition();
+ void FinishTransition();
+
+ // Return a helper for facilitating the selinux transition of snapuserd.
+ // If snapuserd is not in use, null is returned. StartTransition() should
+ // be called after reading policy. FinishTransition() should be called
+ // after loading policy. In between, no reads of /system or other dynamic
+ // partitions are possible.
+ static std::unique_ptr<SnapuserdSelinuxHelper> CreateIfNeeded();
+
+ private:
+ void RelaunchFirstStageSnapuserd();
+ void ExecSnapuserd();
+
+ std::unique_ptr<SnapshotManager> sm_;
+ BlockDevInitializer block_dev_init_;
+ pid_t old_pid_;
+ std::vector<std::string> argv_;
+};
+
+// Remove /dev/socket/snapuserd. This ensures that (1) the existing snapuserd
+// will receive no new requests, and (2) the next copy we transition to can
+// own the socket.
+void CleanupSnapuserdSocket();
+
+// Kill an instance of snapuserd given a pid.
+void KillFirstStageSnapuserd(pid_t pid);
+
+// Save an open fd to /system/bin (in the ramdisk) into an environment. This is
+// used to later execveat() snapuserd.
+void SaveRamdiskPathToSnapuserd();
+
+// Returns true if first-stage snapuserd is running.
+bool IsFirstStageSnapuserdRunning();
+
+// Return the pid of the first-stage instances of snapuserd, if it was started.
+std::optional<pid_t> GetSnapuserdFirstStagePid();
+
+// Save an open fd to /system/bin (in the ramdisk) into an environment. This is
+// used to later execveat() snapuserd.
+void SaveRamdiskPathToSnapuserd();
+
+// Returns true if first-stage snapuserd is running.
+bool IsFirstStageSnapuserdRunning();
+
+} // namespace init
+} // namespace android