Redesign of MagiskSU's sepolicy model

Introduce new domain `magisk_client` and new file type `magisk_exec`.

Connection to magiskd's always-on socket is restricted to magisk_client
only. Whitelisted process domains can transit to magisk_client through
executing files labelled magisk_exec. The main magisk binary shall be
the only file labelled as magisk_exec throughout the whole system.
All processes thus are no longer allowed to connect to magiskd directly
without going through the proper magisk binary.

Connection failures are silenced from audit logs with dontaudit rules,
so crazy processes which traverse through all unix domain sockets to try
connection can no longer check logcat to know the actual reason behind
EACCES, leaking the denied process policy (which is u:r:magisk:s0).

This also allows us to remove many rules that open up holes in
untrusted_app domains that were used to make remote shell work properly.
Since all processes establishing the remote shell are now restricted to
the magisk_client domain, all these rules are moved to magisk_client.
This makes Magisk require fewer compromises in Android's security model.

Note: as of this commit, requesting new root access via Magisk Manager
will stop working as Magisk Manager can no longer communicate with
magiskd directly. This will be addressed in a future commit that
involves changes in both native and application side.

native/jni/core/restorecon.cpp

@@ -10,7 +10,8 @@ using namespace std;
 #define SYSTEM_CON  "u:object_r:system_file:s0"
 #define ADB_CON     "u:object_r:adb_data_file:s0"
 #define ROOT_CON    "u:object_r:rootfs:s0"
-#define MAGISK_CON  "u:object_r:" SEPOL_FILE_DOMAIN ":s0"
+#define MAGISK_CON  "u:object_r:" SEPOL_FILE_TYPE ":s0"
+#define EXEC_CON    "u:object_r:" SEPOL_EXEC_TYPE ":s0"
 
 static void restore_syscon(int dirfd) {
 	struct dirent *entry;
@@ -86,9 +87,9 @@ void restore_tmpcon() {
 	int dfd = dirfd(dir.get());
 
 	for (dirent *entry; (entry = xreaddir(dir.get()));) {
-		if (entry->d_name == "magisk"sv || entry->d_name == "magiskinit"sv)
+		if (entry->d_name == "magisk"sv)
-			setfilecon_at(dfd, entry->d_name, MAGISK_CON);
+			setfilecon_at(dfd, entry->d_name, EXEC_CON);
 		else
-			setfilecon_at(dfd, entry->d_name, ROOT_CON);
+			setfilecon_at(dfd, entry->d_name, SYSTEM_CON);
 	}
 }

native/jni/core/scripting.cpp

@@ -83,7 +83,7 @@ rm -f $APK
 )EOF";
 
 void install_apk(const char *apk) {
-	setfilecon(apk, "u:object_r:" SEPOL_FILE_DOMAIN ":s0");
+	setfilecon(apk, "u:object_r:" SEPOL_FILE_TYPE ":s0");
 	exec_t exec {
 		.pre_exec = set_standalone,
 		.fork = fork_no_zombie

native/jni/magiskpolicy/rules.cpp

@@ -1,33 +1,8 @@
-#include <initializer_list>
-
 #include <logging.hpp>
 #include <magiskpolicy.hpp>
 
 #include "sepolicy.hpp"
 
-void sepol_impl::allow_su_client(const char *type) {
-	if (!exists(type))
-		return;
-	allow(type, SEPOL_PROC_DOMAIN, "unix_stream_socket", "connectto");
-	allow(type, SEPOL_PROC_DOMAIN, "unix_stream_socket", "getopt");
-
-	// Allow binder service
-	allow(type, SEPOL_PROC_DOMAIN, "binder", "call");
-	allow(type, SEPOL_PROC_DOMAIN, "binder", "transfer");
-
-	// Allow termios ioctl
-	allow(type, "devpts", "chr_file", "ioctl");
-	allow(type, "untrusted_app_devpts", "chr_file", "ioctl");
-	allow(type, "untrusted_app_25_devpts", "chr_file", "ioctl");
-	allow(type, "untrusted_app_all_devpts", "chr_file", "ioctl");
-	if (db->policyvers >= POLICYDB_VERSION_XPERMS_IOCTL) {
-		allowxperm(type, "devpts", "chr_file", "0x5400-0x54FF");
-		allowxperm(type, "untrusted_app_devpts", "chr_file", "0x5400-0x54FF");
-		allowxperm(type, "untrusted_app_25_devpts", "chr_file", "0x5400-0x54FF");
-		allowxperm(type, "untrusted_app_all_devpts", "chr_file", "0x5400-0x54FF");
-	}
-}
-
 void sepolicy::magisk_rules() {
 	// Temp suppress warnings
 	auto bak = log_cb.w;
@@ -37,18 +12,86 @@ void sepolicy::magisk_rules() {
 	deny(ALL, "kernel", "security", "load_policy");
 
 	type(SEPOL_PROC_DOMAIN, "domain");
-	type(SEPOL_FILE_DOMAIN, "file_type");
+	type(SEPOL_CLIENT_DOMAIN, "domain");
-	permissive(SEPOL_PROC_DOMAIN);
+	type(SEPOL_FILE_TYPE, "file_type");
+	type(SEPOL_EXEC_TYPE, "file_type");
+	permissive(SEPOL_PROC_DOMAIN);  /* Just in case something is missing */
 	typeattribute(SEPOL_PROC_DOMAIN, "mlstrustedsubject");
 	typeattribute(SEPOL_PROC_DOMAIN, "netdomain");
 	typeattribute(SEPOL_PROC_DOMAIN, "bluetoothdomain");
-	typeattribute(SEPOL_FILE_DOMAIN, "mlstrustedobject");
+	typeattribute(SEPOL_FILE_TYPE, "mlstrustedobject");
+
+	// Make our root domain unconstrained
+	allow(SEPOL_PROC_DOMAIN, ALL, ALL, ALL);
+	// Allow us to do any ioctl on all block devices
+	if (db->policyvers >= POLICYDB_VERSION_XPERMS_IOCTL)
+		allowxperm(SEPOL_PROC_DOMAIN, ALL, "blk_file", ALL);
+
+	// Create unconstrained file type
+	allow(ALL, SEPOL_FILE_TYPE, "file", ALL);
+	allow(ALL, SEPOL_FILE_TYPE, "dir", ALL);
+	allow(ALL, SEPOL_FILE_TYPE, "fifo_file", ALL);
+	allow(ALL, SEPOL_FILE_TYPE, "chr_file", ALL);
+
+	// Basic su client needs
+	allow(SEPOL_CLIENT_DOMAIN, ALL, "fd", "use");
+	allow(SEPOL_CLIENT_DOMAIN, SEPOL_CLIENT_DOMAIN, ALL, ALL);
+	allow(SEPOL_CLIENT_DOMAIN, SEPOL_EXEC_TYPE, "file", ALL);
+	allow(SEPOL_CLIENT_DOMAIN, SEPOL_PROC_DOMAIN, "unix_stream_socket", "connectto");
+	allow(SEPOL_CLIENT_DOMAIN, SEPOL_PROC_DOMAIN, "unix_stream_socket", "getopt");
+
+	// Allow su client to manipulate pts
+	const char *pts[] {
+		"devpts", "untrusted_app_devpts", "untrusted_app_25_devpts", "untrusted_app_all_devpts" };
+	for (auto type : pts) {
+		allow(SEPOL_CLIENT_DOMAIN, type, "chr_file", "read");
+		allow(SEPOL_CLIENT_DOMAIN, type, "chr_file", "write");
+		allow(SEPOL_CLIENT_DOMAIN, type, "chr_file", "getattr");
+		allow(SEPOL_CLIENT_DOMAIN, type, "chr_file", "ioctl");
+		if (db->policyvers >= POLICYDB_VERSION_XPERMS_IOCTL)
+			allowxperm(SEPOL_CLIENT_DOMAIN, type, "chr_file", "0x5400-0x54FF");
+	}
+
+	// Allow these processes to access MagiskSU
+	const char *clients[] {
+		"init", "shell", "system_app", "priv_app", "platform_app", "untrusted_app",
+		"untrusted_app_25", "untrusted_app_27", "untrusted_app_29", "update_engine" };
+	for (auto type : clients) {
+		if (!exists(type))
+			continue;
+		// exec magisk
+		allow(type, SEPOL_EXEC_TYPE, "file", "read");
+		allow(type, SEPOL_EXEC_TYPE, "file", "open");
+		allow(type, SEPOL_EXEC_TYPE, "file", "getattr");
+		allow(type, SEPOL_EXEC_TYPE, "file", "execute");
+
+		// Auto transit to client domain
+		type_transition(type, SEPOL_EXEC_TYPE, "process", SEPOL_CLIENT_DOMAIN);
+		allow(type, SEPOL_CLIENT_DOMAIN, "process", "transition");
+		dontaudit(type, SEPOL_CLIENT_DOMAIN, "process", "siginh");
+		dontaudit(type, SEPOL_CLIENT_DOMAIN, "process", "rlimitinh");
+		dontaudit(type, SEPOL_CLIENT_DOMAIN, "process", "noatsecure");
+
+		allow(SEPOL_CLIENT_DOMAIN, type, "process", "sigchld");
+		allow(SEPOL_CLIENT_DOMAIN, type, "fifo_file", "read");
+		allow(SEPOL_CLIENT_DOMAIN, type, "fifo_file", "write");
+		allow(SEPOL_CLIENT_DOMAIN, type, "fifo_file", "ioctl");
+	}
+
+	// Allow system_server to manage magisk_client
+	allow("system_server", SEPOL_CLIENT_DOMAIN, "process", "getpgid");
+	allow("system_server", SEPOL_CLIENT_DOMAIN, "process", "sigkill");
+
+	// Don't allow pesky processes to monitor audit deny logs when poking magisk daemon sockets
+	dontaudit(ALL, SEPOL_PROC_DOMAIN, "unix_stream_socket", ALL);
 
 	// Let everyone access tmpfs files (for SAR sbin overlay)
 	allow(ALL, "tmpfs", "file", ALL);
 
-	// For normal rootfs file/directory operations when rw (for SAR / overlay)
+	// For relabelling files
 	allow("rootfs", "labeledfs", "filesystem", "associate");
+	allow(SEPOL_FILE_TYPE, "pipefs", "filesystem", "associate");
+	allow(SEPOL_FILE_TYPE, "devpts", "filesystem", "associate");
 
 	// Let init transit to SEPOL_PROC_DOMAIN
 	allow("kernel", "kernel", "process", "setcurrent");
@@ -60,25 +103,6 @@ void sepolicy::magisk_rules() {
 	allow("init", "tmpfs", "file", "getattr");
 	allow("init", "tmpfs", "file", "execute");
 
-	// Make our domain unconstrained
-	allow(SEPOL_PROC_DOMAIN, ALL, ALL, ALL);
-	// Allow us to do any ioctl on all block devices
-	if (db->policyvers >= POLICYDB_VERSION_XPERMS_IOCTL)
-		allowxperm(SEPOL_PROC_DOMAIN, ALL, "blk_file", ALL);
-
-	// Make our file type unconstrained
-	allow(ALL, SEPOL_FILE_DOMAIN, "file", ALL);
-	allow(ALL, SEPOL_FILE_DOMAIN, "dir", ALL);
-	allow(ALL, SEPOL_FILE_DOMAIN, "fifo_file", ALL);
-	allow(ALL, SEPOL_FILE_DOMAIN, "chr_file", ALL);
-
-	// Allow these processes to access MagiskSU
-	std::initializer_list<const char *> clients {
-		"init", "shell", "system_app", "priv_app", "platform_app", "untrusted_app",
-		"untrusted_app_25", "untrusted_app_27", "untrusted_app_29", "update_engine" };
-	for (auto type : clients)
-		impl->allow_su_client(type);
-
 	// suRights
 	allow("servicemanager", SEPOL_PROC_DOMAIN, "dir", "search");
 	allow("servicemanager", SEPOL_PROC_DOMAIN, "dir", "read");

native/jni/magiskpolicy/sepolicy.hpp

@@ -20,7 +20,6 @@ struct sepol_impl : public sepolicy {
 	void add_typeattribute(type_datum_t *type, type_datum_t *attr);
 	bool add_typeattribute(const char *type, const char *attr);
 	void strip_dontaudit();
-	void allow_su_client(const char *type);
 };
 
 #define impl static_cast<sepol_impl *>(this)

native/jni/su/su_daemon.cpp

@@ -275,9 +275,9 @@ void su_daemon_handler(int client, struct ucred *credential) {
 	xdup2(errfd, STDERR_FILENO);
 
 	// Unleash all streams from SELinux hell
-	setfilecon("/proc/self/fd/0", "u:object_r:" SEPOL_FILE_DOMAIN ":s0");
+	setfilecon("/proc/self/fd/0", "u:object_r:" SEPOL_FILE_TYPE ":s0");
-	setfilecon("/proc/self/fd/1", "u:object_r:" SEPOL_FILE_DOMAIN ":s0");
+	setfilecon("/proc/self/fd/1", "u:object_r:" SEPOL_FILE_TYPE ":s0");
-	setfilecon("/proc/self/fd/2", "u:object_r:" SEPOL_FILE_DOMAIN ":s0");
+	setfilecon("/proc/self/fd/2", "u:object_r:" SEPOL_FILE_TYPE ":s0");
 
 	close(infd);
 	close(outfd);

native/jni/utils/include/selinux.hpp

@@ -16,8 +16,14 @@
 #define SYSEXT_POLICY_DIR   "/system_ext/etc/selinux/"
 #define SPLIT_PLAT_CIL      PLAT_POLICY_DIR "plat_sepolicy.cil"
 
+// Unconstrained domain the daemon and root processes run in
 #define SEPOL_PROC_DOMAIN   "magisk"
-#define SEPOL_FILE_DOMAIN   "magisk_file"
+// Highly constrained domain, sole purpose is to connect to daemon
+#define SEPOL_CLIENT_DOMAIN "magisk_client"
+// Unconstrained file type that anyone can access
+#define SEPOL_FILE_TYPE     "magisk_file"
+// Special file type to allow clients to transit to client domain automatically
+#define SEPOL_EXEC_TYPE     "magisk_exec"
 
 extern void (*freecon)(char *con);
 extern int (*setcon)(const char *con);