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platform_system_core/init/selinux.cpp

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/*
* Copyright (C) 2017 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.
*/
// This file contains the functions that initialize SELinux during boot as well as helper functions
// for SELinux operation for init.
// When the system boots, there is no SEPolicy present and init is running in the kernel domain.
// Init loads the SEPolicy from the file system, restores the context of /system/bin/init based on
// this SEPolicy, and finally exec()'s itself to run in the proper domain.
// The SEPolicy on Android comes in two variants: monolithic and split.
// The monolithic policy variant is for legacy non-treble devices that contain a single SEPolicy
// file located at /sepolicy and is directly loaded into the kernel SELinux subsystem.
// The split policy is for supporting treble devices. It splits the SEPolicy across files on
// /system/etc/selinux (the 'plat' portion of the policy) and /vendor/etc/selinux (the 'nonplat'
// portion of the policy). This is necessary to allow the system image to be updated independently
// of the vendor image, while maintaining contributions from both partitions in the SEPolicy. This
// is especially important for VTS testing, where the SEPolicy on the Google System Image may not be
// identical to the system image shipped on a vendor's device.
// The split SEPolicy is loaded as described below:
// 1) There is a precompiled SEPolicy located at either /vendor/etc/selinux/precompiled_sepolicy or
// /odm/etc/selinux/precompiled_sepolicy if odm parition is present. Stored along with this file
// are the sha256 hashes of the parts of the SEPolicy on /system and /product that were used to
// compile this precompiled policy. The system partition contains a similar sha256 of the parts
// of the SEPolicy that it currently contains. Symmetrically, product paritition contains a
// sha256 of its SEPolicy. System loads this precompiled_sepolicy directly if and only if hashes
// for system policy match and hashes for product policy match.
// 2) If these hashes do not match, then either /system or /product (or both) have been updated out
// of sync with /vendor and the init needs to compile the SEPolicy. /system contains the
// SEPolicy compiler, secilc, and it is used by the LoadSplitPolicy() function below to compile
// the SEPolicy to a temp directory and load it. That function contains even more documentation
// with the specific implementation details of how the SEPolicy is compiled if needed.
#include "selinux.h"
#include <android/api-level.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#include <android-base/chrono_utils.h>
#include <android-base/file.h>
#include <android-base/logging.h>
#include <android-base/parseint.h>
#include <android-base/unique_fd.h>
#include <fs_avb/fs_avb.h>
#include <selinux/android.h>
#include "debug_ramdisk.h"
#include "reboot_utils.h"
#include "util.h"
using namespace std::string_literals;
using android::base::ParseInt;
using android::base::Timer;
using android::base::unique_fd;
using android::fs_mgr::AvbHandle;
namespace android {
namespace init {
namespace {
enum EnforcingStatus { SELINUX_PERMISSIVE, SELINUX_ENFORCING };
EnforcingStatus StatusFromCmdline() {
EnforcingStatus status = SELINUX_ENFORCING;
import_kernel_cmdline(false,
[&](const std::string& key, const std::string& value, bool in_qemu) {
if (key == "androidboot.selinux" && value == "permissive") {
status = SELINUX_PERMISSIVE;
}
});
return status;
}
bool IsEnforcing() {
if (ALLOW_PERMISSIVE_SELINUX) {
return StatusFromCmdline() == SELINUX_ENFORCING;
}
return true;
}
// Forks, executes the provided program in the child, and waits for the completion in the parent.
// Child's stderr is captured and logged using LOG(ERROR).
bool ForkExecveAndWaitForCompletion(const char* filename, char* const argv[]) {
// Create a pipe used for redirecting child process's output.
// * pipe_fds[0] is the FD the parent will use for reading.
// * pipe_fds[1] is the FD the child will use for writing.
int pipe_fds[2];
if (pipe(pipe_fds) == -1) {
PLOG(ERROR) << "Failed to create pipe";
return false;
}
pid_t child_pid = fork();
if (child_pid == -1) {
PLOG(ERROR) << "Failed to fork for " << filename;
return false;
}
if (child_pid == 0) {
// fork succeeded -- this is executing in the child process
// Close the pipe FD not used by this process
close(pipe_fds[0]);
// Redirect stderr to the pipe FD provided by the parent
if (TEMP_FAILURE_RETRY(dup2(pipe_fds[1], STDERR_FILENO)) == -1) {
PLOG(ERROR) << "Failed to redirect stderr of " << filename;
_exit(127);
return false;
}
close(pipe_fds[1]);
if (execv(filename, argv) == -1) {
PLOG(ERROR) << "Failed to execve " << filename;
return false;
}
// Unreachable because execve will have succeeded and replaced this code
// with child process's code.
_exit(127);
return false;
} else {
// fork succeeded -- this is executing in the original/parent process
// Close the pipe FD not used by this process
close(pipe_fds[1]);
// Log the redirected output of the child process.
// It's unfortunate that there's no standard way to obtain an istream for a file descriptor.
// As a result, we're buffering all output and logging it in one go at the end of the
// invocation, instead of logging it as it comes in.
const int child_out_fd = pipe_fds[0];
std::string child_output;
if (!android::base::ReadFdToString(child_out_fd, &child_output)) {
PLOG(ERROR) << "Failed to capture full output of " << filename;
}
close(child_out_fd);
if (!child_output.empty()) {
// Log captured output, line by line, because LOG expects to be invoked for each line
std::istringstream in(child_output);
std::string line;
while (std::getline(in, line)) {
LOG(ERROR) << filename << ": " << line;
}
}
// Wait for child to terminate
int status;
if (TEMP_FAILURE_RETRY(waitpid(child_pid, &status, 0)) != child_pid) {
PLOG(ERROR) << "Failed to wait for " << filename;
return false;
}
if (WIFEXITED(status)) {
int status_code = WEXITSTATUS(status);
if (status_code == 0) {
return true;
} else {
LOG(ERROR) << filename << " exited with status " << status_code;
}
} else if (WIFSIGNALED(status)) {
LOG(ERROR) << filename << " killed by signal " << WTERMSIG(status);
} else if (WIFSTOPPED(status)) {
LOG(ERROR) << filename << " stopped by signal " << WSTOPSIG(status);
} else {
LOG(ERROR) << "waitpid for " << filename << " returned unexpected status: " << status;
}
return false;
}
}
bool ReadFirstLine(const char* file, std::string* line) {
line->clear();
std::string contents;
if (!android::base::ReadFileToString(file, &contents, true /* follow symlinks */)) {
return false;
}
std::istringstream in(contents);
std::getline(in, *line);
return true;
}
bool FindPrecompiledSplitPolicy(std::string* file) {
file->clear();
// If there is an odm partition, precompiled_sepolicy will be in
// odm/etc/selinux. Otherwise it will be in vendor/etc/selinux.
static constexpr const char vendor_precompiled_sepolicy[] =
"/vendor/etc/selinux/precompiled_sepolicy";
static constexpr const char odm_precompiled_sepolicy[] =
"/odm/etc/selinux/precompiled_sepolicy";
if (access(odm_precompiled_sepolicy, R_OK) == 0) {
*file = odm_precompiled_sepolicy;
} else if (access(vendor_precompiled_sepolicy, R_OK) == 0) {
*file = vendor_precompiled_sepolicy;
} else {
PLOG(INFO) << "No precompiled sepolicy";
return false;
}
std::string actual_plat_id;
if (!ReadFirstLine("/system/etc/selinux/plat_sepolicy_and_mapping.sha256", &actual_plat_id)) {
PLOG(INFO) << "Failed to read "
"/system/etc/selinux/plat_sepolicy_and_mapping.sha256";
return false;
}
std::string actual_product_id;
if (!ReadFirstLine("/product/etc/selinux/product_sepolicy_and_mapping.sha256",
&actual_product_id)) {
PLOG(INFO) << "Failed to read "
"/product/etc/selinux/product_sepolicy_and_mapping.sha256";
return false;
}
std::string precompiled_plat_id;
std::string precompiled_plat_sha256 = *file + ".plat_sepolicy_and_mapping.sha256";
if (!ReadFirstLine(precompiled_plat_sha256.c_str(), &precompiled_plat_id)) {
PLOG(INFO) << "Failed to read " << precompiled_plat_sha256;
file->clear();
return false;
}
std::string precompiled_product_id;
std::string precompiled_product_sha256 = *file + ".product_sepolicy_and_mapping.sha256";
if (!ReadFirstLine(precompiled_product_sha256.c_str(), &precompiled_product_id)) {
PLOG(INFO) << "Failed to read " << precompiled_product_sha256;
file->clear();
return false;
}
if (actual_plat_id.empty() || actual_plat_id != precompiled_plat_id ||
actual_product_id.empty() || actual_product_id != precompiled_product_id) {
file->clear();
return false;
}
return true;
}
bool GetVendorMappingVersion(std::string* plat_vers) {
if (!ReadFirstLine("/vendor/etc/selinux/plat_sepolicy_vers.txt", plat_vers)) {
PLOG(ERROR) << "Failed to read /vendor/etc/selinux/plat_sepolicy_vers.txt";
return false;
}
if (plat_vers->empty()) {
LOG(ERROR) << "No version present in plat_sepolicy_vers.txt";
return false;
}
return true;
}
constexpr const char plat_policy_cil_file[] = "/system/etc/selinux/plat_sepolicy.cil";
bool IsSplitPolicyDevice() {
return access(plat_policy_cil_file, R_OK) != -1;
}
bool LoadSplitPolicy() {
// IMPLEMENTATION NOTE: Split policy consists of three CIL files:
// * platform -- policy needed due to logic contained in the system image,
// * non-platform -- policy needed due to logic contained in the vendor image,
// * mapping -- mapping policy which helps preserve forward-compatibility of non-platform policy
// with newer versions of platform policy.
//
// secilc is invoked to compile the above three policy files into a single monolithic policy
// file. This file is then loaded into the kernel.
// See if we need to load userdebug_plat_sepolicy.cil instead of plat_sepolicy.cil.
const char* force_debuggable_env = getenv("INIT_FORCE_DEBUGGABLE");
bool use_userdebug_policy =
((force_debuggable_env && "true"s == force_debuggable_env) &&
AvbHandle::IsDeviceUnlocked() && access(kDebugRamdiskSEPolicy, F_OK) == 0);
if (use_userdebug_policy) {
LOG(WARNING) << "Using userdebug system sepolicy";
}
// Load precompiled policy from vendor image, if a matching policy is found there. The policy
// must match the platform policy on the system image.
std::string precompiled_sepolicy_file;
// use_userdebug_policy requires compiling sepolicy with userdebug_plat_sepolicy.cil.
// Thus it cannot use the precompiled policy from vendor image.
if (!use_userdebug_policy && FindPrecompiledSplitPolicy(&precompiled_sepolicy_file)) {
unique_fd fd(open(precompiled_sepolicy_file.c_str(), O_RDONLY | O_CLOEXEC | O_BINARY));
if (fd != -1) {
if (selinux_android_load_policy_from_fd(fd, precompiled_sepolicy_file.c_str()) < 0) {
LOG(ERROR) << "Failed to load SELinux policy from " << precompiled_sepolicy_file;
return false;
}
return true;
}
}
// No suitable precompiled policy could be loaded
LOG(INFO) << "Compiling SELinux policy";
// We store the output of the compilation on /dev because this is the most convenient tmpfs
// storage mount available this early in the boot sequence.
char compiled_sepolicy[] = "/dev/sepolicy.XXXXXX";
unique_fd compiled_sepolicy_fd(mkostemp(compiled_sepolicy, O_CLOEXEC));
if (compiled_sepolicy_fd < 0) {
PLOG(ERROR) << "Failed to create temporary file " << compiled_sepolicy;
return false;
}
// Determine which mapping file to include
std::string vend_plat_vers;
if (!GetVendorMappingVersion(&vend_plat_vers)) {
return false;
}
std::string plat_mapping_file("/system/etc/selinux/mapping/" + vend_plat_vers + ".cil");
std::string plat_compat_cil_file("/system/etc/selinux/mapping/" + vend_plat_vers +
".compat.cil");
if (access(plat_compat_cil_file.c_str(), F_OK) == -1) {
plat_compat_cil_file.clear();
}
std::string product_policy_cil_file("/product/etc/selinux/product_sepolicy.cil");
if (access(product_policy_cil_file.c_str(), F_OK) == -1) {
product_policy_cil_file.clear();
}
std::string product_mapping_file("/product/etc/selinux/mapping/" + vend_plat_vers + ".cil");
if (access(product_mapping_file.c_str(), F_OK) == -1) {
product_mapping_file.clear();
}
// vendor_sepolicy.cil and plat_pub_versioned.cil are the new design to replace
// nonplat_sepolicy.cil.
std::string plat_pub_versioned_cil_file("/vendor/etc/selinux/plat_pub_versioned.cil");
std::string vendor_policy_cil_file("/vendor/etc/selinux/vendor_sepolicy.cil");
if (access(vendor_policy_cil_file.c_str(), F_OK) == -1) {
// For backward compatibility.
// TODO: remove this after no device is using nonplat_sepolicy.cil.
vendor_policy_cil_file = "/vendor/etc/selinux/nonplat_sepolicy.cil";
plat_pub_versioned_cil_file.clear();
} else if (access(plat_pub_versioned_cil_file.c_str(), F_OK) == -1) {
LOG(ERROR) << "Missing " << plat_pub_versioned_cil_file;
return false;
}
// odm_sepolicy.cil is default but optional.
std::string odm_policy_cil_file("/odm/etc/selinux/odm_sepolicy.cil");
if (access(odm_policy_cil_file.c_str(), F_OK) == -1) {
odm_policy_cil_file.clear();
}
const std::string version_as_string = std::to_string(SEPOLICY_VERSION);
// clang-format off
std::vector<const char*> compile_args {
"/system/bin/secilc",
use_userdebug_policy ? kDebugRamdiskSEPolicy: plat_policy_cil_file,
"-m", "-M", "true", "-G", "-N",
"-c", version_as_string.c_str(),
plat_mapping_file.c_str(),
"-o", compiled_sepolicy,
// We don't care about file_contexts output by the compiler
"-f", "/sys/fs/selinux/null", // /dev/null is not yet available
};
// clang-format on
if (!plat_compat_cil_file.empty()) {
compile_args.push_back(plat_compat_cil_file.c_str());
}
if (!product_policy_cil_file.empty()) {
compile_args.push_back(product_policy_cil_file.c_str());
}
if (!product_mapping_file.empty()) {
compile_args.push_back(product_mapping_file.c_str());
}
if (!plat_pub_versioned_cil_file.empty()) {
compile_args.push_back(plat_pub_versioned_cil_file.c_str());
}
if (!vendor_policy_cil_file.empty()) {
compile_args.push_back(vendor_policy_cil_file.c_str());
}
if (!odm_policy_cil_file.empty()) {
compile_args.push_back(odm_policy_cil_file.c_str());
}
compile_args.push_back(nullptr);
if (!ForkExecveAndWaitForCompletion(compile_args[0], (char**)compile_args.data())) {
unlink(compiled_sepolicy);
return false;
}
unlink(compiled_sepolicy);
LOG(INFO) << "Loading compiled SELinux policy";
if (selinux_android_load_policy_from_fd(compiled_sepolicy_fd, compiled_sepolicy) < 0) {
LOG(ERROR) << "Failed to load SELinux policy from " << compiled_sepolicy;
return false;
}
return true;
}
bool LoadMonolithicPolicy() {
LOG(VERBOSE) << "Loading SELinux policy from monolithic file";
if (selinux_android_load_policy() < 0) {
PLOG(ERROR) << "Failed to load monolithic SELinux policy";
return false;
}
return true;
}
bool LoadPolicy() {
return IsSplitPolicyDevice() ? LoadSplitPolicy() : LoadMonolithicPolicy();
}
void SelinuxInitialize() {
LOG(INFO) << "Loading SELinux policy";
if (!LoadPolicy()) {
LOG(FATAL) << "Unable to load SELinux policy";
}
bool kernel_enforcing = (security_getenforce() == 1);
bool is_enforcing = IsEnforcing();
if (kernel_enforcing != is_enforcing) {
if (security_setenforce(is_enforcing)) {
PLOG(FATAL) << "security_setenforce(%s) failed" << (is_enforcing ? "true" : "false");
}
}
if (auto result = WriteFile("/sys/fs/selinux/checkreqprot", "0"); !result) {
LOG(FATAL) << "Unable to write to /sys/fs/selinux/checkreqprot: " << result.error();
}
}
} // namespace
// The files and directories that were created before initial sepolicy load or
// files on ramdisk need to have their security context restored to the proper
// value. This must happen before /dev is populated by ueventd.
void SelinuxRestoreContext() {
LOG(INFO) << "Running restorecon...";
selinux_android_restorecon("/dev", 0);
selinux_android_restorecon("/dev/kmsg", 0);
if constexpr (WORLD_WRITABLE_KMSG) {
selinux_android_restorecon("/dev/kmsg_debug", 0);
}
selinux_android_restorecon("/dev/null", 0);
selinux_android_restorecon("/dev/ptmx", 0);
selinux_android_restorecon("/dev/socket", 0);
selinux_android_restorecon("/dev/random", 0);
selinux_android_restorecon("/dev/urandom", 0);
selinux_android_restorecon("/dev/__properties__", 0);
selinux_android_restorecon("/dev/block", SELINUX_ANDROID_RESTORECON_RECURSE);
selinux_android_restorecon("/dev/device-mapper", 0);
selinux_android_restorecon("/apex", 0);
}
int SelinuxKlogCallback(int type, const char* fmt, ...) {
android::base::LogSeverity severity = android::base::ERROR;
if (type == SELINUX_WARNING) {
severity = android::base::WARNING;
} else if (type == SELINUX_INFO) {
severity = android::base::INFO;
}
char buf[1024];
va_list ap;
va_start(ap, fmt);
vsnprintf(buf, sizeof(buf), fmt, ap);
va_end(ap);
android::base::KernelLogger(android::base::MAIN, severity, "selinux", nullptr, 0, buf);
return 0;
}
// This function sets up SELinux logging to be written to kmsg, to match init's logging.
void SelinuxSetupKernelLogging() {
selinux_callback cb;
cb.func_log = SelinuxKlogCallback;
selinux_set_callback(SELINUX_CB_LOG, cb);
}
// This function returns the Android version with which the vendor SEPolicy was compiled.
// It is used for version checks such as whether or not vendor_init should be used
int SelinuxGetVendorAndroidVersion() {
static int vendor_android_version = [] {
if (!IsSplitPolicyDevice()) {
// If this device does not split sepolicy files, it's not a Treble device and therefore,
// we assume it's always on the latest platform.
return __ANDROID_API_FUTURE__;
}
std::string version;
if (!GetVendorMappingVersion(&version)) {
LOG(FATAL) << "Could not read vendor SELinux version";
}
int major_version;
std::string major_version_str(version, 0, version.find('.'));
if (!ParseInt(major_version_str, &major_version)) {
PLOG(FATAL) << "Failed to parse the vendor sepolicy major version "
<< major_version_str;
}
return major_version;
}();
return vendor_android_version;
}
// This function initializes SELinux then execs init to run in the init SELinux context.
int SetupSelinux(char** argv) {
SetStdioToDevNull(argv);
InitKernelLogging(argv);
if (REBOOT_BOOTLOADER_ON_PANIC) {
InstallRebootSignalHandlers();
}
boot_clock::time_point start_time = boot_clock::now();
// Set up SELinux, loading the SELinux policy.
SelinuxSetupKernelLogging();
SelinuxInitialize();
// We're in the kernel domain and want to transition to the init domain. File systems that
// store SELabels in their xattrs, such as ext4 do not need an explicit restorecon here,
// but other file systems do. In particular, this is needed for ramdisks such as the
// recovery image for A/B devices.
if (selinux_android_restorecon("/system/bin/init", 0) == -1) {
PLOG(FATAL) << "restorecon failed of /system/bin/init failed";
}
setenv(kEnvSelinuxStartedAt, std::to_string(start_time.time_since_epoch().count()).c_str(), 1);
const char* path = "/system/bin/init";
const char* args[] = {path, "second_stage", nullptr};
execv(path, const_cast<char**>(args));
// execv() only returns if an error happened, in which case we
// panic and never return from this function.
PLOG(FATAL) << "execv(\"" << path << "\") failed";
return 1;
}
} // namespace init
} // namespace android
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