Magisk/native/jni/init/selinux.cpp
topjohnwu 753808a4ce Also hijack plat_file_contexts if necessary
Since Android 13, sepolicy are also loaded from APEX modules. Part
of the change is to run restorecon before SELinux is set to enforce.
In order to support this situation, we also hijack plat_file_contexts
if necessary to properly order our operations.

Original idea credits to @yujincheng08, close #5603
2022-03-18 00:46:34 -07:00

188 lines
6.4 KiB
C++

#include <sys/mount.h>
#include <magisk.hpp>
#include <magiskpolicy.hpp>
#include <utils.hpp>
#include "init.hpp"
using namespace std;
void MagiskInit::patch_sepolicy(const char *file) {
LOGD("Patching monolithic policy\n");
auto sepol = unique_ptr<sepolicy>(sepolicy::from_file("/sepolicy"));
sepol->magisk_rules();
// Custom rules
if (!custom_rules_dir.empty()) {
if (auto dir = xopen_dir(custom_rules_dir.data())) {
for (dirent *entry; (entry = xreaddir(dir.get()));) {
auto rule = custom_rules_dir + "/" + entry->d_name + "/sepolicy.rule";
if (xaccess(rule.data(), R_OK) == 0) {
LOGD("Loading custom sepolicy patch: [%s]\n", rule.data());
sepol->load_rule_file(rule.data());
}
}
}
}
LOGD("Dumping sepolicy to: [%s]\n", file);
sepol->to_file(file);
// Remove OnePlus stupid debug sepolicy and use our own
if (access("/sepolicy_debug", F_OK) == 0) {
unlink("/sepolicy_debug");
link("/sepolicy", "/sepolicy_debug");
}
}
#define MOCK_COMPAT SELINUXMOCK "/compatible"
#define MOCK_LOAD SELINUXMOCK "/load"
#define MOCK_BLOCKING SELINUXMOCK "/blocking"
#define REAL_SELINUXFS SELINUXMOCK "/fs"
void MagiskInit::hijack_sepolicy() {
const char *blocking_target;
string actual_content;
xmkdir(SELINUXMOCK, 0);
if (access("/system/etc/selinux/apex", F_OK) == 0) {
// On devices with apex sepolicy, it runs restorecon before enforcing SELinux.
// To block control flow before that happens, we will have to hijack the
// plat_file_contexts file to achieve that.
if (access("/system/etc/selinux/plat_file_contexts", F_OK) == 0) {
blocking_target = "/system/etc/selinux/plat_file_contexts";
} else if (access("/plat_file_contexts", F_OK) == 0) {
blocking_target = "/plat_file_contexts";
} else {
// Error, should never happen
LOGE("! Cannot find plat_file_contexts\n");
return;
}
actual_content = full_read(blocking_target);
LOGD("Hijack [%s]\n", blocking_target);
mkfifo(MOCK_BLOCKING, 0644);
xmount(MOCK_BLOCKING, blocking_target, nullptr, MS_BIND, nullptr);
} else {
// We block using the "enforce" node
blocking_target = SELINUX_ENFORCE;
actual_content = "0";
}
// Hijack the "load" and "enforce" node in selinuxfs to manipulate
// the actual sepolicy being loaded into the kernel
auto hijack = [&] {
LOGD("Hijack [" SELINUX_LOAD "]\n");
mkfifo(MOCK_LOAD, 0600);
xmount(MOCK_LOAD, SELINUX_LOAD, nullptr, MS_BIND, nullptr);
if (strcmp(blocking_target, SELINUX_ENFORCE) == 0) {
LOGD("Hijack [" SELINUX_ENFORCE "]\n");
mkfifo(MOCK_BLOCKING, 0644);
xmount(MOCK_BLOCKING, SELINUX_ENFORCE, nullptr, MS_BIND, nullptr);
}
};
string dt_compat;
if (access(SELINUX_ENFORCE, F_OK) != 0) {
// selinuxfs not mounted yet. Hijack the dt fstab nodes first
// and let the original init mount selinuxfs for us
// This only happens on Android 8.0 - 9.0
// Preserve sysfs and procfs for hijacking
mount_list.erase(std::remove_if(
mount_list.begin(), mount_list.end(),
[](const string &s) { return s == "/proc" || s == "/sys"; }), mount_list.end());
// Remount procfs with proper options
xmount(nullptr, "/proc", nullptr, MS_REMOUNT, "hidepid=2,gid=3009");
char buf[4096];
snprintf(buf, sizeof(buf), "%s/fstab/compatible", config->dt_dir);
dt_compat = full_read(buf);
LOGD("Hijack [%s]\n", buf);
mkfifo(MOCK_COMPAT, 0444);
xmount(MOCK_COMPAT, buf, nullptr, MS_BIND, nullptr);
} else {
hijack();
}
// Read all custom rules into memory
string rules;
if (!custom_rules_dir.empty()) {
if (auto dir = xopen_dir(custom_rules_dir.data())) {
for (dirent *entry; (entry = xreaddir(dir.get()));) {
auto rule_file = custom_rules_dir + "/" + entry->d_name + "/sepolicy.rule";
if (xaccess(rule_file.data(), R_OK) == 0) {
LOGD("Load custom sepolicy patch: [%s]\n", rule_file.data());
full_read(rule_file.data(), rules);
rules += '\n';
}
}
}
}
// Create a new process waiting for init operations
if (xfork()) {
// In parent, return and continue boot process
return;
}
if (!dt_compat.empty()) {
// This open will block until init calls DoFirstStageMount
// The only purpose here is actually to wait for init to mount selinuxfs for us
int fd = xopen(MOCK_COMPAT, O_WRONLY);
char buf[4096];
snprintf(buf, sizeof(buf), "%s/fstab/compatible", config->dt_dir);
xumount2(buf, MNT_DETACH);
hijack();
xwrite(fd, dt_compat.data(), dt_compat.size());
close(fd);
}
// Read full sepolicy
int fd = xopen(MOCK_LOAD, O_RDONLY);
string policy = fd_full_read(fd);
close(fd);
auto sepol = unique_ptr<sepolicy>(sepolicy::from_data(policy.data(), policy.length()));
sepol->magisk_rules();
sepol->load_rules(rules);
// This open will block until init calls security_getenforce or selinux_android_restorecon
fd = xopen(MOCK_BLOCKING, O_WRONLY);
// Cleanup the hijacks
umount2("/init", MNT_DETACH);
xumount2(SELINUX_LOAD, MNT_DETACH);
xumount2(blocking_target, MNT_DETACH);
// Load patched policy
xmkdir(REAL_SELINUXFS, 0755);
xmount("selinuxfs", REAL_SELINUXFS, "selinuxfs", 0, nullptr);
sepol->to_file(REAL_SELINUXFS "/load");
// Write to mock blocking target ONLY after sepolicy is loaded. We need to make sure
// the actual init process is blocked until sepolicy is loaded, or else
// restorecon will fail and re-exec won't change context, causing boot failure.
// We (ab)use the fact that init reads from our blocking target, and
// because it has been replaced with our FIFO file, init will block until we
// write something into the pipe, effectively hijacking its control flow.
xwrite(fd, actual_content.data(), actual_content.length());
close(fd);
// At this point, the init process will be unblocked
// and continue on with restorecon + re-exec.
// Terminate process
exit(0);
}