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Also hijack plat_file_contexts if necessary

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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
This commit is contained in:
topjohnwu 2022-03-18 00:46:34 -07:00
parent 32cd694ad5
commit 753808a4ce
1 changed files with 60 additions and 29 deletions
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89
native/jni/init/selinux.cpp
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@ -37,36 +37,53 @@ void MagiskInit::patch_sepolicy(const char *file) {
}
}
#define MOCK_LOAD SELINUXMOCK "/load"
#define MOCK_ENFORCE SELINUXMOCK "/enforce"
#define MOCK_COMPAT SELINUXMOCK "/compatible"
#define MOCK_LOAD SELINUXMOCK "/load"
#define MOCK_BLOCKING SELINUXMOCK "/blocking"
#define REAL_SELINUXFS SELINUXMOCK "/fs"
void MagiskInit::hijack_sepolicy() {
// 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';
}
}
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
xmkdir(SELINUXMOCK, 0);
auto hijack = [] {
LOGD("Hijack [" SELINUX_LOAD "] and [" SELINUX_ENFORCE "]\n");
auto hijack = [&] {
LOGD("Hijack [" SELINUX_LOAD "]\n");
mkfifo(MOCK_LOAD, 0600);
mkfifo(MOCK_ENFORCE, 0644);
xmount(MOCK_LOAD, SELINUX_LOAD, nullptr, MS_BIND, nullptr);
xmount(MOCK_ENFORCE, SELINUX_ENFORCE, 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;
@ -94,6 +111,21 @@ void MagiskInit::hijack_sepolicy() {
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
@ -110,6 +142,7 @@ void MagiskInit::hijack_sepolicy() {
xumount2(buf, MNT_DETACH);
hijack();
xwrite(fd, dt_compat.data(), dt_compat.size());
close(fd);
}
@ -123,29 +156,27 @@ void MagiskInit::hijack_sepolicy() {
sepol->magisk_rules();
sepol->load_rules(rules);
// Mount selinuxfs to another path
xmkdir(REAL_SELINUXFS, 0755);
xmount("selinuxfs", REAL_SELINUXFS, "selinuxfs", 0, nullptr);
// This open will block until init calls security_getenforce
fd = xopen(MOCK_ENFORCE, O_WRONLY);
// 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(SELINUX_ENFORCE, 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 "enforce" ONLY after sepolicy is loaded. We need to make sure
// 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 security_getenforce reads the "enforce" file, and
// 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, "0", 1);
xwrite(fd, actual_content.data(), actual_content.length());
close(fd);
// At this point, the init process will be unblocked