Replace use of overlayfs' override_creds with overlay_remounter (6bb52890) · Commits · e / os / android_system_core

fs_mgr/fs_mgr.cpp

+5 −0

Original line number	Diff line number	Diff line
		@@ -2335,6 +2335,11 @@ OverlayfsCheckResult CheckOverlayfs() {
		if (!fs_mgr_filesystem_available("overlay")) {
		return {.supported = false};
		}

		if (!use_override_creds) {
		return {.supported = true};
		}

		struct utsname uts;
		if (uname(&uts) == -1) {
		return {.supported = false};

fs_mgr/include/fs_mgr_overlayfs.h

+6 −0

Original line number	Diff line number	Diff line
		@@ -43,5 +43,11 @@ void MapScratchPartitionIfNeeded(Fstab* fstab,
		// overlays if any partition is flashed or updated.
		void TeardownAllOverlayForMountPoint(const std::string& mount_point = {});

		// Are we using overlayfs's non-upstreamed override_creds feature?
		// b/388912628 removes the need for override_creds
		// Once this bug is fixed and has had enough soak time, remove this variable and hard code to false
		// where it used
		constexpr bool use_override_creds = true;

		} // namespace fs_mgr
		} // namespace android

init/Android.bp

+6 −1

Original line number	Diff line number	Diff line
		@@ -136,6 +136,8 @@ libinit_cc_defaults {
		"-DWORLD_WRITABLE_KMSG=1",
		"-UDUMP_ON_UMOUNT_FAILURE",
		"-DDUMP_ON_UMOUNT_FAILURE=1",
		"-UALLOW_REMOUNT_OVERLAYS",
		"-DALLOW_REMOUNT_OVERLAYS=1",
		],
		},
		eng: {
		@@ -263,7 +265,10 @@ phony {
		name: "init",
		required: [
		"init_second_stage",
		],
		] + select(product_variable("debuggable"), {
		true: ["overlay_remounter"],
		false: [],
		}),
		}

		cc_defaults {

init/first_stage_init.cpp

+1 −1

Original line number	Diff line number	Diff line
		@@ -402,7 +402,7 @@ int FirstStageMain(int argc, char** argv) {

		// /second_stage_resources is used to preserve files from first to second
		// stage init
		CHECKCALL(mount("tmpfs", kSecondStageRes, "tmpfs", MS_NOEXEC \| MS_NOSUID \| MS_NODEV,
		CHECKCALL(mount("tmpfs", kSecondStageRes, "tmpfs", MS_NOSUID \| MS_NODEV,
		"mode=0755,uid=0,gid=0"));

		if (IsMicrodroid() && android::virtualization::IsOpenDiceChangesFlagEnabled()) {

init/selinux.cpp

+65 −0

Original line number	Diff line number	Diff line
		@@ -69,6 +69,7 @@
		#include <android/avf_cc_flags.h>
		#include <fs_avb/fs_avb.h>
		#include <fs_mgr.h>
		#include <fs_mgr_overlayfs.h>
		#include <genfslabelsversion.h>
		#include <libgsi/libgsi.h>
		#include <libsnapshot/snapshot.h>
		@@ -77,6 +78,7 @@
		#include "block_dev_initializer.h"
		#include "debug_ramdisk.h"
		#include "reboot_utils.h"
		#include "second_stage_resources.h"
		#include "snapuserd_transition.h"
		#include "util.h"

		@@ -698,6 +700,65 @@ void LoadSelinuxPolicyAndroid() {
		}
		}

		#ifdef ALLOW_REMOUNT_OVERLAYS
		void SetupOverlays() {
		if (android::fs_mgr::use_override_creds) return;

		bool has_overlays = false;
		std::string contents;
		auto result = android::base::ReadFileToString("/proc/mounts", &contents, true);

		auto lines = android::base::Split(contents, "\n");
		for (auto const& line : lines)
		if (android::base::StartsWith(line, "overlay")) {
		has_overlays = true;
		break;
		}

		if (!has_overlays) return;

		// After adb remount, we mount all r/o volumes with overlayfs to allow writing.
		// However, since overlayfs performs its file operations in the context of the
		// mounting process, this will not work as is - init is in the kernel domain in
		// first stage, which has very limited permissions.

		// In order to fix this, we need to unmount remount all these volumes from a process
		// with sufficient privileges to be able to perform these operations. The
		// overlay_remounter domain has those privileges on debuggable devices.
		// We will call overlay_remounter which will do the unmounts/mounts.
		// But for that to work, the volumes must not be busy, so we need to copy
		// overlay_remounter from system to a ramdisk and run it from there.

		const char* kOverlayRemounter = "overlay_remounter";
		auto or_src = std::filesystem::path("/system/xbin/") / kOverlayRemounter;
		auto or_dest = std::filesystem::path(kSecondStageRes) / kOverlayRemounter;
		std::error_code ec;
		std::filesystem::copy(or_src, or_dest, ec);
		if (ec) {
		LOG(FATAL) << "Failed to copy " << or_src << " to " << or_dest << " " << ec.message();
		}

		if (selinux_android_restorecon(or_dest.c_str(), 0) == -1) {
		PLOG(FATAL) << "restorecon of " << or_dest << " failed";
		}
		auto dest = unique_fd(open(or_dest.c_str(), O_RDONLY \| O_CLOEXEC));
		if (dest.get() == -1) {
		PLOG(FATAL) << "Failed to reopen " << or_dest;
		}
		if (unlink(or_dest.c_str()) == -1) {
		PLOG(FATAL) << "Failed to unlink " << or_dest;
		}
		const char* args[] = {or_dest.c_str(), nullptr};
		fexecve(dest.get(), const_cast<char**>(args), nullptr);

		// execv() only returns if an error happened, in which case we
		// panic and never return from this function.
		PLOG(FATAL) << "execv(\"" << or_dest << "\") failed";
		}
		#else
		void SetupOverlays() {}
		#endif

		int SetupSelinux(char** argv) {
		SetStdioToDevNull(argv);
		InitKernelLogging(argv);
		@@ -738,6 +799,10 @@ int SetupSelinux(char** argv) {

		setenv(kEnvSelinuxStartedAt, std::to_string(start_time.time_since_epoch().count()).c_str(), 1);

		// SetupOverlays does not return if overlays exist, instead it execs overlay_remounter
		// which then execs second stage init
		SetupOverlays();

		const char* path = "/system/bin/init";
		const char* args[] = {path, "second_stage", nullptr};
		execv(path, const_cast<char**>(args));