Make DSU metadata files globally readable

libfstab (fs_mgr_fstab.cpp) could be statically linked into
unpriviledged processes, and calling
fs_mgr_fstab.cpp:ReadFstabFromFile() from an unpriviledged process while
inside of a DSU system would return an incorrect Fstab, because
ReadFstabFromFile() needs to read the DSU metadata files to return the
correct result.

To remedy this, we make DSU metadata files that are required by libfstab
to be globally readable (0644).
We also split the sepolicy label gsi_metadata_filea into two part,
gsi_metadata_file and gsi_public_metadata_file, and make
gsi_public_metadata_file:file readable by other domain (not banned by
neverallow).

Bug: 181110285
Test: Write a small utility program to validate the result of
  ReadFstabFromFile() in DSU.
Change-Id: Ia7c8c584b23752195198cc43698c71a31cabc00e

5 files changed, 28 insertions, 0 deletions

diff --git a/Android.bp b/Android.bp
index 6a7cf06..6b0f829 100644
--- a/Android.bp
+++ b/Android.bp
@@ -101,6 +101,7 @@ cc_binary {
         "libgsi",
         "libgsid",
         "liblp",
+        "libselinux",
         "libutils",
         "libc++fs",
         "libvold_binder",
diff --git a/daemon.cpp b/daemon.cpp
index ca2995d..a1ee809 100644
--- a/daemon.cpp
+++ b/daemon.cpp
@@ -49,6 +49,9 @@ static int DumpDeviceMapper() {
 int main(int argc, char** argv) {
     android::base::InitLogging(argv, android::base::LogdLogger(android::base::SYSTEM));
 
+    // Create globally readable files.
+    umask(0022);
+
     if (argc > 1) {
         if (argv[1] == "run-startup-tasks"s) {
             android::gsi::GsiService::RunStartupTasks();
diff --git a/gsi_service.cpp b/gsi_service.cpp
index 002c51f..939f603 100644
--- a/gsi_service.cpp
+++ b/gsi_service.cpp
@@ -43,6 +43,7 @@
 #include <libfiemap/image_manager.h>
 #include <openssl/sha.h>
 #include <private/android_filesystem_config.h>
+#include <selinux/android.h>
 #include <storage_literals/storage_literals.h>
 
 #include "file_paths.h"
@@ -71,6 +72,16 @@ static constexpr int64_t kDefaultUserdataSize = int64_t(2) * 1024 * 1024 * 1024;
 
 static bool GetAvbPublicKeyFromFd(int fd, AvbPublicKey* dst);
 
+// Fix the file contexts of dsu metadata files.
+// By default, newly created files inherit the file contexts of their parent
+// directory. Since globally readable public metadata files are labeled with a
+// different context, gsi_public_metadata_file, we need to call this function to
+// fix their contexts after creating them.
+static void RestoreconMetadataFiles() {
+    auto flags = SELINUX_ANDROID_RESTORECON_RECURSE | SELINUX_ANDROID_RESTORECON_SKIP_SEHASH;
+    selinux_android_restorecon(DSU_METADATA_PREFIX, flags);
+}
+
 GsiService::GsiService() {
     progress_ = {};
 }
@@ -185,6 +196,7 @@ binder::Status GsiService::createPartition(const ::std::string& name, int64_t si
             *_aidl_return = INSTALL_ERROR_GENERIC;
             return binder::Status::ok();
         }
+        RestoreconMetadataFiles();
     }
 
     installer_ = std::make_unique<PartitionInstaller>(this, install_dir_, name,
@@ -297,6 +309,7 @@ binder::Status GsiService::enableGsi(bool one_shot, const std::string& dsuSlot,
         *_aidl_return = INSTALL_ERROR_GENERIC;
         return binder::Status::ok();
     }
+    RestoreconMetadataFiles();
     if (installer_) {
         ENFORCE_SYSTEM;
         installer_ = {};
diff --git a/gsid.rc b/gsid.rc
index a5d6dd3..2c1a10b 100644
--- a/gsid.rc
+++ b/gsid.rc
@@ -10,6 +10,9 @@ on post-fs
     mkdir /metadata/gsi/dsu 0771 root system
     mkdir /metadata/gsi/ota 0771 root system
     mkdir /metadata/gsi/remount 0771 root system
+    chmod 0664 /metadata/gsi/dsu/active
+    chmod 0664 /metadata/gsi/dsu/booted
+    chmod 0664 /metadata/gsi/dsu/lp_names
 
 on post-fs-data
     write /data/gsi_persistent_data 0
diff --git a/include/libgsi/libgsi.h b/include/libgsi/libgsi.h
index 17066ff..41898df 100644
--- a/include/libgsi/libgsi.h
+++ b/include/libgsi/libgsi.h
@@ -28,6 +28,14 @@ static constexpr char kGsiServiceName[] = "gsiservice";
 
 #define DSU_METADATA_PREFIX "/metadata/gsi/dsu/"
 
+// These files need to be globally readable so that fs_mgr_fstab, which is
+// statically linked into processes, can return consistent result for non-root
+// processes:
+// * kDsuActiveFile
+// * kGsiBootedIndicatorFile
+// * kGsiLpNamesFile
+// * DsuMetadataKeyDirFile(slot)
+
 static constexpr char kGsiBootedIndicatorFile[] = DSU_METADATA_PREFIX "booted";
 
 static constexpr char kGsiLpNamesFile[] = DSU_METADATA_PREFIX "lp_names";