Make transition to "postinstall" domain explicit.

When running the postinstall program, we were relying on the automatic
transition between update_engine domain and postinstall, which required
postinstall_file to be an entrypoint for such domain. This patch makes
the transition explicit by calling setexeccon(3).

Bug: 28008031
TEST=make dist; Deployed an update to edison-eng: postinstall runs as "postinstall" domain.

Change-Id: I27d404886841e0bd39589f0274eaaf98929eeac9

6 files changed, 34 insertions, 10 deletions

diff --git a/Android.mk b/Android.mk
index afb14a62..633baa2b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -149,6 +149,7 @@ ue_libpayload_consumer_exported_static_libraries := \
 ue_libpayload_consumer_exported_shared_libraries := \
     libcrypto-host \
     libcurl-host \
+    libselinux \
     libssl-host \
     $(ue_update_metadata_protos_exported_shared_libraries)
 
diff --git a/common/subprocess.cc b/common/subprocess.cc
index 9738b1d1..9473f10c 100644
--- a/common/subprocess.cc
+++ b/common/subprocess.cc
@@ -32,6 +32,9 @@
 #include <base/strings/stringprintf.h>
 #include <brillo/process.h>
 #include <brillo/secure_blob.h>
+#ifdef __ANDROID__
+#include <selinux/selinux.h>
+#endif  // __ANDROID__
 
 #include "update_engine/common/utils.h"
 
@@ -44,7 +47,9 @@ namespace chromeos_update_engine {
 
 namespace {
 
-bool SetupChild(const std::map<string, string>& env, uint32_t flags) {
+bool SetupChild(const std::map<string, string>& env,
+                uint32_t flags,
+                const char* se_domain) {
   // Setup the environment variables.
   clearenv();
   for (const auto& key_value : env) {
@@ -63,6 +68,15 @@ bool SetupChild(const std::map<string, string>& env, uint32_t flags) {
     return false;
   IGNORE_EINTR(close(fd));
 
+#ifdef __ANDROID__
+  // setexeccon(3) accepts a nullptr to indicate the default context policy.
+  if (setexeccon(se_domain) < 0) {
+    PLOG(ERROR) << "Error setting the SELinux domain to "
+                << (se_domain ? se_domain : "<nullptr>");
+    return false;
+  }
+#endif  // __ANDROID__
+
   return true;
 }
 
@@ -74,6 +88,7 @@ bool SetupChild(const std::map<string, string>& env, uint32_t flags) {
 bool LaunchProcess(const vector<string>& cmd,
                    uint32_t flags,
                    const vector<int>& output_pipes,
+                   const char* se_domain,
                    brillo::Process* proc) {
   for (const string& arg : cmd)
     proc->AddArg(arg);
@@ -92,7 +107,7 @@ bool LaunchProcess(const vector<string>& cmd,
   }
   proc->SetCloseUnusedFileDescriptors(true);
   proc->RedirectUsingPipe(STDOUT_FILENO, false);
-  proc->SetPreExecCallback(base::Bind(&SetupChild, env, flags));
+  proc->SetPreExecCallback(base::Bind(&SetupChild, env, flags, se_domain));
 
   return proc->Start();
 }
@@ -170,16 +185,17 @@ void Subprocess::ChildExitedCallback(const siginfo_t& info) {
 
 pid_t Subprocess::Exec(const vector<string>& cmd,
                        const ExecCallback& callback) {
-  return ExecFlags(cmd, kRedirectStderrToStdout, {}, callback);
+  return ExecFlags(cmd, kRedirectStderrToStdout, {}, nullptr, callback);
 }
 
 pid_t Subprocess::ExecFlags(const vector<string>& cmd,
                             uint32_t flags,
                             const vector<int>& output_pipes,
+                            const char* se_domain,
                             const ExecCallback& callback) {
   unique_ptr<SubprocessRecord> record(new SubprocessRecord(callback));
 
-  if (!LaunchProcess(cmd, flags, output_pipes, &record->proc)) {
+  if (!LaunchProcess(cmd, flags, output_pipes, se_domain, &record->proc)) {
     LOG(ERROR) << "Failed to launch subprocess";
     return 0;
   }
@@ -248,7 +264,7 @@ bool Subprocess::SynchronousExecFlags(const vector<string>& cmd,
   // It doesn't make sense to redirect some pipes in the synchronous case
   // because we won't be reading on our end, so we don't expose the output_pipes
   // in this case.
-  if (!LaunchProcess(cmd, flags, {}, &proc)) {
+  if (!LaunchProcess(cmd, flags, {}, nullptr, &proc)) {
     LOG(ERROR) << "Failed to launch subprocess";
     return false;
   }
diff --git a/common/subprocess.h b/common/subprocess.h
index b655fb74..6c99c8de 100644
--- a/common/subprocess.h
+++ b/common/subprocess.h
@@ -73,6 +73,7 @@ class Subprocess {
   pid_t ExecFlags(const std::vector<std::string>& cmd,
                   uint32_t flags,
                   const std::vector<int>& output_pipes,
+                  const char* se_domain,
                   const ExecCallback& callback);
 
   // Kills the running process with SIGTERM and ignores the callback.
diff --git a/common/subprocess_unittest.cc b/common/subprocess_unittest.cc
index 5ca44e80..d5e33bba 100644
--- a/common/subprocess_unittest.cc
+++ b/common/subprocess_unittest.cc
@@ -148,6 +148,7 @@ TEST_F(SubprocessTest, StderrNotIncludedInOutputTest) {
       {kBinPath "/sh", "-c", "echo on stdout; echo on stderr >&2"},
       0,
       {},
+      nullptr,
       base::Bind(&ExpectedResults, 0, "on stdout\n")));
   loop_.Run();
 }
@@ -158,6 +159,7 @@ TEST_F(SubprocessTest, PipeRedirectFdTest) {
       {kBinPath "/sh", "-c", "echo on pipe >&3"},
       0,
       {3},
+      nullptr,
       base::Bind(&ExpectedDataOnPipe, &subprocess_, &pid, 3, "on pipe\n", 0));
   EXPECT_NE(0, pid);
 
@@ -174,10 +176,7 @@ TEST_F(SubprocessTest, PipeClosedWhenNotRedirectedTest) {
   const vector<string> cmd = {kBinPath "/sh", "-c",
      base::StringPrintf("echo on pipe >/proc/self/fd/%d", pipe.writer)};
   EXPECT_TRUE(subprocess_.ExecFlags(
-      cmd,
-      0,
-      {},
-      base::Bind(&ExpectedResults, 1, "")));
+      cmd, 0, {}, nullptr, base::Bind(&ExpectedResults, 1, "")));
   loop_.Run();
 }
 
diff --git a/p2p_manager.cc b/p2p_manager.cc
index 127e5ff9..bcfe1561 100644
--- a/p2p_manager.cc
+++ b/p2p_manager.cc
@@ -396,7 +396,10 @@ class LookupData {
 
     // We expect to run just "p2p-client" and find it in the path.
     child_pid_ = Subprocess::Get().ExecFlags(
-        cmd, Subprocess::kSearchPath, {},
+        cmd,
+        Subprocess::kSearchPath,
+        {},
+        nullptr,
         Bind(&LookupData::OnLookupDone, base::Unretained(this)));
 
     if (!child_pid_) {
diff --git a/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index db1ec3c8..dc4af713 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -43,6 +43,9 @@ namespace {
 // sample_images.sh file.
 const int kPostinstallStatusFd = 3;
 
+// The SELinux domain where the postinstall program runs.
+const char* kPostinstallDomain = "postinstall";
+
 }  // namespace
 
 namespace chromeos_update_engine {
@@ -167,6 +170,7 @@ void PostinstallRunnerAction::PerformPartitionPostinstall() {
       command,
       Subprocess::kRedirectStderrToStdout,
       {kPostinstallStatusFd},
+      kPostinstallDomain,
       base::Bind(&PostinstallRunnerAction::CompletePartitionPostinstall,
                  base::Unretained(this)));
   // Subprocess::Exec should never return a negative process id.