kernel: start RLIMIT_CPU timers in NewThreadGroup

Before cl/695198313, this bug only affected RLIMIT_CPU soft limits, which were
represented by tg.rlimitCPUSoftSetting and was similarly uninitialized by
Kernel.NewThreadGroup(); the CPU clock ticker fetched RLIMIT_CPU hard limits in
each tick. After cl/695198313, this bug affects both RLIMIT_CPU soft and hard
limits.

Itimers don't have the same issue since they're not preserved across fork().

PiperOrigin-RevId: 695605349

pkg/sentry/kernel/task_acct.go

@@ -90,18 +90,22 @@ func (t *Task) Setitimer(id int32, newitv linux.ItimerVal) (linux.ItimerVal, err
 //
 // Preconditions: The caller must be running on the task goroutine.
 func (t *Task) NotifyRlimitCPUUpdated() {
-	// Lock t.tg.timerMu to synchronize updates to these timers between tasks
-	// in t.tg.
-	t.tg.timerMu.Lock()
-	defer t.tg.timerMu.Unlock()
-	rlimitCPU := t.tg.limits.Get(limits.CPU)
-	t.tg.appSysCPUClockLast.Store(t)
-	t.tg.rlimitCPUSoftTimer.Set(ktime.Setting{
+	t.tg.notifyRlimitCPUUpdated(t)
+}
+
+func (tg *ThreadGroup) notifyRlimitCPUUpdated(t *Task) {
+	// Lock tg.timerMu to synchronize updates to these timers between tasks in
+	// tg.
+	tg.timerMu.Lock()
+	defer tg.timerMu.Unlock()
+	rlimitCPU := tg.limits.Get(limits.CPU)
+	tg.appSysCPUClockLast.Store(t)
+	tg.rlimitCPUSoftTimer.Set(ktime.Setting{
 		Enabled: rlimitCPU.Cur != limits.Infinity,
 		Next:    ktime.FromSeconds(int64(min(rlimitCPU.Cur, math.MaxInt64))),
 		Period:  time.Second,
 	}, nil)
-	t.tg.rlimitCPUHardTimer.Set(ktime.Setting{
+	tg.rlimitCPUHardTimer.Set(ktime.Setting{
 		Enabled: rlimitCPU.Max != limits.Infinity,
 		Next:    ktime.FromSeconds(int64(min(rlimitCPU.Max, math.MaxInt64))),
 	}, nil)

pkg/sentry/kernel/thread_group.go

@@ -305,6 +305,7 @@ func (k *Kernel) NewThreadGroup(pidns *PIDNamespace, sh *SignalHandlers, termina
 	tg.rlimitCPUSoftListener.tg = tg
 	tg.rlimitCPUHardTimer.Init(&tg.appSysCPUClock, &tg.rlimitCPUHardListener)
 	tg.rlimitCPUHardListener.tg = tg
+	tg.notifyRlimitCPUUpdated(nil)
 	tg.oldRSeqCritical.Store(&OldRSeqCriticalRegion{})
 	return tg
 }

test/syscalls/linux/timers.cc

@@ -50,13 +50,15 @@ namespace {
 #define CPUCLOCK_PROF 0
 #endif  // CPUCLOCK_PROF
 
-PosixErrorOr<absl::Duration> ProcessCPUTime(pid_t pid) {
+clockid_t ProcessCPUClock(pid_t pid) {
   // Use pid-specific CPUCLOCK_PROF, which is the clock used to enforce
   // RLIMIT_CPU.
-  clockid_t clockid = (~static_cast<clockid_t>(pid) << 3) | CPUCLOCK_PROF;
+  return (~static_cast<clockid_t>(pid) << 3) | CPUCLOCK_PROF;
+}
 
+PosixErrorOr<absl::Duration> ProcessCPUTime(pid_t pid) {
   struct timespec ts;
-  int ret = clock_gettime(clockid, &ts);
+  int ret = clock_gettime(ProcessCPUClock(pid), &ts);
   if (ret < 0) {
     return PosixError(errno, "clock_gettime failed");
   }
@@ -223,6 +225,61 @@ TEST(TimerTest, ProcessKilledOnCPUHardLimit) {
   EXPECT_GE(cpu, kHardLimit);
 }
 
+TEST(TimerTest, RlimitCpuInherited) {
+  pid_t child_pid = fork();
+  MaybeSave();
+  if (child_pid == 0) {
+    // Ignore SIGXCPU from the RLIMIT_CPU soft limit.
+    struct sigaction new_action;
+    new_action.sa_handler = NoopSignalHandler;
+    new_action.sa_flags = 0;
+    sigemptyset(&new_action.sa_mask);
+    TEST_PCHECK(sigaction(SIGXCPU, &new_action, nullptr) == 0);
+
+    // Set both soft and hard limits to expire a short time from now. (Since we
+    // may not be able to raise RLIMIT_CPU again, this must happen in a
+    // disposable child of the test process.)
+    constexpr int kDelaySeconds = 2;
+    struct timespec ts;
+    TEST_PCHECK(clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts) == 0);
+    struct rlimit cpu_limits;
+    // +1 to round up, presuming that ts.tv_nsec > 0.
+    cpu_limits.rlim_cur = cpu_limits.rlim_max = ts.tv_sec + kDelaySeconds + 1;
+    TEST_PCHECK(setrlimit(RLIMIT_CPU, &cpu_limits) == 0);
+    MaybeSave();
+
+    pid_t grandchild_pid = fork();
+    MaybeSave();
+    if (grandchild_pid == 0) {
+      // Burn CPU.
+      for (;;) {
+        int x = 0;
+        benchmark::DoNotOptimize(x);  // Don't optimize this loop away.
+      }
+    }
+    TEST_PCHECK(grandchild_pid > 0);
+
+    // Wait for the grandchild to exit, but do not reap it. This will allow us
+    // to check its CPU usage while it is zombied.
+    TEST_PCHECK(waitid(P_PID, grandchild_pid, nullptr, WEXITED | WNOWAIT) == 0);
+    MaybeSave();
+    TEST_PCHECK(clock_gettime(ProcessCPUClock(grandchild_pid), &ts) == 0);
+    TEST_CHECK(ts.tv_sec >= cpu_limits.rlim_max);
+    // Reap the grandchild and check that it was SIGKILLed by the RLIMIT_CPU
+    // hard limit.
+    int status;
+    TEST_PCHECK(waitpid(grandchild_pid, &status, 0) == grandchild_pid);
+    TEST_CHECK(WIFSIGNALED(status) && (WTERMSIG(status) == SIGKILL));
+    _exit(0);
+  }
+
+  int status;
+  ASSERT_THAT(waitpid(child_pid, &status, 0),
+              SyscallSucceedsWithValue(child_pid));
+  EXPECT_TRUE(WIFEXITED(status) && (WEXITSTATUS(status) == 0))
+      << "status = " << status;
+}
+
 // See timerfd.cc:TimerSlack() for rationale.
 constexpr absl::Duration kTimerSlack = absl::Milliseconds(500);