[PATCH v2] rcu-tasks: Fix an use-after-free when grace-period kthread touch the task_strcut

[Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>]

trc_del_holdout() invokes put_task_struct() may cause the task_struct
is freed once the task is exiting. If happened, then we shouldn't touch
the task_strcut, or it will triger an use-after-free. Here we only mark
it as checked, and grace-period kthread will romove it from holdout
list.

        CPU0                                        CPU1
trc_add_holdout(t, bhp)
//t->usage==2
......
                                       ......
                                       release_task
                                         ->put_task_struct_rcu_user
                                           ->delayed_put_task_struct
                                             ......
                                             ->put_task_struct(t)
                                             //t->usage==1

check_all_holdout_tasks_trace
  ->trc_wait_for_one_reader
    ->get_task_struct
    ->try_invoke_on_locked_down_task(trc_inspect_reader)
      ->trc_del_holdout
    ->put_task_struct(t)
  //t->usage==0 and task_struct freed
  ->READ_ONCE(t->trc_reader_checked)
  //ops, t had been freed.

BTW, do the same thing in trc_wait_for_one_reader() when the task is a
current running task even if it is safe.

Reported-by: syzbot+7b2b13f4943374609532@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
---
v1->v2:
1. improve the commit messages.
2. change the comments in codes.

 kernel/rcu/tasks.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 36607551f966..2303cb131367 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -872,10 +872,9 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 		in_qs = likely(!t->trc_reader_nesting);
 	}
 
-	// Mark as checked.  Because this is called from the grace-period
-	// kthread, also remove the task from the holdout list.
+	// Mark as checked so that the grace-period kthread will
+	// remove it from the holdout list.
 	t->trc_reader_checked = true;
-	trc_del_holdout(t);
 
 	if (in_qs)
 		return true;  // Already in quiescent state, done!!!
@@ -899,10 +898,11 @@ static void trc_wait_for_one_reader(struct task_struct *t,
 	if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
 		return;
 
-	// The current task had better be in a quiescent state.
+	// The current task had better be in a quiescent state, and
+	// mark as checked so that the grace-period kthread will
+	// remove it from the holdout list.
 	if (t == current) {
 		t->trc_reader_checked = true;
-		trc_del_holdout(t);
 		WARN_ON_ONCE(t->trc_reader_nesting);
 		return;
 	}
-- 
2.27.0