[PATCH AUTOSEL 5.13 05/19] rcu-tasks: Don't delete holdouts within trc_inspect_reader()

Sasha Levin
2021-07-23T03:57:36Z

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 1d10bf55d85d34eb73dd8263635f43fd72135d2d ]

As Yanfei pointed out, although invoking trc_del_holdout() is safe
from the viewpoint of the integrity of the holdout list itself,
the put_task_struct() invoked by trc_del_holdout() can result in
use-after-free errors due to later accesses to this task_struct structure
by the RCU Tasks Trace grace-period kthread.

This commit therefore removes this call to trc_del_holdout() from
trc_inspect_reader() in favor of the grace-period thread's existing call
to trc_del_holdout(), thus eliminating that particular class of
use-after-free errors.

Reported-by: "Xu, Yanfei" <yanfei.xu@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/tasks.h | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 350ebf5051f9..71e9d625371a 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -908,10 +908,9 @@ 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!!!
--
2.30.2