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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, "Xu, Yanfei" <yanfei.xu@windriver.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.13 08/22] rcu-tasks: Dont delete holdouts within trc_inspect_reader()
Date:   Thu, 29 Jul 2021 15:54:39 +0200
Message-Id: <20210729135137.603219354@linuxfoundation.org>
In-Reply-To: <20210729135137.336097792@linuxfoundation.org>
References: <20210729135137.336097792@linuxfoundation.org>
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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