linux-kernel - [PATCH v3 2/2] signal: Don't disable preemption in ptrace_stop() on PREEMPT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-Id: <20230615105627.1311437-3-bigeasy@linutronix.de>
Date:   Thu, 15 Jun 2023 12:56:27 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v3 2/2] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT.

On PREEMPT_RT keeping preemption disabled during the invocation of
cgroup_enter_frozen() is a problem because the function acquires css_set_lock
which is a sleeping lock on PREEMPT_RT and must not be acquired with disabled
preemption.
The preempt-disabled section is only for performance optimisation
reasons and can be avoided.

Extend the comment and don't disable preemption before scheduling on
PREEMPT_RT.

Acked-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/signal.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index da017a5461163..e887cd684d17a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2328,11 +2328,20 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
 	 * The preempt-disable section ensures that there will be no preemption
 	 * between unlock and schedule() and so improving the performance since
 	 * the ptracer has no reason to sleep.
+	 *
+	 * On PREEMPT_RT locking tasklist_lock does not disable preemption.
+	 * Therefore the task can be preempted (after
+	 * do_notify_parent_cldstop()) before unlocking tasklist_lock so there
+	 * is no benefit in doing this. The optimisation is harmful on
+	 * PEEMPT_RT because the spinlock_t (in cgroup_enter_frozen()) must not
+	 * be acquired with disabled preemption.
 	 */
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	read_unlock(&tasklist_lock);
 	cgroup_enter_frozen();
-	preempt_enable_no_resched();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable_no_resched();
 	schedule();
 	cgroup_leave_frozen(true);
 
-- 
2.40.1