linux-kernel - [PATCH] livepatch: add locking to force and signal functions

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Date:   Wed, 20 Dec 2017 10:28:07 +0100
From:   Miroslav Benes <mbenes@...e.cz>
To:     jpoimboe@...hat.com, jeyu@...nel.org, jikos@...nel.org
Cc:     pmladek@...e.com, jbaron@...mai.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>
Subject: [PATCH] livepatch: add locking to force and signal functions

klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
because it seemed to be superfluous. A potential race in
klp_send_signals() was harmless and there was nothing in
klp_force_transition() which needed to be synchronized. That changed
with the addition of klp_forced variable during the review process.

There is a small window now, when klp_complete_transition() does not see
klp_forced set to true while all tasks have been already transitioned to
the target state. module_put() is called and the module can be removed.

Acquire klp_mutex to prevent it. Do the same in klp_send_signals() just
to be sure. There is no real downside to that.

Reported-by: Jason Baron <jbaron@...mai.com>
Signed-off-by: Miroslav Benes <mbenes@...e.cz>
---
 kernel/livepatch/transition.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index be5bfa533ee8..3f932ff607cd 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -625,6 +625,8 @@ void klp_send_signals(void)

 	pr_notice("signaling remaining tasks\n");

+	mutex_lock(&klp_mutex);
+
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task) {
 		if (!klp_patch_pending(task))
@@ -653,6 +655,8 @@ void klp_send_signals(void)
 		}
 	}
 	read_unlock(&tasklist_lock);
+
+	mutex_unlock(&klp_mutex);
 }

 /*
@@ -671,6 +675,8 @@ void klp_force_transition(void)

 	pr_warn("forcing remaining tasks to the patched state\n");

+	mutex_lock(&klp_mutex);
+
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task)
 		klp_update_patch_state(task);
@@ -680,4 +686,6 @@ void klp_force_transition(void)
 		klp_update_patch_state(idle_task(cpu));

 	klp_forced = true;
+
+	mutex_unlock(&klp_mutex);
 }
-- 
2.15.1