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 in sysfs callback to prevent it. Do the same for the
signal sending just to be sure. There is no real downside to that.

Fixes: c99a2be7 ("livepatch: force transition to finish")
Fixes: 43347d56 ("livepatch: send a fake signal to all blocking tasks")
Reported-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/livepatch/core.c

@@ -537,23 +537,25 @@ static ssize_t signal_store(struct kobject *kobj, struct kobj_attribute *attr,
 	int ret;
 	bool val;
 
-	patch = container_of(kobj, struct klp_patch, kobj);
-
-	/*
-	 * klp_mutex lock is not grabbed here intentionally. It is not really
-	 * needed. The race window is harmless and grabbing the lock would only
-	 * hold the action back.
-	 */
-	if (patch != klp_transition_patch)
-		return -EINVAL;
-
 	ret = kstrtobool(buf, &val);
 	if (ret)
 		return ret;
 
-	if (val)
+	if (!val)
+		return count;
+
+	mutex_lock(&klp_mutex);
+
+	patch = container_of(kobj, struct klp_patch, kobj);
+	if (patch != klp_transition_patch) {
+		mutex_unlock(&klp_mutex);
+		return -EINVAL;
+	}
+
 	klp_send_signals();
 
+	mutex_unlock(&klp_mutex);
+
 	return count;
 }
 
@@ -564,23 +566,25 @@ static ssize_t force_store(struct kobject *kobj, struct kobj_attribute *attr,
 	int ret;
 	bool val;
 
-	patch = container_of(kobj, struct klp_patch, kobj);
-
-	/*
-	 * klp_mutex lock is not grabbed here intentionally. It is not really
-	 * needed. The race window is harmless and grabbing the lock would only
-	 * hold the action back.
-	 */
-	if (patch != klp_transition_patch)
-		return -EINVAL;
-
 	ret = kstrtobool(buf, &val);
 	if (ret)
 		return ret;
 
-	if (val)
+	if (!val)
+		return count;
+
+	mutex_lock(&klp_mutex);
+
+	patch = container_of(kobj, struct klp_patch, kobj);
+	if (patch != klp_transition_patch) {
+		mutex_unlock(&klp_mutex);
+		return -EINVAL;
+	}
+
 	klp_force_transition();
 
+	mutex_unlock(&klp_mutex);
+
 	return count;
 }