kobject: Don't trigger kobject_uevent(KOBJ_REMOVE) twice.

syzbot is hitting use-after-free bug in uinput module [1]. This is because
kobject_uevent(KOBJ_REMOVE) is called again due to commit 0f4dafc0
("Kobject: auto-cleanup on final unref") after memory allocation fault
injection made kobject_uevent(KOBJ_REMOVE) from device_del() from
input_unregister_device() fail, while uinput_destroy_device() is expecting
that kobject_uevent(KOBJ_REMOVE) is not called after device_del() from
input_unregister_device() completed.

That commit intended to catch cases where nobody even attempted to send
"remove" uevents. But there is no guarantee that an event will ultimately
be sent. We are at the point of no return as far as the rest of the kernel
is concerned; there are no repeats or do-overs.

Also, it is not clear whether some subsystem depends on that commit.
If no subsystem depends on that commit, it will be better to remove
the state_{add,remove}_uevent_sent logic. But we don't want to risk
a regression (in a patch which will be backported) by trying to remove
that logic. Therefore, as a first step, let's avoid the use-after-free bug
by making sure that kobject_uevent(KOBJ_REMOVE) won't be triggered twice.

[1] https://syzkaller.appspot.com/bug?id=8b17c134fe938bbddd75a45afaa9e68af43a362dReported-by: syzbot <syzbot+f648cfb7e0b52bf7ae32@syzkaller.appspotmail.com>
Analyzed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Fixes: 0f4dafc0 ("Kobject: auto-cleanup on final unref")
Cc: Kay Sievers <kay@vrfy.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/kobject_uevent.c

@@ -466,6 +466,13 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 	int i = 0;
 	int retval = 0;
 
+	/*
+	 * Mark "remove" event done regardless of result, for some subsystems
+	 * do not want to re-trigger "remove" event via automatic cleanup.
+	 */
+	if (action == KOBJ_REMOVE)
+		kobj->state_remove_uevent_sent = 1;
+
 	pr_debug("kobject: '%s' (%p): %s\n",
 		 kobject_name(kobj), kobj, __func__);
 
@@ -567,10 +574,6 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 		kobj->state_add_uevent_sent = 1;
 		break;
 
-	case KOBJ_REMOVE:
-		kobj->state_remove_uevent_sent = 1;
-		break;
-
 	case KOBJ_UNBIND:
 		zap_modalias_env(env);
 		break;