mrp: introduce active flags to prevent UAF when applicant uninit

The caller of del_timer_sync must prevent restarting of the timer, If
we have no this synchronization, there is a small probability that the
cancellation will not be successful.

And syzbot report the fellowing crash:
==================================================================
BUG: KASAN: use-after-free in hlist_add_head include/linux/list.h:929 [inline]
BUG: KASAN: use-after-free in enqueue_timer+0x18/0xa4 kernel/time/timer.c:605
Write at addr f9ff000024df6058 by task syz-fuzzer/2256
Pointer tag: [f9], memory tag: [fe]

CPU: 1 PID: 2256 Comm: syz-fuzzer Not tainted 6.1.0-rc5-syzkaller-00008-
ge01d50cb #0
Hardware name: linux,dummy-virt (DT)
Call trace:
 dump_backtrace.part.0+0xe0/0xf0 arch/arm64/kernel/stacktrace.c:156
 dump_backtrace arch/arm64/kernel/stacktrace.c:162 [inline]
 show_stack+0x18/0x40 arch/arm64/kernel/stacktrace.c:163
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x68/0x84 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:284 [inline]
 print_report+0x1a8/0x4a0 mm/kasan/report.c:395
 kasan_report+0x94/0xb4 mm/kasan/report.c:495
 __do_kernel_fault+0x164/0x1e0 arch/arm64/mm/fault.c:320
 do_bad_area arch/arm64/mm/fault.c:473 [inline]
 do_tag_check_fault+0x78/0x8c arch/arm64/mm/fault.c:749
 do_mem_abort+0x44/0x94 arch/arm64/mm/fault.c:825
 el1_abort+0x40/0x60 arch/arm64/kernel/entry-common.c:367
 el1h_64_sync_handler+0xd8/0xe4 arch/arm64/kernel/entry-common.c:427
 el1h_64_sync+0x64/0x68 arch/arm64/kernel/entry.S:576
 hlist_add_head include/linux/list.h:929 [inline]
 enqueue_timer+0x18/0xa4 kernel/time/timer.c:605
 mod_timer+0x14/0x20 kernel/time/timer.c:1161
 mrp_periodic_timer_arm net/802/mrp.c:614 [inline]
 mrp_periodic_timer+0xa0/0xc0 net/802/mrp.c:627
 call_timer_fn.constprop.0+0x24/0x80 kernel/time/timer.c:1474
 expire_timers+0x98/0xc4 kernel/time/timer.c:1519

To fix it, we can introduce a new active flags to make sure the timer will
not restart.

Reported-by: syzbot+6fd64001c20aa99e34a4@syzkaller.appspotmail.com
Signed-off-by: Schspa Shi <schspa@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/mrp.h

@@ -124,6 +124,7 @@ struct mrp_applicant {
 	struct sk_buff		*pdu;
 	struct rb_root		mad;
 	struct rcu_head		rcu;
+	bool			active;
 };
 
 struct mrp_port {

net/802/mrp.c

@@ -606,7 +606,10 @@ static void mrp_join_timer(struct timer_list *t)
 	spin_unlock(&app->lock);
 
 	mrp_queue_xmit(app);
-	mrp_join_timer_arm(app);
+	spin_lock(&app->lock);
+	if (likely(app->active))
+		mrp_join_timer_arm(app);
+	spin_unlock(&app->lock);
 }
 
 static void mrp_periodic_timer_arm(struct mrp_applicant *app)
@@ -620,11 +623,12 @@ static void mrp_periodic_timer(struct timer_list *t)
 	struct mrp_applicant *app = from_timer(app, t, periodic_timer);
 
 	spin_lock(&app->lock);
-	mrp_mad_event(app, MRP_EVENT_PERIODIC);
-	mrp_pdu_queue(app);
+	if (likely(app->active)) {
+		mrp_mad_event(app, MRP_EVENT_PERIODIC);
+		mrp_pdu_queue(app);
+		mrp_periodic_timer_arm(app);
+	}
 	spin_unlock(&app->lock);
-
-	mrp_periodic_timer_arm(app);
 }
 
 static int mrp_pdu_parse_end_mark(struct sk_buff *skb, int *offset)
@@ -872,6 +876,7 @@ int mrp_init_applicant(struct net_device *dev, struct mrp_application *appl)
 	app->dev = dev;
 	app->app = appl;
 	app->mad = RB_ROOT;
+	app->active = true;
 	spin_lock_init(&app->lock);
 	skb_queue_head_init(&app->queue);
 	rcu_assign_pointer(dev->mrp_port->applicants[appl->type], app);
@@ -900,6 +905,9 @@ void mrp_uninit_applicant(struct net_device *dev, struct mrp_application *appl)
 
 	RCU_INIT_POINTER(port->applicants[appl->type], NULL);
 
+	spin_lock_bh(&app->lock);
+	app->active = false;
+	spin_unlock_bh(&app->lock);
 	/* Delete timer and generate a final TX event to flush out
 	 * all pending messages before the applicant is gone.
 	 */