linkwatch: use __dev_put() in callers to prevent UAF

From: Jiayuan Chen <[email protected]>

After linkwatch_do_dev() calls __dev_put() to release the linkwatch reference, the device refcount may drop to 1. At this point, netdev_run_todo() can proceed (since linkwatch_sync_dev() sees an empty list and returns without blocking), wait for the refcount to become 1 via netdev_wait_allrefs_any(), and then free the device via kobject_put().

This creates a use-after-free when __linkwatch_run_queue() tries to call netdev_unlock_ops() on the already-freed device.

Note that adding netdev_lock_ops()/netdev_unlock_ops() pair in netdev_run_todo() before kobject_put() would not work, because netdev_lock_ops() is conditional - it only locks when netdev_need_ops_lock() returns true. If the device doesn't require ops_lock, linkwatch won't hold any lock, and netdev_run_todo() acquiring the lock won't provide synchronization.

Fix this by moving __dev_put() from linkwatch_do_dev() to its callers. The device reference logically pairs with de-listing the device, so it's reasonable for the caller that did the de-listing to release it. This allows placing __dev_put() after all device accesses are complete, preventing UAF.

The bug can be reproduced by adding mdelay(2000) after linkwatch_do_dev() in __linkwatch_run_queue(), then running:

ip tuntap add mode tun name tun_test
ip link set tun_test up
ip link set tun_test carrier off
ip link set tun_test carrier on
sleep 0.5
ip tuntap del mode tun name tun_test

KASAN report:

==================================================================

 BUG: KASAN: use-after-free in netdev_need_ops_lock include/net/netdev_lock.h:33 [inline]
 BUG: KASAN: use-after-free in netdev_unlock_ops include/net/netdev_lock.h:47 [inline]
 BUG: KASAN: use-after-free in __linkwatch_run_queue+0x865/0x8a0 net/core/link_watch.c:245
 Read of size 8 at addr ffff88804de5c008 by task kworker/u32:10/8123

 CPU: 0 UID: 0 PID: 8123 Comm: kworker/u32:10 Not tainted syzkaller #0 PREEMPT(full)
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
 Workqueue: events_unbound linkwatch_event
 Call Trace:
  <TASK>
  __dump_stack lib/dump_stack.c:94 [inline]
  dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
  print_address_description mm/kasan/report.c:378 [inline]
  print_report+0x156/0x4c9 mm/kasan/report.c:482
  kasan_report+0xdf/0x1a0 mm/kasan/report.c:595
  netdev_need_ops_lock include/net/netdev_lock.h:33 [inline]
  netdev_unlock_ops include/net/netdev_lock.h:47 [inline]
  __linkwatch_run_queue+0x865/0x8a0 net/core/link_watch.c:245
  linkwatch_event+0x8f/0xc0 net/core/link_watch.c:304
  process_one_work+0x9c2/0x1840 kernel/workqueue.c:3257
  process_scheduled_works kernel/workqueue.c:3340 [inline]
  worker_thread+0x5da/0xe40 kernel/workqueue.c:3421
  kthread+0x3b3/0x730 kernel/kthread.c:463
  ret_from_fork+0x754/0xaf0 arch/x86/kernel/process.c:158
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
  </TASK>
 ==================================================================

---
v2 -> v3: Add comments for each __dev_put() call explaining why
__dev_put() is used instead of dev_put(), suggested by Eric Dumazet
https://lore.kernel.org/netdev/[email protected]/#t
v1 -> v2: Move __dev_put() to callers instead of adding extra
reference, suggested by Jakub Kicinski
https://lore.kernel.org/netdev/[email protected]/T/#t

---
 net/core/link_watch.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

index 212cde35affa..25c455c10a01 100644
@@ -185,10 +185,6 @@ static void linkwatch_do_dev(struct net_device *dev)
 
 		netif_state_change(dev);
 	}
-	/* Note: our callers are responsible for calling netdev_tracker_free().
-	 * This is the reason we use __dev_put() instead of dev_put().
-	 */
-	__dev_put(dev);
 }
 
 static void __linkwatch_run_queue(int urgent_only)

@@ -243,6 +239,11 @@ static void __linkwatch_run_queue(int urgent_only)
 		netdev_lock_ops(dev);
 		linkwatch_do_dev(dev);
 		netdev_unlock_ops(dev);
+		/* Use __dev_put() because netdev_tracker_free() was already
+		 * called above. Must be after netdev_unlock_ops() to prevent
+		 * netdev_run_todo() from freeing the device while still in use.
+		 */
+		__dev_put(dev);
 		do_dev--;
 		spin_lock_irq(&lweventlist_lock);
 	}

@@ -278,8 +279,13 @@ void __linkwatch_sync_dev(struct net_device *dev)
 {
 	netdev_ops_assert_locked(dev);
 
-	if (linkwatch_clean_dev(dev))
+	if (linkwatch_clean_dev(dev)) {
 		linkwatch_do_dev(dev);
+		/* Use __dev_put() because netdev_tracker_free() was already
+		 * called inside linkwatch_clean_dev().
+		 */
+		__dev_put(dev);
+	}
 }
 
 void linkwatch_sync_dev(struct net_device *dev)

@@ -288,6 +294,10 @@ void linkwatch_sync_dev(struct net_device *dev)
 		netdev_lock_ops(dev);
 		linkwatch_do_dev(dev);
 		netdev_unlock_ops(dev);
+		/* Use __dev_put() because netdev_tracker_free() was already
+		 * called inside linkwatch_clean_dev().
+		 */
+		__dev_put(dev);
 	}
 }
 

2.43.0

Thanks a lot for this fix.