From: Jiayuan Chen 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: __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 ================================================================== Fixes: 04efcee6ef8d ("net: hold instance lock during NETDEV_CHANGE") Reported-by: syzbot+1ec2f6a450f0b54af8c8@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6824d064.a70a0220.3e9d8.001a.GAE@google.com/T/ Signed-off-by: Jiayuan Chen Signed-off-by: Jiayuan Chen --- 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/20260130141455.63201-1-jiayuan.chen@linux.dev/#t v1 -> v2: Move __dev_put() to callers instead of adding extra reference, suggested by Jakub Kicinski https://lore.kernel.org/netdev/20260129190918.4d63d241@kernel.org/T/#t --- net/core/link_watch.c | 20 +++++++++++++++----- 1 file changed, 15 insertions(+), 5 deletions(-) diff --git a/net/core/link_watch.c b/net/core/link_watch.c index 212cde35affa..25c455c10a01 100644 --- a/net/core/link_watch.c +++ b/net/core/link_watch.c @@ -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