slip_devs[] stores bare net_device pointers and takes no reference on them. sl_sync() and sl_alloc() walk that table from slip_open() under rtnl_lock(), while an entry is dropped by sl_free_netdev(), which sl_setup() installs as dev->priv_destructor. priv_destructor is called from netdev_run_todo(), which deliberately runs with the RTNL semaphore released so that it can sleep while waiting for the device refcount to drop: /* Snapshot list, allow later requests */ list_replace_init(&net_todo_list, &list); __rtnl_unlock(); ... if (dev->priv_destructor) dev->priv_destructor(dev); /* slip_devs[i] = NULL */ if (dev->needs_free_netdev) free_netdev(dev); ... /* Free network device */ kobject_put(&dev->dev.kobj); So rtnl_lock() does not serialise slip_open() against the teardown at all. sl_sync() can load slip_devs[i] while the entry is still published and dereference it after netdev_run_todo() has run the destructor and released the device: CPU0 (slip_open) CPU1 (slip_close) unregister_netdev() rtnl_unlock() netdev_run_todo() __rtnl_unlock() rtnl_lock() sl_sync() dev = slip_devs[i] priv_destructor(dev) slip_devs[i] = NULL kobject_put(&dev->dev.kobj) /* dev is freed */ sl = netdev_priv(dev) if (sl->tty || sl->leased) /* use-after-free */ BUG: KASAN: use-after-free in sl_sync drivers/net/slip/slip.c:730 [inline] BUG: KASAN: use-after-free in slip_open+0xef4/0x1210 drivers/net/slip/slip.c:806 Read of size 1 at addr ffff8880712dac71 by task syz-executor.2/6506 CPU: 2 PID: 6506 Comm: syz-executor.2 Not tainted 6.1.134-syzkaller-00260-g0c8fc3469765 #0 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 Call Trace: sl_sync drivers/net/slip/slip.c:730 [inline] slip_open+0xef4/0x1210 drivers/net/slip/slip.c:806 tty_ldisc_open+0xa2/0x120 drivers/tty/tty_ldisc.c:433 tty_set_ldisc+0x324/0x720 drivers/tty/tty_ldisc.c:564 tiocsetd drivers/tty/tty_io.c:2428 [inline] tty_ioctl+0x5f0/0x1530 drivers/tty/tty_io.c:2712 Allocated by task 6502: alloc_netdev_mqs+0x98/0xfe0 net/core/dev.c:10719 sl_alloc drivers/net/slip/slip.c:756 [inline] slip_open+0x36d/0x1210 drivers/net/slip/slip.c:817 tty_ldisc_open+0xa2/0x120 drivers/tty/tty_ldisc.c:433 tty_set_ldisc+0x324/0x720 drivers/tty/tty_ldisc.c:564 Freed by task 6497: device_release+0xa2/0x240 drivers/base/core.c:2507 kobject_put+0x179/0x280 lib/kobject.c:729 netdev_run_todo+0x6c8/0xef0 net/core/dev.c:10509 slip_close+0x166/0x1c0 drivers/net/slip/slip.c:906 tty_ldisc_close+0x113/0x1a0 drivers/tty/tty_ldisc.c:456 tty_ldisc_kill+0x94/0x160 drivers/tty/tty_ldisc.c:614 tty_ldisc_release+0xe3/0x2b0 drivers/tty/tty_ldisc.c:782 tty_release+0xbcc/0xe70 drivers/tty/tty_io.c:1860 Commit e58c19124189 ("slip: Fix use-after-free Read in slip_open") fixed a different source of stale entries - a device left in slip_devs[] after slip_open() freed it on the registration error path - and does not address this race, which is why the report survives it. Drop the entry from ndo_uninit instead. unregister_netdevice() calls ndo_uninit under RTNL, before the device is queued to netdev_run_todo(), so an entry that sl_sync() can still see while holding RTNL belongs to a device that cannot be freed until RTNL is dropped. sl_free_netdev() stays only for the slip_open() error path, where register_netdevice() may have failed before ndo_init and ndo_uninit is then not called either. Both running for the same device is harmless: they run under the same RTNL section, so the slot cannot have been reused in between. This also removes the second symptom of the missing exclusion: a destructor running after sl_alloc() had already handed the slot out to another channel used to clear a live entry, so sl_sync() stopped at that NULL, sl_alloc() returned the same index again, and register_netdevice() failed with -EEXIST because slN was still there. Reproduced on x86_64 with several threads looping over open("/dev/ptmx") + ioctl(TIOCSETD, N_SLIP) + close(). Found by Linux Verification Center (linuxtesting.org) with Syzkaller. Fixes: 5342b77c4123 ("slip: Clean up create and destroy") Cc: stable@vger.kernel.org Suggested-by: Jakub Kicinski Signed-off-by: Aleksandr Khromov --- v2: - drop the slip_devs[] entry from ndo_uninit, under RTNL, instead of adding a dedicated mutex (Jakub Kicinski) - commit message updated accordingly v1: https://lore.kernel.org/netdev/20260817070952.1155579-1-haa@amicon.ru/ --- drivers/net/slip/slip.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c index faae711cf793..85b2438e8923 100644 --- a/drivers/net/slip/slip.c +++ b/drivers/net/slip/slip.c @@ -628,9 +628,15 @@ static void sl_uninit(struct net_device *dev) struct slip *sl = netdev_priv(dev); sl_free_bufs(sl); + /* Drop the slip_devs[] entry here rather than from the destructor: + * ndo_uninit runs under RTNL, so it cannot race sl_sync(). + */ + slip_devs[dev->base_addr] = NULL; } -/* Hook the destructor so we can free slip devices at the right point in time */ +/* Only for the slip_open() error path: register_netdevice() can fail before + * ndo_init, and then ndo_uninit is not called either. + */ static void sl_free_netdev(struct net_device *dev) { int i = dev->base_addr; @@ -657,7 +663,6 @@ static void sl_setup(struct net_device *dev) { dev->netdev_ops = &sl_netdev_ops; dev->needs_free_netdev = true; - dev->priv_destructor = sl_free_netdev; dev->hard_header_len = 0; dev->addr_len = 0; @@ -908,7 +913,7 @@ static void slip_close(struct tty_struct *tty) #endif /* Flush network side */ unregister_netdev(sl->dev); - /* This will complete via sl_free_netdev */ + /* sl_uninit() has dropped the slip_devs[] entry by now */ } static void slip_hangup(struct tty_struct *tty) -- 2.48.1