slip: fix use-after-free in sl_sync()

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 <kuba@kernel.org>
Signed-off-by: Aleksandr Khromov <haa@amicon.ru>
Link: https://patch.msgid.link/20260824100547.164773-1-haa@amicon.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

0 files changed, 0 insertions, 0 deletions