serial: caif: fix use-after-free in caif_serial ldisc_close()

serial: caif: fix use-after-free in caif_serial ldisc_close()

There is a use-after-free bug in caif_serial where handle_tx() may
access ser->tty after the tty has been freed.

The race condition occurs between ldisc_close() and packet transmission:

    CPU 0 (close)                     CPU 1 (xmit)
    -------------                     ------------
    ldisc_close()
      tty_kref_put(ser->tty)
      [tty may be freed here]
                     <-- race window -->
                                      caif_xmit()
                                        handle_tx()
                                          tty = ser->tty  // dangling ptr
                                          tty->ops->write() // UAF!
      schedule_work()
        ser_release()
          unregister_netdevice()

The root cause is that tty_kref_put() is called in ldisc_close() while
the network device is still active and can receive packets.

Since ser and tty have a 1:1 binding relationship with consistent
lifecycles (ser is allocated in ldisc_open and freed in ser_release
via unregister_netdevice, and each ser binds exactly one tty), we can
safely defer the tty reference release to ser_release() where the
network device is unregistered.

Fix this by moving tty_kref_put() from ldisc_close() to ser_release(),
after unregister_netdevice(). This ensures the tty reference is held
as long as the network device exists, preventing the UAF.

Note: We save ser->tty before unregister_netdevice() because ser is
embedded in netdev's private data and will be freed along with netdev
(needs_free_netdev = true).

How to reproduce: Add mdelay(500) at the beginning of ldisc_close()
to widen the race window, then run the reproducer program [1].

Note: There is a separate deadloop issue in handle_tx() when using
PORT_UNKNOWN serial ports (e.g., /dev/ttyS3 in QEMU without proper
serial backend). This deadloop exists even without this patch,
and is likely caused by inconsistency between uart_write_room() and
uart_write() in serial core. It has been addressed in a separate
patch [2].

KASAN report:

==================================================================
BUG: KASAN: slab-use-after-free in handle_tx+0x5d1/0x620
Read of size 1 at addr ffff8881131e1490 by task caif_uaf_trigge/9929

Call Trace:
 <TASK>
 dump_stack_lvl+0x10e/0x1f0
 print_report+0xd0/0x630
 kasan_report+0xe4/0x120
 handle_tx+0x5d1/0x620
 dev_hard_start_xmit+0x9d/0x6c0
 __dev_queue_xmit+0x6e2/0x4410
 packet_xmit+0x243/0x360
 packet_sendmsg+0x26cf/0x5500
 __sys_sendto+0x4a3/0x520
 __x64_sys_sendto+0xe0/0x1c0
 do_syscall_64+0xc9/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f615df2c0d7

Allocated by task 9930:

Freed by task 64:

Last potentially related work creation:

The buggy address belongs to the object at ffff8881131e1000
 which belongs to the cache kmalloc-cg-2k of size 2048
The buggy address is located 1168 bytes inside of
 freed 2048-byte region [ffff8881131e1000, ffff8881131e1800)

The buggy address belongs to the physical page:
page_owner tracks the page as allocated
page last free pid 9778 tgid 9778 stack trace:

Memory state around the buggy address:
 ffff8881131e1380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8881131e1400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8881131e1480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                         ^
 ffff8881131e1500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8881131e1580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================
[1]: https://gist.github.com/mrpre/f683f244544f7b11e7fa87df9e6c2eeb
[2]: https://lore.kernel.org/linux-serial/20260204074327.226165-1-jiayuan.chen@linux.dev/T/#u

Reported-by: syzbot+827272712bd6d12c79a4@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000a4a7550611e234f5@google.com/T/
Fixes: 56e0ef527b18 ("drivers/net: caif: fix wrong rtnl_is_locked() usage")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: Jijie Shao <shaojijie@huawei.com>
Link: https://patch.msgid.link/20260206074450.154267-1-jiayuan.chen@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>