In the Linux kernel, the following vulnerability has been resolved:

serial: caif: fix use-after-free in caifserial ldiscclose()

There is a use-after-free bug in caifserial where handletx() 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 ttykrefput() 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 ldiscopen and freed in serrelease via unregisternetdevice, and each ser binds exactly one tty), we can safely defer the tty reference release to serrelease() where the network device is unregistered.

Fix this by moving ttykrefput() from ldiscclose() to serrelease(), 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 unregisternetdevice() because ser is embedded in netdev's private data and will be freed along with netdev (needsfree_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 handletx() when using PORTUNKNOWN 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 uartwriteroom() and uart_write() in serial core. It has been addressed in a separate patch [2].

KASAN report:

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

Call Trace: <TASK> dumpstacklvl+0x10e/0x1f0 printreport+0xd0/0x630 kasanreport+0xe4/0x120 handletx+0x5d1/0x620 devhardstartxmit+0x9d/0x6c0 __devqueuexmit+0x6e2/0x4410 packetxmit+0x243/0x360 packetsendmsg+0x26cf/0x5500 __sys_sendto+0x4a3/0x520 __x64syssendto+0xe0/0x1c0 dosyscall64+0xc9/0xf80 entrySYSCALL64afterhwframe+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