CVE-2026-45866

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

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 netdevs 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: 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

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/308e7e4d0a846359685f40aade023aee7b27284c
• https://git.kernel.org/stable/c/331e2b7051635780edea248dd08ae2026c126f4a
• https://git.kernel.org/stable/c/40962f2bf8cdba63af23aec95ad3f49b689e58e2
• https://git.kernel.org/stable/c/4e63d6f68544ae5269ac9735ae5b69b59b5b8725
• https://git.kernel.org/stable/c/52731ef4438155cea782fac74e547a327ab9e7c5
• https://git.kernel.org/stable/c/5e266ba8d330d3b8e5bc198f238cd8901826cfa1
• https://git.kernel.org/stable/c/c8c197aaa56b25a2d54f3aa07e27e228d6c08546
• https://git.kernel.org/stable/c/d3c75db4e0460641dbcd274b40867e252d801da1