Describe CVE-2021-4439.
What is CVE-2021-4439?
In the Linux kernel, the following vulnerability has been resolved:

isdn: cpai: check ctr->cnr to avoid array index out of bound

The cmtp_add_connection() would add a cmtp session to a controller
and run a kernel thread to process cmtp.

	__module_get(THIS_MODULE);
	session->task = kthread_run(cmtp_session, session, "kcmtpd_ctr_%d",
								session->num);

During this process, the kernel thread would call detach_capi_ctr()
to detach a register controller. if the controller
was not attached yet, detach_capi_ctr() would
trigger an array-index-out-bounds bug.

[   46.866069][ T6479] UBSAN: array-index-out-of-bounds in
drivers/isdn/capi/kcapi.c:483:21
[   46.867196][ T6479] index -1 is out of range for type 'capi_ctr *[32]'
[   46.867982][ T6479] CPU: 1 PID: 6479 Comm: kcmtpd_ctr_0 Not tainted
5.15.0-rc2+ #8
[   46.869002][ T6479] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.14.0-2 04/01/2014
[   46.870107][ T6479] Call Trace:
[   46.870473][ T6479]  dump_stack_lvl+0x57/0x7d
[   46.870974][ T6479]  ubsan_epilogue+0x5/0x40
[   46.871458][ T6479]  __ubsan_handle_out_of_bounds.cold+0x43/0x48
[   46.872135][ T6479]  detach_capi_ctr+0x64/0xc0
[   46.872639][ T6479]  cmtp_session+0x5c8/0x5d0
[   46.873131][ T6479]  ? __init_waitqueue_head+0x60/0x60
[   46.873712][ T6479]  ? cmtp_add_msgpart+0x120/0x120
[   46.874256][ T6479]  kthread+0x147/0x170
[   46.874709][ T6479]  ? set_kthread_struct+0x40/0x40
[   46.875248][ T6479]  ret_from_fork+0x1f/0x30
[   46.875773][ T6479].