CVE-2021-4439: isdn: cpai: check ctr->cnr to avoid array index out of bound

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Description
===========

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]

The Linux kernel CVE team has assigned CVE-2021-4439 to this issue.


Affected and fixed versions
===========================

	Fixed in 4.4.290 with commit e8b8de17e164
	Fixed in 4.9.288 with commit 24219a977bfe
	Fixed in 4.14.253 with commit 9b6b2db77bc3
	Fixed in 4.19.214 with commit 7d91adc0ccb0
	Fixed in 5.4.156 with commit 285e9210b1fa
	Fixed in 5.10.76 with commit 7f221ccbee4e
	Fixed in 5.14.15 with commit cc20226e218a
	Fixed in 5.15 with commit 1f3e2e97c003

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-4439
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/isdn/capi/kcapi.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/e8b8de17e164c9f1b7777f1c6f99d05539000036
	https://git.kernel.org/stable/c/24219a977bfe3d658687e45615c70998acdbac5a
	https://git.kernel.org/stable/c/9b6b2db77bc3121fe435f1d4b56e34de443bec75
	https://git.kernel.org/stable/c/7d91adc0ccb060ce564103315189466eb822cc6a
	https://git.kernel.org/stable/c/285e9210b1fab96a11c0be3ed5cea9dd48b6ac54
	https://git.kernel.org/stable/c/7f221ccbee4ec662e2292d490a43ce6c314c4594
	https://git.kernel.org/stable/c/cc20226e218a2375d50dd9ac14fb4121b43375ff
	https://git.kernel.org/stable/c/1f3e2e97c003f80c4b087092b225c8787ff91e4d