CVE-2021-47085: hamradio: improve the incomplete fix to avoid NPD

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

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

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

hamradio: improve the incomplete fix to avoid NPD

The previous commit 3e0588c291d6 ("hamradio: defer ax25 kfree after
unregister_netdev") reorder the kfree operations and unregister_netdev
operation to prevent UAF.

This commit improves the previous one by also deferring the nullify of
the ax->tty pointer. Otherwise, a NULL pointer dereference bug occurs.
Partial of the stack trace is shown below.

BUG: kernel NULL pointer dereference, address: 0000000000000538
RIP: 0010:ax_xmit+0x1f9/0x400
...
Call Trace:
 dev_hard_start_xmit+0xec/0x320
 sch_direct_xmit+0xea/0x240
 __qdisc_run+0x166/0x5c0
 __dev_queue_xmit+0x2c7/0xaf0
 ax25_std_establish_data_link+0x59/0x60
 ax25_connect+0x3a0/0x500
 ? security_socket_connect+0x2b/0x40
 __sys_connect+0x96/0xc0
 ? __hrtimer_init+0xc0/0xc0
 ? common_nsleep+0x2e/0x50
 ? switch_fpu_return+0x139/0x1a0
 __x64_sys_connect+0x11/0x20
 do_syscall_64+0x33/0x40
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The crash point is shown as below

static void ax_encaps(...) {
  ...
  set_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags); // ax->tty = NULL!
  ...
}

By placing the nullify action after the unregister_netdev, the ax->tty
pointer won't be assigned as NULL net_device framework layer is well
synchronized.

The Linux kernel CVE team has assigned CVE-2021-47085 to this issue.


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

	Fixed in 4.4.297 with commit 371a874ea06f
	Fixed in 4.9.295 with commit 83ba6ec97c74
	Fixed in 4.14.260 with commit a7b0ae2cc486
	Fixed in 4.19.223 with commit b68f41c6320b
	Fixed in 5.4.169 with commit a5c6a13e9056
	Fixed in 5.10.89 with commit 7dd52af1eb57
	Fixed in 5.15.12 with commit 03d00f7f1815
	Fixed in 5.16 with commit b2f37aead1b8

Please see https://www.kernel.org or 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-2021-47085
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/net/hamradio/mkiss.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/371a874ea06f147d6ca30be43dad33683965eba6
	https://git.kernel.org/stable/c/83ba6ec97c74fb1a60f7779a26b6a94b28741d8a
	https://git.kernel.org/stable/c/a7b0ae2cc486fcb601f9f9d87d98138cc7b7f7f9
	https://git.kernel.org/stable/c/b68f41c6320b2b7fbb54a95f07a69f3dc7e56c59
	https://git.kernel.org/stable/c/a5c6a13e9056d87805ba3042c208fbd4164ad22b
	https://git.kernel.org/stable/c/7dd52af1eb5798f590d9d9e1c56ed8f5744ee0ca
	https://git.kernel.org/stable/c/03d00f7f1815ec00dab5035851b3de83afd054a8
	https://git.kernel.org/stable/c/b2f37aead1b82a770c48b5d583f35ec22aabb61e