Subject: CVE-2021-47084: hamradio: defer ax25 kfree after unregister_netdev

From: Greg Kroah-Hartman <[email-masked]>

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

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

hamradio: defer ax25 kfree after unregister_netdev

There is a possible race condition (use-after-free) like below

 (USE)                       |  (FREE)
ax25_sendmsg                 |
 ax25_queue_xmit             |
  dev_queue_xmit             |
   __dev_queue_xmit          |
    __dev_xmit_skb           |
     sch_direct_xmit         | ...
      xmit_one               |
       netdev_start_xmit     | tty_ldisc_kill
        __netdev_start_xmit  |  mkiss_close
         ax_xmit             |   kfree
          ax_encaps          |
                             |

Even though there are two synchronization primitives before the kfree:
1. wait_for_completion(&ax->dead). This can prevent the race with
routines from mkiss_ioctl. However, it cannot stop the routine coming
from upper layer, i.e., the ax25_sendmsg.

2. netif_stop_queue(ax->dev). It seems that this line of code aims to
halt the transmit queue but it fails to stop the routine that already
being xmit.

This patch reorder the kfree after the unregister_netdev to avoid the
possible UAF as the unregister_netdev() is well synchronized and won't
return if there is a running routine.

The Linux kernel CVE team has assigned CVE-2021-47084 to this issue.

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

	Fixed in 4.4.297 with commit b5b193d0c671
	Fixed in 4.9.295 with commit 8a1a314965a1
	Fixed in 4.14.260 with commit eaa816a86e62
	Fixed in 4.19.223 with commit 896193a02a29
	Fixed in 5.4.169 with commit ef5f7bfa19e3
	Fixed in 5.10.89 with commit 450121075a6a
	Fixed in 5.15.12 with commit cb6c99aedd2c
	Fixed in 5.16 with commit 3e0588c291d6

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-2021-47084
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/b5b193d0c67180fefdc664650138e3b7959df615
	https://git.kernel.org/stable/c/8a1a314965a17c62084a056b4f2cb7a770854c90
	https://git.kernel.org/stable/c/eaa816a86e629cbcc0a94f38391fee09231628c7
	https://git.kernel.org/stable/c/896193a02a2981e60c40d4614fd095ce92135ccd
	https://git.kernel.org/stable/c/ef5f7bfa19e3fc366f4c6d1a841ceaddf7a9f5d4
	https://git.kernel.org/stable/c/450121075a6a6f1d50f97225d3396315309d61a1
	https://git.kernel.org/stable/c/cb6c99aedd2c843056a598a8907a6128cb07603b
	https://git.kernel.org/stable/c/3e0588c291d6ce225f2b891753ca41d45ba42469