Vulnerability CVE-2021-47084

Vulnerability CVE-2021-47084

Published: 2024-03-04

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.

References:

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

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