Subject: CVE-2021-47377: xen/balloon: use a kernel thread instead a workqueue

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

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

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

xen/balloon: use a kernel thread instead a workqueue

Today the Xen ballooning is done via delayed work in a workqueue. This
might result in workqueue hangups being reported in case of large
amounts of memory are being ballooned in one go (here 16GB):

BUG: workqueue lockup - pool cpus=6 node=0 flags=0x0 nice=0 stuck for 64s!
Showing busy workqueues and worker pools:
workqueue events: flags=0x0
  pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=2/256 refcnt=3
    in-flight: 229:balloon_process
    pending: cache_reap
workqueue events_freezable_power_: flags=0x84
  pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
    pending: disk_events_workfn
workqueue mm_percpu_wq: flags=0x8
  pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
    pending: vmstat_update
pool 12: cpus=6 node=0 flags=0x0 nice=0 hung=64s workers=3 idle: 2222 43

This can easily be avoided by using a dedicated kernel thread for doing
the ballooning work.

The Linux kernel CVE team has assigned CVE-2021-47377 to this issue.

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

	Fixed in 4.14.249 with commit c5d5a43dd2b6
	Fixed in 4.19.209 with commit 6bba79c6a073
	Fixed in 5.4.150 with commit 29917bbb07c3
	Fixed in 5.10.70 with commit 372d3e6ea1e1
	Fixed in 5.14.9 with commit 922fd5b6bb13
	Fixed in 5.15 with commit 8480ed9c2bbd

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-2021-47377
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/xen/balloon.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/c5d5a43dd2b649a0a290bfed00fb76d1aff89be6
	https://git.kernel.org/stable/c/6bba79c6a073741b672b0bf86a1f03c0fe47f973
	https://git.kernel.org/stable/c/29917bbb07c30be295dece245c7c21872e1a6fbb
	https://git.kernel.org/stable/c/372d3e6ea1e115942fdfb4b25f7003d822d071be
	https://git.kernel.org/stable/c/922fd5b6bb13ad31ff36e86e2eba2f26d8135272
	https://git.kernel.org/stable/c/8480ed9c2bbd56fc86524998e5f2e3e22f5038f6