Subject: CVE-2021-47318: arch_topology: Avoid use-after-free for scale_freq_data

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

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

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

arch_topology: Avoid use-after-free for scale_freq_data

Currently topology_scale_freq_tick() (which gets called from
scheduler_tick()) may end up using a pointer to "struct
scale_freq_data", which was previously cleared by
topology_clear_scale_freq_source(), as there is no protection in place
here. The users of topology_clear_scale_freq_source() though needs a
guarantee that the previously cleared scale_freq_data isn't used
anymore, so they can free the related resources.

Since topology_scale_freq_tick() is called from scheduler tick, we don't
want to add locking in there. Use the RCU update mechanism instead
(which is already used by the scheduler's utilization update path) to
guarantee race free updates here.

synchronize_rcu() makes sure that all RCU critical sections that started
before it is called, will finish before it returns. And so the callers
of topology_clear_scale_freq_source() don't need to worry about their
callback getting called anymore.

The Linux kernel CVE team has assigned CVE-2021-47318 to this issue.

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

	Issue introduced in 5.13 with commit 01e055c120a4 and fixed in 5.13.4 with commit ccdf7e073170
	Issue introduced in 5.13 with commit 01e055c120a4 and fixed in 5.14 with commit 83150f5d05f0

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-2021-47318
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/base/arch_topology.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/ccdf7e073170886bc370c613e269de610a794c4a
	https://git.kernel.org/stable/c/83150f5d05f065fb5c12c612f119015cabdcc124