Subject: CVE-2021-47255: kvm: LAPIC: Restore guard to prevent illegal APIC register access

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

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

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

kvm: LAPIC: Restore guard to prevent illegal APIC register access

Per the SDM, "any access that touches bytes 4 through 15 of an APIC
register may cause undefined behavior and must not be executed."
Worse, such an access in kvm_lapic_reg_read can result in a leak of
kernel stack contents. Prior to commit 01402cf81051 ("kvm: LAPIC:
write down valid APIC registers"), such an access was explicitly
disallowed. Restore the guard that was removed in that commit.

The Linux kernel CVE team has assigned CVE-2021-47255 to this issue.

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

	Issue introduced in 5.3 with commit 01402cf81051 and fixed in 5.4.128 with commit bf99ea52970c
	Issue introduced in 5.3 with commit 01402cf81051 and fixed in 5.10.46 with commit 018685461a5b
	Issue introduced in 5.3 with commit 01402cf81051 and fixed in 5.12.13 with commit a2aff09807fb
	Issue introduced in 5.3 with commit 01402cf81051 and fixed in 5.13 with commit 218bf772bddd

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-2021-47255
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:
	arch/x86/kvm/lapic.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/bf99ea52970caeb4583bdba1192c1f9b53b12c84
	https://git.kernel.org/stable/c/018685461a5b9a9a70e664ac77aef0d7415a3fd5
	https://git.kernel.org/stable/c/a2aff09807fbe4018c269d3773a629949058b210
	https://git.kernel.org/stable/c/218bf772bddd221489c38dde6ef8e917131161f6