CVE-2021-46990: powerpc/64s: Fix crashes when toggling entry flush barrier

CVE-2021-46990: powerpc/64s: Fix crashes when toggling entry flush barrier

[Greg Kroah-Hartman]

From: gregkh@kernel.org

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

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

powerpc/64s: Fix crashes when toggling entry flush barrier

The entry flush mitigation can be enabled/disabled at runtime via a
debugfs file (entry_flush), which causes the kernel to patch itself to
enable/disable the relevant mitigations.

However depending on which mitigation we're using, it may not be safe to
do that patching while other CPUs are active. For example the following
crash:

  sleeper[15639]: segfault (11) at c000000000004c20 nip c000000000004c20 lr c000000000004c20

Shows that we returned to userspace with a corrupted LR that points into
the kernel, due to executing the partially patched call to the fallback
entry flush (ie. we missed the LR restore).

Fix it by doing the patching under stop machine. The CPUs that aren't
doing the patching will be spinning in the core of the stop machine
logic. That is currently sufficient for our purposes, because none of
the patching we do is to that code or anywhere in the vicinity.

The Linux kernel CVE team has assigned CVE-2021-46990 to this issue.


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

	Issue introduced in 4.4.245 with commit 4a1e90af718d and fixed in 4.4.269 with commit 8382b15864e5
	Issue introduced in 4.9.245 with commit fa4bf9f38184 and fixed in 4.9.269 with commit 0c25a7bb697f
	Issue introduced in 4.14.208 with commit db01cad9efe3 and fixed in 4.14.233 with commit ee4b7aab93c2
	Issue introduced in 4.19.159 with commit f69bb4e51f41 and fixed in 4.19.191 with commit 2db22ba4e0e1
	Issue introduced in 5.4.79 with commit b65458b6be80 and fixed in 5.4.120 with commit 0b4eb172cc12
	Issue introduced in 5.10 with commit f79643787e0a and fixed in 5.10.38 with commit d2e3590ca39c
	Issue introduced in 5.10 with commit f79643787e0a and fixed in 5.11.22 with commit dd0d6117052f
	Issue introduced in 5.10 with commit f79643787e0a and fixed in 5.12.5 with commit 5bc00fdda1e9
	Issue introduced in 5.10 with commit f79643787e0a and fixed in 5.13 with commit aec86b052df6

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-2021-46990
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/powerpc/lib/feature-fixups.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/8382b15864e5014261b4f36c2aa89723612ee058
	https://git.kernel.org/stable/c/0c25a7bb697f2e6ee65b6d63782f675bf129511a
	https://git.kernel.org/stable/c/ee4b7aab93c2631c3bb0753023c5dda592bb666b
	https://git.kernel.org/stable/c/2db22ba4e0e103f00e0512e0ecce36ac78c644f8
	https://git.kernel.org/stable/c/0b4eb172cc12dc102cd0ad013e53ee4463db9508
	https://git.kernel.org/stable/c/d2e3590ca39ccfd8a5a46d8c7f095cb6c7b9ae92
	https://git.kernel.org/stable/c/dd0d6117052faace5440db20fc37175efe921c7d
	https://git.kernel.org/stable/c/5bc00fdda1e934c557351a9c751a205293e68cbf
	https://git.kernel.org/stable/c/aec86b052df6541cc97c5fca44e5934cbea4963b