CVE-2026-64369: s390: Revert support for DCACHE_WORD_ACCESS

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

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

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

s390: Revert support for DCACHE_WORD_ACCESS

load_unaligned_zeropad() reads eight bytes from unaligned addresses and may
cross page boundaries. It handles exceptions which may happen if reading
from the second page results in an exception.

For pages which are donated to the Ultravisor for secure execution purposes
the do_secure_storage_access() exception handler however does not handle
such exceptions correctly. Such an exception may result in an endless
exception loop which will never be resolved.

An attempt to fix this [1] turned out to be not sufficient. For now revert
load_unaligned_zeropad() until this problem has been resolved in a proper
way.

Note that the implementation of load_unaligned_zeropad() itself is
correct. The revert is just a temporary workaround until there is complete
fix for secure storage access exceptions.

[1] commit b00be77302d7 ("s390/mm: Add missing secure storage access fixups for donated memory")

The Linux kernel CVE team has assigned CVE-2026-64369 to this issue.


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

	Issue introduced in 6.7 with commit 802ba53eefc592a6a82231f74e19bafe3256f172 and fixed in 6.12.97 with commit c9e0f1517631ac08987f8385817119bccf2f1f12
	Issue introduced in 6.7 with commit 802ba53eefc592a6a82231f74e19bafe3256f172 and fixed in 6.18.39 with commit be79d285bea70d0edd5015bd487311bfa8cbebc9
	Issue introduced in 6.7 with commit 802ba53eefc592a6a82231f74e19bafe3256f172 and fixed in 7.1.4 with commit c94806905e02cc8e17a69c822d93c41743b7ffc5
	Issue introduced in 6.7 with commit 802ba53eefc592a6a82231f74e19bafe3256f172 and fixed in 7.2-rc1 with commit 37540b8c287fc817bdbd0c62bb75ad6eab0e5d03

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-2026-64369
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/s390/Kconfig
	arch/s390/include/asm/asm-extable.h
	arch/s390/include/asm/word-at-a-time.h
	arch/s390/mm/extable.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/c9e0f1517631ac08987f8385817119bccf2f1f12
	https://git.kernel.org/stable/c/be79d285bea70d0edd5015bd487311bfa8cbebc9
	https://git.kernel.org/stable/c/c94806905e02cc8e17a69c822d93c41743b7ffc5
	https://git.kernel.org/stable/c/37540b8c287fc817bdbd0c62bb75ad6eab0e5d03