CVE-2026-64369: s390: Revert support for DCACHE_WORD_ACCESS
In the Linux kernel, the following vulnerability has been resolved:
s390: Revert support for DCACHEWORDACCESS
loadunalignedzeropad() 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 dosecurestorageaccess() 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 loadunalignedzeropad() until this problem has been resolved in a proper way.
Note that the implementation of loadunalignedzeropad() 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")
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Patch b00be77302d7
Event History
Frequently Asked Questions
Which systems are exposed to this issue?
The issue applies to Linux kernel systems on s390 where pages are donated to the Ultravisor for secure execution purposes. The failure occurs when an unaligned eight-byte read crosses into a second page and that access raises a secure-storage exception.
What is the practical impact if the issue is triggered?
A secure storage access exception on the second page may enter an endless exception loop that cannot be resolved. This results in an availability impact.
Does exploitation require remote access or user interaction?
No. The supplied CVSS vector identifies this as locally exploitable with low attack complexity and low privileges required, with no user interaction.
What mitigation is available if a complete fix is not yet available?
The documented workaround is to revert support for DCACHE_WORD_ACCESS by reverting load_unaligned_zeropad(). This is described as temporary until secure storage access exceptions are properly fixed.