Bug 2393520 (CVE-2025-38733)

Summary: CVE-2025-38733 kernel: s390/mm: Do not map lowcore with identity mapping
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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OS: Linux   
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Description OSIDB Bzimport 2025-09-05 18:03:58 UTC 
In the Linux kernel, the following vulnerability has been resolved:

s390/mm: Do not map lowcore with identity mapping

Since the identity mapping is pinned to address zero the lowcore is always
also mapped to address zero, this happens regardless of the relocate_lowcore
command line option. If the option is specified the lowcore is mapped
twice, instead of only once.

This means that NULL pointer accesses will succeed instead of causing an
exception (low address protection still applies, but covers only parts).
To fix this never map the first two pages of physical memory with the
identity mapping.
Comment 1 Jon Moroney 2025-09-05 21:14:48 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025090543-CVE-2025-38733-65a1@gregkh/T