In the Linux kernel's s390 crypto subsystem, PHMAC requests forwarded to the cryptographic engine may return EBUSY to indicate that the engine is currently busy. The original code interpreted only EINPROGRESS as a successful transfer and treated EBUSY as an error. The missing handling means that when an EBUSY return occurs, cryptographic operations fail or hang, which can result in a denial of service to processes relying on PHMAC functions.

The vulnerability affects any Linux kernel that includes the s390 crypto subsystem before the patch that added the correct treatment of EBUSY. The kernel commit referenced in the description (203702f0013f9b7324e8c5facb6cd6e9c0942b13) addresses the issue. All builds of the Linux kernel for the s390 architecture, as identified by the CPE string for the Linux kernel, potentially contain the flaw if they precede that commit. Systems running such kernels on IBM S390 hardware are most likely impacted.

The CVSS score of 5.5 indicates a moderate severity, and the EPSS score of <1% reflects a very low probability of real-world exploitation. The issue is not listed in CISA’s KEV catalog. The description does not state that privileges are required to trigger PHMAC operations, so it is unclear whether the flaw can be exercised by ordinary users or only by privileged code. Every scenario leads to a failure or suspension of cryptographic services, but does not expose data or allow code execution. Therefore the risk remains primarily a local denial of service unless elevated privileges can trigger PHMAC repeatedly.