CVE-2025-39726: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-39726 affects the Linux kernel's s390/ism driver, specifically related to concurrency management in the ism_cmd() function. The vulnerability was discovered and disclosed on September 5, 2025. The issue exists because the driver does not honor the s390x ISM device data sheet requirement that only one request-response sequence is allowable per ISM function at any point in time (NVD).

The vulnerability stems from improper synchronization in the s390/ism driver, where commands (inputs to the firmware) may be partially or fully overwritten by another CPU attempting to issue commands on the same function. This can result in DMB token values being used as DMB IOVAs, leading to PCI events indicating invalid DMA. The vulnerability has been assigned a CVSS v3.1 Base Score of 5.5, indicating moderate severity (Red Hat).

When exploited, the vulnerability can cause ISM functions to enter an error state (with PEC 2 or PEC 3A) after certain workloads. This results in broken connections handled by the affected function, and future connection requests fail to consider the ISM device due to its dysfunctional state. The issue can also lead to command execution errors where one command might be lost while another is executed twice (NVD).

The vulnerability has been fixed in newer kernel versions. Red Hat has marked this for deferred fixes in Red Hat Enterprise Linux versions 8, 9, and 10. Some Linux distributions like Debian have already implemented fixes in their trixie, forky, and sid releases (Debian Tracker).